abc transporters

7/31/2019 ABC Transporters

1/21

ATP-binding cassette transporter 1

ATP-binding cassette transporter

ABC Transporter

Vitamin B12

transporter, BtuCD PDB 1l7v[1]

Identifiers

Symbol ABC_tran

Pfam PF00005[2]

InterPro IPR003439[3]

PROSITE PDOC00185[4]

SCOP 1b0u[5]

SUPERFAMILY 1b0u[6]

TCDB 3.A.1[7]

OPM superfamily 17[8]

OPM protein 3g5u[9]

Available protein structures:

Pfam structures[10]

PDB RCSB PDB[11]

; PDBe[12]

PDBsum structure summary[13]
http://www.ebi.ac.uk/thornton-srv/databases/cgi-bin/pdbsum/GetPfamStr.pl?pfam_id=PF00005http://en.wikipedia.org/w/index.php?title=PDBsumhttp://www.ebi.ac.uk/pdbe-srv/PDBeXplore/pfam/?pfam=PF00005http://www.rcsb.org/pdb/search/smartSubquery.do?smartSearchSubtype=PfamIdQuery&pfamID=PF00005http://en.wikipedia.org/w/index.php?title=Protein_Data_Bankhttp://pfam.sanger.ac.uk/family/PF00005?tab=pdbBlockhttp://en.wikipedia.org/w/index.php?title=Pfamhttp://opm.phar.umich.edu/protein.php?search=3g5uhttp://en.wikipedia.org/w/index.php?title=Orientations_of_Proteins_in_Membranes_databasehttp://opm.phar.umich.edu/families.php?superfamily=17http://en.wikipedia.org/w/index.php?title=Orientations_of_Proteins_in_Membranes_databasehttp://www.tcdb.org/search/result.php?tc=3.A.1http://en.wikipedia.org/w/index.php?title=TCDBhttp://supfam.org/SUPERFAMILY/cgi-bin/search.cgi?search_field=1b0uhttp://en.wikipedia.org/w/index.php?title=SUPERFAMILYhttp://scop.mrc-lmb.cam.ac.uk/scop/search.cgi?tlev=fa;&pdb=1b0uhttp://en.wikipedia.org/w/index.php?title=Structural_Classification_of_Proteinshttp://www.expasy.org/cgi-bin/prosite-search-ac?PDOC00185http://en.wikipedia.org/w/index.php?title=PROSITEhttp://www.ebi.ac.uk/interpro/DisplayIproEntry?ac=IPR003439http://en.wikipedia.org/w/index.php?title=InterProhttp://pfam.sanger.ac.uk/family?acc=PF00005http://en.wikipedia.org/w/index.php?title=Pfamhttp://www.pdbe.org/1l7vhttp://en.wikipedia.org/w/index.php?title=File%3A1l7v_opm.gif


2/21


Lipid flippase MsbA

Molybdate transporter AB2C

2complex, open

state

ATP-binding cassette transporters (ABC-transporter) are members

of a protein superfamily that is one of the largest and most ancient

families with representatives in all extant phyla from prokaryotes to

humans.[14][15] ABC transporters are transmembrane proteins that

utilize the energy of adenosine triphosphate (ATP) hydrolysis to carry

out certain biological processes including translocation of varioussubstrates across membranes and non-transport-related processes such

as translation of RNA and DNA repair.[16][17] They transport a wide

variety of substrates across extra- and intracellular membranes,

including metabolic products, lipids and sterols, and drugs. Proteins are

classified as ABC transporters based on the sequence and organization

of their ATP-binding cassette (ABC) domain(s). ABC transporters

are involved in tumor resistance, cystic fibrosis and a range of other

inherited human diseases along with both bacterial (prokaryotic) and

eukaryotic (including human) developement of resistance to multiple

drugs.

Function

ABC transporters utilize the energy of ATP hydrolysis to transport

various substrates across cellular membranes. They are divided into

three main functional categories. In prokaryotes, importers mediate the

uptake of nutrients into the cell. The substrates that can be transported

include ions, amino acids, peptides, sugars, and other molecules that

are mostly hydrophilic. The membrane-spanning region of the ABC

transporter protects hydrophilic substrates from the lipids of themembrane bilayer thus providing a pathway across the cell membrane.

Eukaryotes do not possess any importers.Exporters or effluxers, which

are both present in prokaryotes and eukaryotes, function as pumps that

extrude toxins and drugs out of the cell. In gram-negative bacteria,

exporters transport lipids and some polysaccharides from the

cytoplasm to the periplasm. The third subgroup of ABC proteins do not

function as transporters, but are rather involved in translation and DNA

repair processes.[16]

Prokaryotic ABC proteins

Bacterial ABC transporters are essential in cell viability, virulence, and

pathogenicity.[16] Iron ABC uptake systems, for example, are important effectors of virulence.[18] Pathogens use

siderophores, such as FepA, to scavenge iron that is in complex with high-affinity iron-binding proteins or

erythrocytes. These are high-affinity iron-complexing molecules that are secreted by bacteria and reabsorb iron into

iron-siderophore complexes. The chvE-gguAB gene in Agrobacterium tumefaciens encodes glucose and galactose

importers that are also associated with virulence.[19][20] Transporters are extremely vital in cell survival such that

they function as protein systems that counteract any undesirable change occurring in the cell. For instance, a

potential lethal increase in osmotic strength is counterbalanced by activation of osmosensing ABC transporters that

mediate uptake of solutes.[21] Other than functioning in transport, some bacterial ABC proteins are also involved inthe regulation of several physiological processes.[16]
http://en.wikipedia.org/w/index.php?title=Osmosishttp://en.wikipedia.org/w/index.php?title=Galactosehttp://en.wikipedia.org/w/index.php?title=Glucosehttp://en.wikipedia.org/w/index.php?title=Genehttp://en.wikipedia.org/w/index.php?title=Erythrocyteshttp://en.wikipedia.org/w/index.php?title=FepAhttp://en.wikipedia.org/w/index.php?title=Siderophoreshttp://en.wikipedia.org/w/index.php?title=Pathogenshttp://en.wikipedia.org/w/index.php?title=Virulencehttp://en.wikipedia.org/w/index.php?title=Periplasmhttp://en.wikipedia.org/w/index.php?title=Cytoplasmhttp://en.wikipedia.org/w/index.php?title=Polysaccharidehttp://en.wikipedia.org/w/index.php?title=Gram-negative_bacteriahttp://en.wikipedia.org/w/index.php?title=Eukaryoteshttp://en.wikipedia.org/w/index.php?title=Lipid_bilayerhttp://en.wikipedia.org/w/index.php?title=Hydrophilichttp://en.wikipedia.org/w/index.php?title=Sugarhttp://en.wikipedia.org/w/index.php?title=Peptidehttp://en.wikipedia.org/w/index.php?title=Amino_acidhttp://en.wikipedia.org/w/index.php?title=Ionhttp://en.wikipedia.org/w/index.php?title=Nutrienthttp://en.wikipedia.org/w/index.php?title=Cell_membranehttp://en.wikipedia.org/w/index.php?title=Substrate_%28biochemistry%29http://en.wikipedia.org/w/index.php?title=Cystic_fibrosishttp://en.wikipedia.org/w/index.php?title=Medicationhttp://en.wikipedia.org/w/index.php?title=Sterolhttp://en.wikipedia.org/w/index.php?title=Lipidhttp://en.wikipedia.org/w/index.php?title=Metabolismhttp://en.wikipedia.org/w/index.php?title=Cell_membranehttp://en.wikipedia.org/w/index.php?title=Adenosine_triphosphatehttp://en.wikipedia.org/w/index.php?title=Transmembrane_proteinhttp://en.wikipedia.org/w/index.php?title=Prokaryotehttp://en.wikipedia.org/w/index.php?title=Phylumhttp://en.wikipedia.org/w/index.php?title=Extant_taxonhttp://en.wikipedia.org/w/index.php?title=Proteinhttp://en.wikipedia.org/w/index.php?title=Gene_cassettehttp://en.wikipedia.org/w/index.php?title=File%3A2onk.gifhttp://en.wikipedia.org/w/index.php?title=File%3A3b60.gif


3/21


In bacterial efflux systems, certain substances that need to be extruded from the cell include surface components of

the bacterial cell (e.g. capsular polysaccharides, lipopolysaccharides, and teichoic acid), proteins involved in

bacterial pathogenesis (e.g. hemolysis, heme-binding protein, and alkaline protease), heme, hydrolytic enzymes,

S-layer proteins, competence factors, toxins, antibiotics, bacteriocins, peptide antibiotics, drugs and siderophores.[22]

They also play important roles in biosynthetic pathways, including extracellular polysaccharide biosynthesis [23] and

cytochrome biogenesis.[24]

Eukaryotic ABC proteins

Although most eukaryotic ABC transporters are effluxers, some are not directly involved in transporting substrates.

In the cystic fibrosis transmembrane regulator (CFTR) and in the sulfonylurea receptor (SUR), ATP hydrolysis is

associated with the regulation of opening and closing of ion channels carried by the ABC protein itself or other

proteins.[17]

Human ABC transporters are involved in several diseases that arise from polymorphisms in ABC genes and rarely

due to complete loss of function of single ABC proteins.[25] Such diseases include Mendelian diseases and complex

genetic disorders such as cystic fibrosis, adrenoleukodystrophy, Stargardt disease, Tangier disease, immune

deficiencies, progressive familial intraheptic cholestasis, Dubin-Johnson syndrome, Pseudoxanthoma elasticum,persistent hyperinsulinemic hypoglycemia of infancy due to focal adenomatous hyperplasia, X-linked sideroblastosis

and anemia, age-related macular degeneration, familial hypoapoproteinemia, Retinitis pigmentosum, cone rod

dystrophy, and others.[17] The human ABCB (MDR/TAP) family is responsible for multiple drug resistance (MDR)

against a variety of structurally unrelated drugs. ABCB1 or MDR1 P-glycoprotein is also involved in other

biological processes for which lipid transport is the main function. It is found to mediate the secretion of the steroid

aldosterone by the adrenals, and its inhibition blocked the migration of dendritic immune cells, possibly related to

the outward transport of the lipid platelet activating factor (PAF). It has also been reported that ABCB1 mediates

transport of cortisol and dexamethasone, but not of progesterone in ABCB1 transfected cells. MDR1 can also

transport cholesterol, short-chain and long-chain analogs of phosphatidylcholine (PC), phosphatidylethanolamine

(PE), phosphatidylserine (PS), sphingomyelin (SM), and glucosylceramide (GlcCer). Multispecific transport ofdiverse endogenous lipids through the MDR1 transporter can possibly affect the transbilayer distribution of lipids, in

particular of species normally predominant on the inner plasma membrane leaflet such as PS and PE.[25]

More recently, ABC-transporters have been shown to exist within the placenta, indicating they could play a

protective role for the developing fetus against xenobiotics.[26]
http://en.wikipedia.org/w/index.php?title=Xenobiotichttp://en.wikipedia.org/w/index.php?title=Placentahttp://en.wikipedia.org/w/index.php?title=Glucosylceramidehttp://en.wikipedia.org/w/index.php?title=Sphingomyelinhttp://en.wikipedia.org/w/index.php?title=Phosphatidylserinehttp://en.wikipedia.org/w/index.php?title=Phosphatidylethanolaminehttp://en.wikipedia.org/w/index.php?title=Phosphatidylcholinehttp://en.wikipedia.org/w/index.php?title=Cholesterolhttp://en.wikipedia.org/w/index.php?title=Progesteronehttp://en.wikipedia.org/w/index.php?title=Dexamethasonehttp://en.wikipedia.org/w/index.php?title=Cortisolhttp://en.wikipedia.org/w/index.php?title=Platelet_activating_factorhttp://en.wikipedia.org/w/index.php?title=Dendritichttp://en.wikipedia.org/w/index.php?title=Aldosteronehttp://en.wikipedia.org/w/index.php?title=P-glycoproteinhttp://en.wikipedia.org/w/index.php?title=Multiple_drug_resistancehttp://en.wikipedia.org/w/index.php?title=Corneal_dystrophyhttp://en.wikipedia.org/w/index.php?title=Corneal_dystrophyhttp://en.wikipedia.org/w/index.php?title=Macular_degenerationhttp://en.wikipedia.org/w/index.php?title=Sideroblastic_anemiahttp://en.wikipedia.org/w/index.php?title=Sideroblastic_anemiahttp://en.wikipedia.org/w/index.php?title=Hyperplasiahttp://en.wikipedia.org/w/index.php?title=Hyperinsulinemic_hypoglycemiahttp://en.wikipedia.org/w/index.php?title=Pseudoxanthoma_elasticumhttp://en.wikipedia.org/w/index.php?title=Dubin-Johnson_syndromehttp://en.wikipedia.org/w/index.php?title=Cholestasishttp://en.wikipedia.org/w/index.php?title=Tangier_diseasehttp://en.wikipedia.org/w/index.php?title=Stargardt_diseasehttp://en.wikipedia.org/w/index.php?title=Adrenoleukodystrophyhttp://en.wikipedia.org/w/index.php?title=Mendelianhttp://en.wikipedia.org/w/index.php?title=Polymorphism_%28biology%29http://en.wikipedia.org/w/index.php?title=Sulfonylureahttp://en.wikipedia.org/w/index.php?title=Cystic_fibrosis_transmembrane_conductance_regulatorhttp://en.wikipedia.org/w/index.php?title=Cystic_fibrosishttp://en.wikipedia.org/w/index.php?title=Cytochromehttp://en.wikipedia.org/w/index.php?title=Antibioticshttp://en.wikipedia.org/w/index.php?title=Bacteriocinshttp://en.wikipedia.org/w/index.php?title=Antibioticshttp://en.wikipedia.org/w/index.php?title=Toxinshttp://en.wikipedia.org/w/index.php?title=Hydrolytic_enzymeshttp://en.wikipedia.org/w/index.php?title=Proteasehttp://en.wikipedia.org/w/index.php?title=Hemehttp://en.wikipedia.org/w/index.php?title=Hemolysishttp://en.wikipedia.org/w/index.php?title=Teichoic_acidhttp://en.wikipedia.org/w/index.php?title=Lipopolysaccharides


4/21


Structure

Structure of an ABC importer: BtuCD with binding protein (PDB 2qi9[27]

)

Structure of an ABC exporter: Sav1866 with bound nucleotide (PDB 2onj[28]

)

The common feature of all ABC transporters

is that they consist of two distinct domains,

the transmembrane domain (TMD) and the

nucleotide-binding domain (NBD). The

TMD, also known as membrane-spanningdomain (MSD) or integral membrane (IM)

domain, consists of alpha helices, embedded

in the membrane bilayer. It recognizes a

variety of substrates and undergoes

conformational changes to transport the

substrate across the membrane. The

sequence and architecture of TMDs is

variable, reflecting the chemical diversity of

substrates that can be translocated. The

NBD or ATP-binding cassette (ABC)

domain, on the other hand, is located in the

cytoplasm and has a highly conserved

sequence. The NBD is the site for ATP

binding.[29] In most exporters, the

N-terminal transmembrane domain and the

C-terminal ABC domains are fused as a

single polypeptide chain, arranged as

TMD-NBD-TMD-NBD. An example is the

E. coli hemolysin exporter HlyB. Importers

have an inverted organization, that is,

NBD-TMD-NBD-TMD, where the ABC

domain is N-terminal whereas the TMD is

C-terminal, such as in the E. coli MacB

protein responsible for macrolide

resistance.[16][17]

The structural architecture of ABC

transporters consists minimally of two

TMDs and two ABCs. Four individual

polypeptide chains including two TMD and two NBD subunits, may combine to form a full transportersuch as in

the E. coli BtuCD[30][31] importer involved in the uptake of vitamin B12

. Most exporters, such as in the multidrug

exporter Sav1866[32] from Staphylococcus aureus, are made up of a homodimer consisting of two half transporters

or monomers of a TMD fused to a nucleotide-binding domain (NBD). A full transporter is often required to gain

functionality. Some ABC transporters have additional elements that contribute to the regulatory function of this class

of proteins. In particular, importers have a high-affinity binding protein (BP) that specifically associates with the

substrate in the periplasm for delivery to the appropriate ABC transporter. Exporters do not have the binding protein

but have an intracellular domain (ICD) that joins the membrane-spanning helices and the ABC domain. The ICD is

believed to be responsible for communication between the TMD and NBD.[29]
http://en.wikipedia.org/w/index.php?title=Monomershttp://en.wikipedia.org/w/index.php?title=Homodimerhttp://en.wikipedia.org/w/index.php?title=Vitamin_B12http://en.wikipedia.org/w/index.php?title=Macrolidehttp://en.wikipedia.org/w/index.php?title=Alpha_heliceshttp://en.wikipedia.org/w/index.php?title=ATP-binding_domain_of_ABC_transportershttp://en.wikipedia.org/w/index.php?title=File%3AAbc-sav.jpghttp://www.rcsb.org/pdb/explore/explore.do?structureId=2onjhttp://en.wikipedia.org/w/index.php?title=Protein_Data_Bankhttp://en.wikipedia.org/w/index.php?title=File%3ABtucd.jpghttp://www.rcsb.org/pdb/explore/explore.do?structureId=2qi9http://en.wikipedia.org/w/index.php?title=Protein_Data_Bank


5/21


Transmembrane domain (TMD)

Most transporters have transmembrane domains that consist of a total of 12 -helices with 6 -helices per monomer.

Since TMDs are structurally diverse, some transporters have varying number of helices (between six to eleven). The

TM domains are categorized into three distinct sets of folds: type I ABC importer, type II ABC importerand ABC

exporterfolds. The classification of importer folds is based on detailed characterization of the sequences.[29] The

type I ABC importer fold was originally observed in the ModB TM subunit of the molybdate transporter.[33]

Thisdiagnostic fold can also be found in the MalF and MalG TM subunits of MalFGK

2[34] and the Met transporter

MetI.[35] In the MetI transporter, a minimal set of 5 transmembrane helices constitute this fold while an additional

helix is present for both ModB and MalG. The common organization of the fold is the up-down topology of the

TM2-5 helices that lines the translocation pathway and the TM1 helix wrapped around the outer, membrane-facing

surface and contacts the other TM helices. The type II ABC importer fold is observed in the twenty TM

helix-domain of BtuCD[30] and in Hi1471,[36] a homologous transporter from Haemophilus influenzae. In BtuCD,

the packing of the helices is complex. The noticeable pattern is that the TM2 helix is positioned through the center of

the subunit where it is surrounded in close proximity by the other helices. Meanwhile, the TM5 and TM10 helices

are positioned in the TMD interface. The membrane spanning region of ABC exporters is organized into two wings

that are composed of helices TM1 and TM2 from one subunit and TM3-6 of the other, in a domain-swappedarrangement. A prominent pattern is that helices TM1-3 are related to TM4-6 by an approximate twofold rotation

around an axis in the plane of the membrane.[29]

Nucleotide-binding domain (NBD)

Structure of the NBD of ABC transporters with bound nucleotide (PDB 2onj [28]). Linear

representation of protein sequence above shows the relative positions of the conserved

amino acid motifs in the structure (colors match with 3D structure)

The ABC domain consists of two

domains, the catalytic core domain

similar to RecA-like motor ATPases

and a smaller, structurally diverse

-helical subdomain that is unique to

ABC transporters. The larger domain

typically consists of two -sheets and

six helices, where the catalytic

Walker A motif (GXXGXGKS/T

where X is any amino acid) or P-loop

and Walker B motif (D, of

which is a hydrophobic residue) is

situated. The helical domain consists

of three or four helices and the ABC

signature motif, also known asLSGGQmotif, linker peptide or C motif. The

ABC domain also has a glutamine

residue residing in a flexible loop

called Q loop, lid or -phosphate switch, that connects the TMD and ABC. The Q loop is presumed to be involved in

the interaction of the NBD and TMD, particularly in the coupling of nucleotide hydrolysis to the conformational

changes of the TMD during substrate translocation. The H motif or switch region contains a highly conserved

histidine residue that is also important in the interaction of the ABC domain with ATP. The name ATP-binding

cassette is derived from the diagnostic arrangement of the folds or motifs of this class of proteins upon formation of

the ATP sandwich and ATP hydrolysis.[16][22][29]
http://en.wikipedia.org/w/index.php?title=Protein_Data_Bankhttp://www.rcsb.org/pdb/explore/explore.do?structureId=2onjhttp://en.wikipedia.org/w/index.php?title=Hydrolysishttp://en.wikipedia.org/w/index.php?title=Hydrolysishttp://en.wikipedia.org/w/index.php?title=Histidinehttp://en.wikipedia.org/w/index.php?title=Histidinehttp://en.wikipedia.org/w/index.php?title=Histidinehttp://en.wikipedia.org/w/index.php?title=Hydrolysishttp://en.wikipedia.org/w/index.php?title=Walker_motifshttp://en.wikipedia.org/w/index.php?title=ATPaseshttp://en.wikipedia.org/w/index.php?title=RecAhttp://en.wikipedia.org/w/index.php?title=File%3AAbc_domain.jpghttp://www.rcsb.org/pdb/explore/explore.do?structureId=2onjhttp://en.wikipedia.org/w/index.php?title=Protein_Data_Bankhttp://en.wikipedia.org/w/index.php?title=Molybdate


6/21


ATP binding and hydrolysis

Dimer formation of the two ABC domains of transporters requires ATP binding.[37] It is generally observed that the

ATP bound state is associated with the most extensive interface between ABC domains, whereas the structures of

nucleotide-free transporters exhibit conformations with greater separations between the ABC domains.[29] Structures

of the ATP-bound state of isolated NBDs have been reported for importers including HisP, [38] GlcV,[39] MJ1267,[40]

E. coli MalK (E.c.MalK),[41]

T. litoralis MalK (TlMalK),[42]

and exporters such as TAP,[43]

HlyB,[44]

MJ0796,[45][46] Sav1866,[32] and MsbA.[47] In these transporters, ATP is bound to the ABC domain. Two molecules

of ATP are positioned at the interface of the dimer, sandwiched between the Walker A motif of one subunit and the

LSGGQ motif of the other.[29] This was first observed in Rad50[48] and reported in structures of MJ0796, the NBD

subunit of the LolD transporter fromMethanococcus jannaschii[46] and E.c.MalK of a maltose transporter.[41] These

structures were also consistent with results from biochemical studies revealing that ATP is in close contact with

residues in the P-loop and LSGGQ motif during catalysis.[49]

Nucleotide binding is required to ensure the electrostatic and/or structural integrity of the active site and contribute to

the formation of an active NBD dimer.[50] Binding of ATP is stabilized by the following interactions: (1)

ring-stacking interaction of a conserved aromatic residue preceding the Walker A motif and the adenosine ring of

ATP,[51][52](2) hydrogen-bonds between a conserved lysine residue in the Walker A motif and the oxygen atoms ofthe - and -phosphates of ATP and coordination of these phosphates and some residues in the Walker A motif with

Mg2+ ion,[39][43] and (3) -phosphate coordination with side chain of serine and backbone amide groups of glycine

residues in the LSGGQ motif.[53] In addition, a residue that suggests the tight coupling of ATP binding and

dimerization, is the conserved histidine in the H-loop. This histidine contacts residues across the dimer interface in

the Walker A motif and the D loop, a conserved sequence following the Walker B motif.[41][46][48][54]

The enzymatic hydrolysis of ATP requires proper binding of the phosphates and positioning of the -phosphate to

the attacking water.[29] In the nucleotide binding site, the oxygen atoms of the - and -phosphates of ATP are

stabilized by residues in the Walker A motif[55][56] and coordinate with Mg2+.[29] This Mg2+ ion also coordinates

with the terminal aspartate residue in the Walker B motif through the attacking H2O.[39][40][45] A general base, which

may be the glutamate residue adjacent to the Walker B motif, [37][46][52] glutamine in the Q-loop,[36][42][46] or a

histidine in the switch region that forms a hydrogen bond with the -phosphate of ATP, is found to catalyze the rate

of ATP hydrolysis by promoting the attacking H2O.[41][42][46][54] The precise molecular mechanism of ATP

hydrolysis is still controversial.[16]

Mechanism of transport

ABC transporters are active transporters, that is, they require energy in the form of adenosine triphosphate (ATP) to

translocate substrates across cell membranes. These proteins harness the energy of ATP binding and/or hydrolysis to

drive conformational changes in the transmembrane domain (TMD) and consequently transports molecules.[57] Both

ABC importers and exporters have a common mechanism in transporting substrates because of the similarities intheir structures. The mechanism that describes the conformational changes associated with binding of substrate is the

alternating-access model. In this model, the substrate binding site alternates between outward- and inward-facing

conformations. The relative binding affinities of the two conformations for the substrate largely determines the net

direction of transport. For importers, since translocation is directed from the periplasm to the cytoplasm, then the

outward-facing conformation will have higher binding affinity for substrate. In contrast, the substrate binding

affinity in exporters will be greater in the inward-facing conformation.[29] A model that describes the conformational

changes in the nucleotide-binding domain (NBD) as a result of ATP binding and hydrolysis is theATP-switch model.

This model presents two principal conformations of the NBDs: formation of a closed dimer upon binding two ATP

molecules and dissociation to an open dimer facilitated by ATP hydrolysis and release of inorganic phosphate (Pi)

and adenosine diphosphate (ADP). Switching between the open and closed dimer conformations inducesconformational changes in the TMD resulting in substrate translocation.[58]
http://en.wikipedia.org/w/index.php?title=Adenosine_diphosphatehttp://en.wikipedia.org/w/index.php?title=Adenosine_diphosphatehttp://en.wikipedia.org/w/index.php?title=Adenosine_diphosphatehttp://en.wikipedia.org/w/index.php?title=Adenosine_diphosphatehttp://en.wikipedia.org/w/index.php?title=Adenosine_diphosphatehttp://en.wikipedia.org/w/index.php?title=Adenosine_diphosphatehttp://en.wikipedia.org/w/index.php?title=Adenosine_diphosphatehttp://en.wikipedia.org/w/index.php?title=Adenosine_diphosphatehttp://en.wikipedia.org/w/index.php?title=Adenosine_diphosphatehttp://en.wikipedia.org/w/index.php?title=Phosphatehttp://en.wikipedia.org/w/index.php?title=Active_transporthttp://en.wikipedia.org/w/index.php?title=Glutaminehttp://en.wikipedia.org/w/index.php?title=Glutamatehttp://en.wikipedia.org/w/index.php?title=Aspartatehttp://en.wikipedia.org/w/index.php?title=Glycinehttp://en.wikipedia.org/w/index.php?title=Amidehttp://en.wikipedia.org/w/index.php?title=Serinehttp://en.wikipedia.org/w/index.php?title=Lysinehttp://en.wikipedia.org/w/index.php?title=Catalysis


7/21


The general mechanism for the transport cycle of ABC transporters has not been fully elucidated but substantial

structural and biochemical data has accumulated to support a model in which ATP binding and hydrolysis is coupled

to conformational changes in the transporter. The resting state of all ABC transporters has the NBDs in an open

dimer configuration, with low affinity for ATP. This open conformation possesses a chamber accessible to the

interior of the transporter. The transport cycle is initiated by binding of substrate to the high-affinity site on the

TMDs, which induces conformational changes in the NBDs and enhances the binding of ATP. Two molecules of

ATP bind, cooperatively, to form the closed dimer configuration. The closed NBD dimer induces a conformational

change in the TMDs such that the TMD opens, forming a chamber with an opening opposite to that of the initial

state. The affinity of the substrate to the TMD is reduced, thereby releasing the substrate. Hydrolysis of ATP follows

and then sequential release of Pi

and then ADP restores the transporter to its basal configuration. Although a

common mechanism has been suggested, the order of substrate binding, nucleotide binding and hydrolysis, and

conformational changes, as well as interactions between the domains is still

debated.[16][22][25][29][47][50][57][58][59][60][61]

Several groups studying ABC transporters have differing assumptions on the driving force of transporter function. It

is generally assumed that ATP hydrolysis provides the principal energy input or power stroke for transport and that

the NBDs operate alternately and are possibly involved in different steps in the transport cycle.[62]

However, recentstructural and biochemical data shows that ATP binding, rather than ATP hydrolysis, provides the power stroke. It

may also be that since ATP binding triggers NBD dimerization, the formation of the dimer may represent the power

stroke. In addition, some transporters have NBDs that do not have similar abilities in binding and hydrolyzing ATP

and that the interface of the NBD dimer consists of two ATP binding pockets suggests a concurrent function of the

two NBDs in the transport cycle.[58]

Some evidence to show that ATP binding is indeed the power stroke of the transport cycle was reported.[58] It has

been shown that ATP binding induces changes in the substrate-binding properties of the TMDs. The affinity of ABC

transporters for substrates has been difficult to measure directly, and indirect measurements, for instance through

stimulation of ATPase activity, often reflects other rate-limiting steps. Recently, direct measurement of vinblastine

binding to permease-glycoprotein (P-glycoprotein) in the presence of nonhydrolyzable ATP analogs, e.g.5-adenylyl---imidodiphosphate (AMP-PNP), showed that ATP binding, in the absence of hydrolysis, is sufficient

to reduce substrate-binding affinity.[63] Also, ATP binding induces substantial conformational changes in the TMDs.

Spectroscopic, protease accessibility and crosslinking studies have shown that ATP binding to the NBDs induces

conformational changes in multidrug resistance-associated protein-1 (MRP1),[64] HisPMQ,[65] LmrA,[66] and

Pgp.[67] Two dimensional crystal structures of AMP-PNP-bound Pgp showed that the major conformational change

during the transport cycle occurs upon ATP binding and that subsequent ATP hydrolysis introduces more limited

changes.[68] Rotation and tilting of transmembrane -helices may both contribute to these conformational changes.

Other studies have focused on confirming that ATP binding induces NBD closed dimer formation. Biochemical

studies of intact transport complexes suggest that the conformational changes in the NBDs are relatively small. In the

absence of ATP, the NBDs may be relatively flexible, but they do not involve a major reorientation of the NBDswith respect to the other domains. ATP binding induces a rigid body rotation of the two ABC subdomains with

respect to each other, which allows the proper alignment of the nucleotide in the active site and interaction with the

designated motifs. There is strong biochemical evidence that binding of two ATP molecules can be cooperative, that

is, ATP must bind to the two active site pockets before the NBDs can dimerize and form the closed, catalytically

active conformation.[58]
http://en.wikipedia.org/w/index.php?title=Crosslinkinghttp://en.wikipedia.org/w/index.php?title=Proteasehttp://en.wikipedia.org/w/index.php?title=Spectroscopyhttp://en.wikipedia.org/w/index.php?title=P-glycoproteinhttp://en.wikipedia.org/w/index.php?title=Permeasehttp://en.wikipedia.org/w/index.php?title=Vinblastine


8/21


ABC importers

Most ABC transporters that mediate the uptake of nutrients and other molecules in bacteria rely on a high-affinity

solute binding protein (BP). BPs are soluble proteins located in the periplasmic space between the inner and outer

membranes of gram-negative bacteria. Gram-positive microorganisms lack a periplasm such that their binding

protein is often a lipoprotein bound to the external face of the cell membrane. Some gram-positive bacteria have BPs

fused to the transmembrane domain of the transporter itself.[16] The first successful x-ray crystal structure of anintact ABC importer is the molybdenum transporter (ModBC-A) from Archaeoglobus fulgidus.[33]

Atomic-resolution structures of three other bacterial importers, E. coli BtuCD,[30]E. coli maltose transporter

(MalFGK2-E),[34] and the putative metal-chelate transporter ofHaemophilus influenza, HI1470/1,[36] have also been

determined. The structures provided detailed pictures of the interaction of the transmembrane and ABC domains as

well as revealed two different conformations with an opening in two opposite directions. Another common feature of

importers is that each NBD is bound to one TMD primarily through a short cytoplasmic helix of the TMD, the

coupling helix. This portion of the EAA loop docks in a surface cleft formed between the RecA-like and helical

ABC subdomains and lies approximately parallel to the membrane bilayer.[60]

Large ABC importers

The BtuCD and HI1470/1 are classified as large ABC importers. The transmembrane subunit of the vitamin B12

importer, BtuCD, contains 10 TM helices and the functional unit consists of two copies each of the nucleotide

binding domain (NBD) and transmembrane domain (TMD). The TMD and NBD interact with one another via the

cytoplasmic loop between two TM helices and the Q loop in the ABC. In the absence of nucleotide, the two ABC

domains are folded and the dimer interface is open. A comparison of the structures with (BtuCDF) and without

(BtuCD) binding protein reveals that BtuCD has an opening that faces the periplasm whereas in BtuCDF, the

outward-facing conformation is closed to both sides of the membrane. The structures of BtuCD and the BtuCD

homolog, HI1470/1, represent two different conformational states of an ABC transporter. The predicted translocation

pathway in BtuCD is open to the periplasm and closed at the cytoplasmic side of the membrane while that of

HI1470/1 faces the opposite direction and open only to the cytoplasm. The difference in the structures is a 9 twist of

one TM subunit relative to the other.[16][29][60]

Small ABC importers

Structures of the ModBC-A and MalFGK2-E, which are in complex with their binding protein, correspond to small

ABC importers. The TMDs of ModBC-A and MalFGK2-E have only six helices per subunit. The homodimer of

ModBC-A is in a conformation in which the TM subunits (ModB) orient in an inverted V-shape with a cavity

accessible to the cytoplasm. The ABC subunits (ModC), on the other hand, are arranged in an open, nucleotide-free

conformation, in which the P-loop of one subunit faces but is detached from the LSGGQ motif of the other. The

binding protein ModA is in a closed conformation with substrate bound in a cleft between its two lobes and attachedto the extracellular loops of ModB, wherein the substrate is sitting directly above the closed entrance of the

transporter. The MalFGK2-E structure resembles the catalytic transition state for ATP hydrolysis. It is in a closed

conformation where it contains two ATP molecules, sandwiched between the Walker A and B motifs of one subunit

and the LSGGQ motif of the other subunit. The maltose binding protein (MBP or MalE) is docked on the

periplasmic side of the TM subunits (MalF and MalG) and a large, occluded cavity can be found at the interface of

MalF and MalG. The arrangement of the TM helices is in a conformation which is closed toward the cytoplasm but

with an opening that faces outward. The structure suggests a possibility that MBP may stimulate the ATPase activity

of the transporter upon binding.[16][29][60]
http://en.wikipedia.org/w/index.php?title=ATPasehttp://en.wikipedia.org/w/index.php?title=ATPasehttp://en.wikipedia.org/w/index.php?title=Transition_statehttp://en.wikipedia.org/w/index.php?title=Maltosehttp://en.wikipedia.org/w/index.php?title=Molybdenumhttp://en.wikipedia.org/w/index.php?title=X-ray_crystallographyhttp://en.wikipedia.org/w/index.php?title=Cell_membranehttp://en.wikipedia.org/w/index.php?title=Lipoproteinhttp://en.wikipedia.org/w/index.php?title=Periplasmhttp://en.wikipedia.org/w/index.php?title=Gram-positivehttp://en.wikipedia.org/w/index.php?title=Gram-negative_bacteria


9/21


Mechanism of transport for importers

Proposed mechanism of transport for ABC importers. This alternating-access model was

based on the crystal structures of ModBC-A[33]

and HI1470/1.[36]

The mechanism of transport for

importers supports the

alternating-access model. The resting

state of importers is inward-facing,

where the nucleotide binding domain(NBD) dimer interface is held open by

the TMDs and facing outward but

occluded from the cytoplasm. Upon

docking of the closed, substrate-loaded

binding protein towards the

periplasmic side of the transmembrane

domains, ATP binds and the NBD dimer closes. This switches the resting state of transporter into an outward-facing

conformation, in which the TMDs have reoriented to receive substrate from the binding protein. After hydrolysis of

ATP, the NBD dimer opens and substrate is released into the cytoplasm. Release of ADP and Pi

reverts the

transporter into its resting state. The only inconsistency of this mechanism to the ATP-switch model is that theconformation in its resting, nucleotide-free state is different from the expected outward-facing conformation.

Although that is the case, the key point is that the NBD does not dimerize unless ATP and binding protein is bound

to the transporter.[16][22][29][58][60]

ABC exporters

Prokaryotic ABC exporters are abundant and have close homologues in eukaryotes. This class of transporters is

studied based on the type of substrate that is transported. One class is involved in the protein (e.g. toxins, hydrolytic

enzymes, S-layer proteins, lantibiotics, bacteriocins, and competence factors) export and the other in drug efflux.

ABC transporters have gained extensive attention because they contribute to the resistance of cells to antibiotics andanticancer agents by pumping drugs out of the cells.[16]

In gram-negative organisms, ABC transporters mediate secretion of protein substrates across inner and outer

membranes simultaneously without passing through the periplasm. This type of secretion is referred to as type I

secretion which involves three components that function in concert: an ABC exporter, a membrane fusion protein

(MFP), and an outer membrane factor (OMF). An example is the secretion of hemolysin (HlyA) fromE. coli where

the inner membrane ABC transporter HlyB interacts with an inner membrane fusion protein HlyD and an outer

membrane facilitator TolC. TolC allows hemolysin to be transported across the two membranes, bypassing the

periplasm.[22]

Bacterial drug resistance has become an increasingly major health problem. One of the mechanisms for drugresistance is associated with an increase in antibiotic efflux from the bacterial cell. Drug resistance associated with

drug efflux, mediated by P-glycoprotein, was originally reported in mammalian cells. In bacteria, Levy and

colleagues presented the first evidence that antibiotic resistance was caused by active efflux of a drug. [69]

P-glycoprotein is the best-studied efflux pump and as such has offered important insights into the mechanism of

bacterial pumps.[16] Although some exporters transport a specific type of substrate, most transporters extrude a

diverse class of drugs with varying structure.[25] These transporters are commonly called multi-drug resistant (MDR)

ABC transporters and sometimes referred to as hydrophobic vacuum cleaners.[61]
http://en.wikipedia.org/w/index.php?title=Multi-drug_resistanthttp://en.wikipedia.org/w/index.php?title=P-glycoproteinhttp://en.wikipedia.org/w/index.php?title=Hemolysinhttp://en.wikipedia.org/w/index.php?title=Membrane_fusion_proteinhttp://en.wikipedia.org/w/index.php?title=Anticancer_agentshttp://en.wikipedia.org/w/index.php?title=Antibioticshttp://en.wikipedia.org/w/index.php?title=Bacteriocinshttp://en.wikipedia.org/w/index.php?title=Lantibioticshttp://en.wikipedia.org/w/index.php?title=Hydrolasehttp://en.wikipedia.org/w/index.php?title=Hydrolasehttp://en.wikipedia.org/w/index.php?title=Toxinshttp://en.wikipedia.org/w/index.php?title=File%3AAbc_importer.jpg


10/21


Human ABCB1/MDR1 P-glycoprotein

P-glycoprotein is a well-studied protein associated with multi-drug resistance. It belongs to the human ABCB

(MDR/TAP) family and is also known as ABCB1 orMDR1 Pgp. MDR1 consists of a functional monomer with two

transmembrane domains (TMD) and two nucleotide-binding domains (NBD). This protein can transport mainly

cationic or electrically neutral substrates as well as a broad spectrum of amphiphilic substrates. The structure of the

full-size ABCB1 monomer was obtained in the presence and absence of nucleotide using electron cryocrystallography. Without the nucleotide, the TMDs are approximately parallel and form a barrel surrounding a

central pore, with the opening facing towards the extracellular side of the membrane and closed at the intracellular

face. In the presence of the nonhydrolyzable ATP analog, AMP-PNP, the TMDs have a substantial reorganization

with three clearly segregated domains. A central pore, which is enclosed between the TMDs, is slightly open towards

the intracellular face with a gap between two domains allowing access of substrate from the lipid phase. Substantial

repacking and possible rotation of the TM helices upon nucleotide binding suggests a helix rotation model for the

transport mechanism.[25]

Sav1866

The first high-resolution structure reported for an ABC exporter was that of Sav1866 from Staphylococcus

aureus.[25][70] Sav1866 is a homolog of multidrug ABC transporters. It shows significant sequence similarity to

human ABC transporters of subfamily B that includes MDR1 and TAP1/TAP2. The ATPase activity of Sav1866 is

known to be stimulated by cancer drugs such as doxorubicin, vinblastine and others,[71] which suggests similar

substrate specificity to P-glycoprotein and therefore a possible common mechanism of substrate translocation.

Sav1866 is a homodimer of half transporters, and each subunit contains an N-terminal TMD with six helices and a

C-terminal NBD. The NBDs are similar in structure to those of other ABC transporters, in which the two ATP

binding sites are formed at the dimer interface between the Walker A motif of one NBD and the LSGGQ motif of the

other. The ADP-bound structure of Sav1866 shows the NBDs in a closed dimer and the TM helices split into two

wings oriented towards the periplasm, forming the outward-facing conformation. Each wing consists of helices

TM1-2 from one subunit and TM3-6 from the other subunit. It contains long intracellular loops (ICLs or ICD)

connecting the TMDs that extend beyond the lipid bilayer into the cytoplasm and interacts with the 8=D. Whereas

the importers contain a short coupling helix that contact a single NBD, Sav1866 has two intracellular coupling

helices, one (ICL1) contacting the NBDs of both subunits and the other (ICL2) interacting with only the opposite

NBD subunit.[29][32][60]

MsbA

MsbA is a multi-drug resistant (MDR) ABC transporter and possibly a lipid flippase. It is an ATPase that transports

lipid A, the hydrophobic moiety of lipopolysaccharide (LPS), a glucosamine-based saccharolipid that makes up the

outer monolayer of the outer membranes of most gram-negative bacteria. Lipid A is an endotoxin and so loss ofMsbA from the cell membrane or mutations that disrupt transport results in the accumulation of lipid A in the inner

cell membrane resulting to cell death. It is a close bacterial homolog of P-glycoprotein (Pgp) by protein sequence

homology and has overlapping substrate specificities with the MDR-ABC transporter LmrA from Lactococcus

lactis.[72] MsbA from E. coli is 36% identical to the NH2-terminal half of human MDR1, suggesting a common

mechanism for transport of amphiphatic and hydrophobic substrates. The MsbA gene encodes a half transporter that

contains a transmembrane domain (TMD) fused with a nucleotide-binding domain (NBD). It is assembled as a

homodimer with a total molecular mass of 129.2 kD. MsbA contains 6 TMDs on the periplasmic side, an NBD

located on the cytoplasmic side of the cell membrane, and an intracellular domain (ICD), bridging the TMD and

NBD. This conserved helix extending from the TMD segments into or near the active site of the NBD is largely

responsible for crosstalk between TMD and NBD. In particular, ICD1 serves as a conserved pivot about which theNBD can rotate, therefore allowing the NBD to disassociate and dimerize during ATP binding and
http://en.wikipedia.org/w/index.php?title=Mutationshttp://en.wikipedia.org/w/index.php?title=Endotoxinhttp://en.wikipedia.org/w/index.php?title=Lipopolysaccharidehttp://en.wikipedia.org/w/index.php?title=Lipid_Ahttp://en.wikipedia.org/w/index.php?title=ATPasehttp://en.wikipedia.org/w/index.php?title=Flippasehttp://en.wikipedia.org/w/index.php?title=Vinblastinehttp://en.wikipedia.org/w/index.php?title=Doxorubicinhttp://en.wikipedia.org/w/index.php?title=Electron_crystallographyhttp://en.wikipedia.org/w/index.php?title=Electron_crystallography


11/21


hydrolysis.[16][22][25][29][50][60][61][73]

Structures of MsbA depicting the three conformational states: open apo (PDB 3b5w[74]

),

closed apo (PDB 3b5x[75]

), and nucleotide-bound (PDB 3b60[76]

)

Previously published (and now

retracted) X-ray structures of MsbA

were inconsistent with the bacterial

homolog Sav1866.[77][78] The

structures were reexamined and foundto have an error in the assignment of

the hand resulting to incorrect models

of MsbA. Recently, the errors have

been rectified and new structures have

been reported.[47] The resting state of

E. coli MsbA exhibits an inverted V

shape with a chamber accessible to the interior of the transporter suggesting an open, inward-facing conformation.

The dimer contacts are concentrated between the extracellular loops and while the NBDs are ~50 apart, the

subunits are facing each other. The distance between the residues in the site of the dimer interface have been verified

by cross-linking experiments[79]

and EPR spectroscopy studies.[80]

The relatively large chamber allows it toaccommodate large head groups such as that present in lipid A. Significant conformational changes are required to

move the large sugar head groups across the membrane. The difference between the two nucleotide-free (apo)

structures is the ~30 pivot of TM4/TM5 helices relative to the TM3/TM6 helices. In the closed apo state (from V.

cholerae MsbA), the NBDs are aligned and although closer, have not formed an ATP sandwich, and the P loops of

opposing monomers are positioned next to one another. In comparison to the open conformation, the dimer interface

of the TMDs in the closed, inward-facing conformation has extensive contacts. For both apo conformations of

MsbA, the chamber opening is facing inward. The structure of MsbA-AMP-PNP

(5-adenylyl---imidodiphosphate), obtained from S. typhimurium, is similar to Sav1866. The NBDs in this

nucleotide-bound, outward-facing conformation, come together to form a canonical ATP dimer sandwich, that is, the

nucleotide is situated in between the P-loop and LSGGQ motif. The conformational transition fromMsbA-closed-apo to MsbA-AMP-PNP involves two steps which are more likely concerted: a ~10 pivot of

TM4/TM5 helices towards TM3/TM6, bringing the NBDs closer but not into alignment followed by tilting of

TM4/TM5 helices ~20 out of plane. The twisting motion results in the separation of TM3/TM6 helices away from

TM1/TM2 leading to a change from an inward- to an outward- facing conformation. Thus, changes in both the

orientation and spacing of the NBDs dramatically rearrange the packing of transmembrane helices and effectively

switch access to the chamber from the inner to the outer leaflet of the membrane.[47] The structures determined for

MsbA is basis for the tilting model of transport.[25] The structures described also highlight the dynamic nature of

ABC exporters as also suggested by fluorescence and EPR studies.[60][80][81]
http://en.wikipedia.org/w/index.php?title=Fluorescencehttp://en.wikipedia.org/w/index.php?title=EPR_spectroscopyhttp://en.wikipedia.org/w/index.php?title=Cross-linkinghttp://en.wikipedia.org/w/index.php?title=File%3AMsba.jpghttp://www.rcsb.org/pdb/explore/explore.do?structureId=3b60http://en.wikipedia.org/w/index.php?title=Protein_Data_Bankhttp://www.rcsb.org/pdb/explore/explore.do?structureId=3b5xhttp://en.wikipedia.org/w/index.php?title=Protein_Data_Bankhttp://www.rcsb.org/pdb/explore/explore.do?structureId=3b5whttp://en.wikipedia.org/w/index.php?title=Protein_Data_Bank


12/21


Mechanism of transport for exporters

Proposed mechanism of transport for ABC exporters. This model was based on structural

and biochemical studies on MsbA.

ABC exporters have a transport

mechanism that is consistent with both

the alternating-access model and

ATP-switch model. In the apo states of

exporters, the conformation isinward-facing and the TMDs and

NBDs are relatively far apart to

accommodate amphiphilic or

hydrophobic substrates. For MsbA, in

particular, the size of the chamber is

large enough to accommodate the

sugar groups from lipopolysaccharides (LPS). As has been suggested by several groups, binding of substrate initiates

the transport cycle. The power stroke, that is, ATP binding that induces NBD dimerization and formation of the

ATP sandwich, drives the conformational changes in the TMDs. In MsbA, the sugar head groups are sequestered

within the chamber during the power stroke. The cavity is lined with charged and polar residues that are likelysolvated creating an energetically unfavorable environment for hydrophobic substrates and energetically favorable

for polar moieties in amphiphilic compounds or sugar groups from LPS. Since the lipid cannot be stable for a long

time in the chamber environment, lipid A and other hydrophobic molecules may flip into an energetically more

favorable position within the outer membrane leaflet. The flipping may also be driven by the rigid-body shearing of

the TMDs while the hydrophobic tails of the LPS are dragged through the lipid bilayer. Repacking of the helices

switches the conformation into an outward-facing state. ATP hydrolysis may widen the periplasmic opening and

push the substrate towards the outer leaflet of the lipid bilayer. Hydrolysis of the second ATP molecule and release

of Piseparates the NBDs followed by restoration of the resting state, opening the chamber towards the cytoplasm for

another cycle.[47][50][58][61][77][78][80][82]

Role in multidrug resistance

ABC transporters are known to play a crucial role in the development of multidrug resistance (MDR). In MDR,

patients that are on medication eventually develop resistance not only to the drug they are taking but also to several

different types of drugs. This is caused by several factors, one of which is increased excretion of the drug from the

cell by ABC transporters. For example, the ABCB1 protein (P-glycoprotein) functions in pumping tumor

suppression drugs out of the cell. Pgp also called MDR1, ABCB1, is the prototype of ABC transporters and also the

most extensively-studied gene. Pgp is known to transport organic cationic or neutral compounds. A few ABCC

family members, also known as MRP, have also been demonstrated to confer MDR to organic anion compounds.

The most-studied member in ABCG family is ABCG2, also known as BCRP (breast cancer resistance protein)

confer resistance to most of Topoisomerase I or II inhibitors such as topotecan, irinotecan, and doxorubicin.

It is unclear exactly how these proteins can translocate such a wide variety of drugs, however one model (the

hydrophobic vacuum cleaner model) states that, in P-glycoprotein, the drugs are bound indiscriminately from the

lipid phase based on their hydrophobicity.
http://en.wikipedia.org/w/index.php?title=P-glycoproteinhttp://en.wikipedia.org/w/index.php?title=Multidrug_resistancehttp://en.wikipedia.org/w/index.php?title=File%3AAbc_exporter.jpg


13/21


Reversal of multidrug resistance

Drug resistance is a common clinical problem that occurs in patients suffering from infectious diseases and in

patients suffering from cancer. Prokaryotic and eukaryotic microorganisms as well as neoplastic cells are often found

to be resistant to drugs. MDR is frequently associated with overexpression of ABC transporters. Inhibition of ABC

transporters by low-molecular weight compounds has been extensively investigated in cancer patients; however, the

clinical results have been disappointing. Recently various RNAi strategies have been applied to reverse MDR indifferent tumor models and this technology is effective in reversing ABC-transporter-mediated MDR in cancer cells

and is therefore a promising strategy for overcoming MDR by gene therapeutic applications. RNAi technology could

also be considered for overcoming MDR in infectious diseases caused by microbial pathogens.[83]

Physiological role

In addition to conferring MDR in tumor cells, ABC transporters are also expressed in the membranes of healthy

cells, where they facilitate the transport of various endogenous substances, as well as of substances foreign to the

body. For instance, ABC transporters such as Pgp, the MRPs and BCRP limit the absorption of many drugs from the

intestine, and pump drugs from the liver cells to the bile as a means of removing foreign substances from the body. Alarge number of drugs are either transported by ABC transporters themselves or affect the transport of other drugs.

The latter scenario can lead to drug-drug interactions, sometimes resulting in altered effects of the drugs.[84]

Methods to characterize ABC transporter interactions

There are a number of assay types that allow the detection of ABC transporter interactions with endogenous and

xenobiotic compounds.[85] The complexity of assay range from relatively simple membrane assays [86] like vesicular

transport assay, ATPase assay to more complex cell based assays up to intricate in vivo[87] detection

methodologies.[88]

Membrane assays

The vesicular transport assay detects the translocation of molecules by ABC transporters.[89] Membranes prepared

under suitable conditions contain inside-out oriented vesicles with the ATP binding site and substrate binding site of

the transporter facing the buffer outside. Substrates of the transporter are taken up into the vesicles in an ATP

dependent manner. Rapid filtration using glass fiber filters or nitrocellulose membranes are used to separate the

vesicles from the incubation solution and the test compound trapped inside the vesicles is retained on the filter. The

quantity of the transported unlabelled molecules is determined by HPLC, LC/MS, LC/MS/MS. Alternatively, the

compounds are radiolabeled, fluorescent or have a fluorescent tag so that the radioactivity or fluorescence retained

on the filter can be quantified.

Various types of membranes from different sources (e.g. insect cells, transfected or selected mammalian cell lines)are used in vesicular transport studies. Membranes are commercially available [90] or can be prepared from various

cells or even tissues e.g. liver canalicular membranes. This assay type has the advantage of measuring the actual

disposition of the substrate across the cell membrane. Its disadvantage is that compounds with medium-to-high

passive permeability are not retained inside the vesicles making direct transport measurements with this class of

compounds difficult to perform.

The vesicular transport assay can be performed in an indirect setting, where interacting test drugs modulate the

transport rate of a reporter compound. This assay type is particularly suitable for the detection of possible drug-drug

interactions and drug-endogenous substrate interactions. It is not sensitive to the passive permeability of the

compounds and therefore detects all interacting compounds. Yet, it does not provide information on whether the

compound tested is an inhibitor of the transporter, or a substrate of the transporter inhibiting its function in acompetitive fashion. A typical example of an indirect vesicular transport assay is the detection of the inhibition of
http://en.wikipedia.org/w/index.php?title=ATPase_assayhttp://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-8371http://en.wikipedia.org/w/index.php?title=Drug_interactionhttp://en.wikipedia.org/w/index.php?title=RNAihttp://en.wikipedia.org/w/index.php?title=Eukaryotichttp://en.wikipedia.org/w/index.php?title=Prokaryotic


14/21


taurocholate transport by ABCB11 (BSEP).

Whole cell based assays

Efflux transporter-expressing cells actively pump substrates out of the cell, which results in a lower rate of substrate

accumulation, lower intracellular concentration at steady state, or a faster rate of substrate elimination from cells

loaded with the substrate. Transported radioactive substrates or labeled fluorescent dyes can be directly measured, orin an indirect set up, the modulation of the accumulation of a probe substrate (e.g. fluorescent dye, like Rho123, or

calcein) can be determined in the presence of a test drug.

Calcein-AM, a highly permeable derivative of calcein readily penetrates into intact cells, where the endogenous

esterases rapidly hydrolyze it to the fluorescent calcein. In contrast to calcein-AM, calcein has low permeability and

therefore gets trapped in the cell and accumulates. As calcein-AM is an excellent substrate of the MDR1 and MRP1

efflux transporters, cells expressing MDR1 and/or MRP1 transporters pump the calcein-AM out of the cell before

esterases can be hydrolyzed it. This results in a lower cellular accumulation rate of calcein. The higher the MDR

activity is in the cell membrane, the less Calcein is accumulated in the cytoplasm. In MDR-expressing cells, the

addition of an MDR inhibitor or an MDR substrate in excess dramatically increases the rate of Calcein accumulation.

Activity of multidrug transporter is reflected by the difference between the amounts of dye accumulated in thepresence and the absence of inhibitor. Using selective inhibitors, transport activity of MDR1 and MRP1 can be easily

distinguished. This assay can be used to screen drugs for transporter interactions, and also to quantify the MDR

activity of cells. The calcein assay is the proprietary assay of SOLVO Biotechnology.

Subfamilies

Human subfamilies

There are 48 known ABC transporters present in humans, which are classified into seven families by the Human

Genome Organization.Family Members Function Examples

ABCA This family contains some of the largest

transporters (over 2,100 amino acids long). Five

of them are located in a cluster in the 17q24

chromosome.

Responsible for the transportation of cholesterol and lipids,

among other things.

ABCA12

ABCA1

ABCB Consists of 4 full and 7 half transporters. Some are located in the bloodbrain barrier, liver, mitochondria

and transports peptides and bile, for example.

ABCB5

ABCC Consists of 12 full transporters. Used in ion transport, cell-surface receptors, toxin secretion.

Includes the CFTR protein, which causes cystic fibrosis when

deficient.

ABCC6

ABCD Consists of 4 half transporters Are all used in peroxisomes. ABCD1

ABCE/ABCF Consists of 1 ABCE and 3 ABCF proteins. These are not actually transporters but merely ATP-binding

domains that were derived from the ABC family, but without the

transmembrane domains. These proteins mainly regulate protein

synthesis or expression.

ABCE,

ABCF1,

ABCF2

ABCG Consists of 6 reverse half-transporters, with the

NBF at the NH3+ end and the TM at the COO-

end.

Transports lipids, diverse drug substrates, bile, cholesterol, and

other steroids.

ABCG2

ABCG1-

A full list of human ABC transporters can be found at [91].
http://nutrigene.4t.com/humanabc.htmhttp://nutrigene.4t.com/humanabc.htmhttp://en.wikipedia.org/w/index.php?title=ABCG1http://en.wikipedia.org/w/index.php?title=ABCG2http://en.wikipedia.org/w/index.php?title=ABCF2http://en.wikipedia.org/w/index.php?title=ABCF1http://en.wikipedia.org/w/index.php?title=ABCD1http://en.wikipedia.org/w/index.php?title=Peroxisomehttp://en.wikipedia.org/w/index.php?title=ABCC6http://en.wikipedia.org/w/index.php?title=Cystic_fibrosishttp://en.wikipedia.org/w/index.php?title=ABCB5http://en.wikipedia.org/w/index.php?title=ABCA1http://en.wikipedia.org/w/index.php?title=ABCA12http://en.wikipedia.org/w/index.php?title=Calceinhttp://en.wikipedia.org/w/index.php?title=ABCB11


15/21


Prokaryotic subfamilies

Importers

Carbohydrate Uptake Transporter-1 (CUT1)

Carbohydrate Uptake Transporter-2 (CUT2)

Polar Amino Acid Uptake Transporter (PAAT)

Peptide/Opine/Nickel Uptake Transporter (PepT)

Hydrophobic Amino Acid Uptake Transporter (HAAT)

Sulfate/Tungstate Uptake Transporter (SulT)

Phosphate Uptake Transporter (PhoT)

Molybdate Uptake Transporter (MolT)

Phosphonate Uptake Transporter (PhnT)

Ferric Iron Uptake Transporter (FeT)

Polyamine/Opine/Phosphonate Uptake Transporter (POPT)

Quaternary Amine Uptake Transporter (QAT)

Vitamin B12

Uptake Transporter (B12T)

Iron Chelate Uptake Transporter (FeCT)

Manganese/Zinc/Iron Chelate Uptake Transporter (MZT)

Nitrate/Nitrite/Cyanate Uptake Transporter (NitT)

Taurine Uptake Transporter (TauT)

Cobalt Uptake Transporter (CoT)

Thiamin Uptake Transporter (ThiT)

Brachyspira Iron Transporter (BIT)

Siderophore-Fe3+ Uptake Transporter (SIUT)

Nickel Uptake Transporter (NiT)

Nickel/Cobalt Uptake Transporter (NiCoT) Methionine Uptake Transporter (MUT)

Lipid Exporter (LipidE)

Exporters

Capsular Polysaccharide Exporter (CPSE)

Lipooligosaccharide Exporter (LOSE)

Lipopolysaccharide Exporter (LPSE)

Teichoic Acid Exporter (TAE)

Drug Exporter-1 (DrugE1)

Lipid Exporter (LipidE) Putative Heme Exporter (HemeE)

-Glucan Exporter (GlucanE)

Protein-1 Exporter (Prot1E)

Protein-2 Exporter (Prot2E)

Peptide-1 Exporter (Pep1E)



Probable Glycolipid Exporter (DevE)

Na+ Exporter (NatE)

Microcin B17 Exporter (McbE) Drug Exporter-2 (DrugE2)


16/21


Microcin J25 Exporter (McjD)

Drug/Siderophore Exporter-3 (DrugE3)

(Putative) Drug Resistance ATPase-1 (Drug RA1)

(Putative) Drug Resistance ATPase-2 (Drug RA2)

Macrolide Exporter (MacB)


3-component Peptide-5 Exporter (Pep5E)

Lipoprotein Translocase (LPT)

-Exotoxin I Exporter (ETE)

AmfS Peptide Exporter (AmfS-E)

SkfA Peptide Exporter (SkfA-E)

CydDC Cysteine and glutathione Exporter (CydDC-E)

[92]

Images

Many structures of water-soluble domains of ABC proteins have been produced in recent years.[14]

More Readings

The ABC Transporters of Human Physiology and Disease (WSPC 2011) ISBN 978-981-4280-06-8 [93].

References

[1] http://www.pdbe.org/1l7v

[2] http://pfam.sanger.ac.uk/family?acc=PF00005

[3] http://www.ebi.ac.uk/interpro/DisplayIproEntry?ac=IPR003439

[4] http://www.expasy.org/cgi-bin/prosite-search-ac?PDOC00185

[5] http://scop.mrc-lmb.cam.ac.uk/scop/search.cgi?tlev=fa;&pdb=1b0u

[6] http://supfam.org/SUPERFAMILY/cgi-bin/search.cgi?search_field=1b0u

[7] http://www.tcdb.org/search/result.php?tc=3.A.1

[8] http://opm.phar.umich. edu/families.php?superfamily=17

[9] http://opm.phar.umich. edu/protein.php?search=3g5u

[10] http://pfam.sanger.ac.uk/family/PF00005?tab=pdbBlock

[11] http://www.rcsb.org/pdb/search/smartSubquery.do?smartSearchSubtype=PfamIdQuery&pfamID=PF00005

[12] http://www.ebi.ac.uk/pdbe-srv/PDBeXplore/pfam/?pfam=PF00005

[13] http://www.ebi.ac.uk/thornton-srv/databases/cgi-bin/pdbsum/GetPfamStr.pl?pfam_id=PF00005

[14] Jones PM, George AM (Mar 2004). "The ABC transporter structure and mechanism: perspectives on recent research". Cell Mol Life Sci.61

(6): 68299. doi:10.1007/s00018-003-3336-9. PMID 15052411.

[15] Ponte-Sucre, A (editor) (2009).ABC Transporters in Microorganisms. Caister Academic Press. ISBN 978-1-904455-49-3.[16] Davidson A.L., Dassa E., Orelle C., Chen J. (2008). "Structure, function, and evolution of bacterial ATP-binding cassette systems".

Microbiol. Mol. Biol. Rev.72 (2): 317364. doi:10.1128/MMBR.00031-07. PMC 2415747. PMID 18535149.

[17][17] Goffeau, A.; B. de Hertogh; and P.V. Baret. 2004. ABC Transporters. In: Encyclopedia of Biological Chemistry. Vol. 1, 1-5.

[18] Henderson D.P., Payne S.M. (1994). "Vibrio cholerae iron transport system: roles of heme and siderophore iron transport in virulence and

identification of a gene associated with multiple iron transport systems".Infect. Immun62 (11): 51205. PMC 303233. PMID 7927795.

[19] Cangelosi G.A., Ankenbauer R.G., Nester E.W. (1990). "Sugars induce the Agrobacterium virulence genes through a periplasmic binding

protein and a transmembrane signal protein".Proc. Natl. Acad. Sci. USA87 (17): 670812. doi:10.1073/pnas.87.17.6708. PMC 54606.

PMID 2118656.

[20] Kemner J.M., Liang X., Nester E.W. (1997). "The Agrobacterium tumefaciens virulence gene chvE is part of a putative ABC-type sugar

transport operon".J. Bacteriol179 (7): 24528. PMC 178989. PMID 9079938.

[21] Poolman B., Spitzer J.J., Wood J.M. (2004). "Bacterial osmosensing: roles of membrane structure and electrostatics in lipid-protein and

protein-protein interactions".Biochim. Biophys. Acta1666 (12): 88104. doi:10.1016/j.bbamem.2004.06.013. PMID 15519310.

[22] Davidson A.L., Chen J. (2004). "ATP-binding cassette transporters in bacteria".Annu. Rev. Biochem73: 241268.

doi:10.1146/annurev.biochem.73.011303.073626. PMID 15189142.
http://www.ebi.ac.uk/thornton-srv/databases/cgi-bin/pdbsum/GetPfamStr.pl?pfam_id=PF00005http://www.ebi.ac.uk/pdbe-srv/PDBeXplore/pfam/?pfam=PF00005http://www.rcsb.org/pdb/search/smartSubquery.do?smartSearchSubtype=PfamIdQuery&pfamID=PF00005http://pfam.sanger.ac.uk/family/PF00005?tab=pdbBlockhttp://opm.phar.umich.edu/protein.php?search=3g5uhttp://opm.phar.umich.edu/families.php?superfamily=17http://www.tcdb.org/search/result.php?tc=3.A.1http://supfam.org/SUPERFAMILY/cgi-bin/search.cgi?search_field=1b0uhttp://scop.mrc-lmb.cam.ac.uk/scop/search.cgi?tlev=fa;&pdb=1b0uhttp://www.expasy.org/cgi-bin/prosite-search-ac?PDOC00185http://www.ebi.ac.uk/interpro/DisplayIproEntry?ac=IPR003439http://pfam.sanger.ac.uk/family?acc=PF00005http://www.pdbe.org/1l7v


17/21


[23] Zhou Z.M., White K.A., Polissi A., Georgopoulos C., Raetz C.R.H. (1998). "Function of Escherichia coli MsbA, an essential ABC family

transporter, in lipid A and phospholipid biosynthesis".J. Biol. Chem.273 (20): 1246675. doi:10.1074/jbc.273.20.12466. PMID 9575204.

[24] Poole R.K., Gibson F., Wu G.H. (1994). "The cydD gene product, component of a heterodimeric ABC transporter, is required for assembly

of periplasmic cytochrome-c and of cytochrome-bd in Escherichia coli".FEMS Microbiol. Lett.117 (2): 217224.

doi:10.1111/j.1574-6968.1994.tb06768.x. PMID 8181727.

[25] Pohl A., Devaux P.F., Herrmann A. (2005). "Function of prokaryotic and eukaryotic ABC proteins in lipid transport".Biochim. Biophys.

Acta1733 (1): 2952. doi:10.1016/j.bbalip.2004.12.007. PMID 15749056.

[26] Gedeon C, Behravan J, Koren G, Piquette-Miller M (2006). "Transport of glyburide by placental ABC transporters: implications in fetaldrug exposure".Placenta27 (1112): 1096102. doi:10.1016/j.placenta.2005.11.012. PMID 16460798.

[27] http://www.rcsb.org/pdb/explore/explore.do?structureId=2qi9

[28] http://www.rcsb.org/pdb/explore/explore.do?structureId=2onj

[29] Rees D.C., Johnson E., Lewinson O. (2009). "ABC transporters: the power to change".Nat. Rev. Mol. Cell Biol.10 (3): 218227.

doi:10.1038/nrm2646. PMC 2830722. PMID 19234479.

[30] Locher K.P., Lee A.T., Rees D.C. (2002). "The E. coli BtuCD structure: framework for ABC transporter architecture and mechanism".

Science296 (5570): 10918. doi:10.1126/science.1071142. PMID 12004122.

[31] Hvorup R.N. et al. (2007). "Asymmetry in the structure of the ABC transporter binding protein complex BtuCD-BtuF". Science317 (5843):

138790. doi:10.1126/science.1145950. PMID 17673622.

[32] Dawson R.J.P., Locher K.P. (2006). "Structure of a bacterial multidrug ABC transporter".Nature443 (7108): 1805.

doi:10.1038/nature05155. PMID 16943773.

[33] Hollenstein K., Frei D.C., Locher K.P. (2007). "Structure of an ABC transporter in complex with its binding protein".Nature446 (7132):2136. doi:10.1038/nature05626. PMID 17322901.

[34] Oldham M.L., Khare D., Quiocho F.A., Davidson A.L., Chen J. (2007). "Crystal structure of a catalytic intermediate of the maltose

transporter".Nature450 (7169): 515522. doi:10.1038/nature06264.

[35] Kadaba N.S., Kaiser J.T., Johnson E., Lee A., Rees D.C. (2008). "The high-affinity E. coli methionine ABC transporter: structure and

allosteric regulation". Science321 (5886): 2503. doi:10.1126/science.1157987. PMC 2527972. PMID 18621668.

[36] Pinkett H.W., Lee A.T., Lum P., Locher K.P., Rees D.C. (2007). "An inward-facing conformation of a putative metal-chelate type ABC

transporter". Science315 (5810): 3737. doi:10.1126/science.1133488. PMID 17158291.

[37] Moody J.E., Millen L., Binns D., Hunt J.F., Thomas P.J. (2002). "Cooperative, ATP-dependent association of the nucleotide binding

cassettes during the catalytic cycle of ATP-binding cassette transporters".J. Biol. Chem.277 (24): 211114. doi:10.1074/jbc.C200228200.

PMID 11964392.

[38] Hung L., Wang I.X., Nikaido K., Liu P., Ames G. F., Kim S. (1998). "Crystal structure of the ATP-binding subunit of an ABC transporter".

Nature396 (6712): 7037. doi:10.1038/25393. PMID 9872322.

[39] Verdon G., Albers S. V., Dijkstra B. W., Driessen A. J., Thunnissen A. M. (2003). "Crystal structures of the ATPase subunit of the glucose

ABC transporter from Sulfolobus solfataricus: nucleotide-free and nucleotide-bound conformations".J. Mol. Biol.330 (2): 343358.

doi:10.1016/S0022-2836(03)00575-8. PMID 12823973.

[40] Karpowich O., Martsinkevish L., Millen L., Yuan Y.R., MacVey K., Thomas P.J., Hunt J.F. (2001). "Crystal structures of MJ1267 reveal an

induced-fit effect at the ATPase active site of an ABC transporter". Structure9 (7): 571586. doi:10.1016/S0969-2126(01)00617-7.

PMID 11470432.

[41] Chen J., Lu G., Lin J., Davidson A.L., Quiocho F.A. (2003). "A tweezers-like motion of the ATP-binding cassette dimer in an ABC

transport cycle".Mol. Cell12 (3): 651661. doi:10.1016/j.molcel.2003.08.004. PMID 14527411.

[42] Diederichs K., Diez J., Greller G., Muller C., Breed J., Schnell C., Vonrhein C., Boos W., Welte W. et al. (2000). "Crystal structure of

MalK, the ATPase subunit of the trehalose/maltose ABC transporter of the archaeon Thermococcus litoralis".EMBO J19 (22): 595161.

doi:10.1093/emboj/19.22.5951. PMC 305842. PMID 11080142.

[43] Gaudet R., Wiley D. C. (2001). "Structure of the ABC ATPase domain of human TAP1, the transporter associated with antigen processing".

EMBO J.20 (17): 49644972. doi:10.1093/emboj/20.17.4964. PMC 125601. PMID 11532960.[44] Schmitt L., Benabdelhak H., Blight M. A., Holland I. B., Syubbs M. T. (2003). "Crystal structure of the nucleotide binding domain of the

ABC-transporter haemolysin B: identification of a variable region within ABC helical domains".J. Mol. Biol.330 (2): 333342.

doi:10.1016/S0022-2836(03)00592-8. PMID 12823972.

[45] Yuan Y.R., Blecker S., Martsinkevich O., Millen L., Thomas P.J., Hunt J.F. (2001). "The crystal structure of the MJ0796 ATP-binding

cassette: implications for the structural consequences of ATP hydrolysis in the active site of an ABC transporter".J. Biol. Chem.34 (276):

3231332321.

[46] Smith P.C., Karpowich N., Millen L., Moody J.E., Rosen J., Thomas P.J., Hunt J.F. (2002). "ATP binding to the motor domain from an

ABC transporter drives formation of a nucleotide sandwich dimer".Mol Cell10 (1): 13949. doi:10.1016/S1097-2765(02)00576-2.

PMID 12150914.

[47] Ward A, Reyes CL, Yu J, Roth CB, Chang G (November 2007). "Flexibility in the ABC transporter MsbA: Alternating access with a twist"

(http://www.pnas.org/cgi/pmidlookup?view=long&pmid=18024585). Proc. Natl. Acad. Sci. U.S.A.104 (48): 1900510.

doi:10.1073/pnas.0709388104. PMC 2141898. PMID 18024585. .

[48] Hopfner K.P., Karcher A., Shin D.S., Craig L., Arthur L.M., Carney J.P., Tainer J.A. (2000). "Structural biology of Rad50 ATPase:

ATP-driven conformational control in DNA double-strand break repair and the ABC-ATPase superfamily". Cell101 (7): 789800.
http://www.pnas.org/cgi/pmidlookup?view=long&pmid=18024585http://www.rcsb.org/pdb/explore/explore.do?structureId=2onjhttp://www.rcsb.org/pdb/explore/explore.do?structureId=2qi9http://en.wikipedia.org/w/index.php?title=Gideon_Koren


18/21


doi:10.1016/S0092-8674(00)80890-9. PMID 10892749.

[49] Fetsch E.E., Davidson A.L. (2002). "Vanadate-catalyzed photocleavage of the signature motif of an ATP-binding cassette (ABC)

transporter".Proc. Natl. Acad. Sci. USA99 (15): 968590. doi:10.1073/pnas.152204499. PMC 124977. PMID 12093921.

[50] Reyes C.L., Ward A., Yu J., Chang G. (2006). "The structures of MsbA: Insight into ABC transporter-mediated multidrug efflux".FEBS

Lett.580 (4): 10428. doi:10.1016/j.febslet.2005.11.033. PMID 16337944.

[51] Ambudkar S.V., Kim I.W., Xia D., Sauna Z.E. (2006). "The A-loop, a novel conserved aromatic acid subdomain upstream of the Walker A

motif in ABC transporters, is critical for ATP binding".FEBS Lett.580 (4): 104955. doi:10.1016/j.febslet.2005.12.051. PMID 16412422.

[52] Geourjon C., Orelle C., Steinfels E., Blanchet C., Deleage G., Pietro A. Di, Jault J.M. (2001). "A common mechanism for ATP hydrolysis inABC transporter and helicase superfamilies". Trends Biochem Sci26 (9): 539544. doi:10.1016/S0968-0004(01)01907-7. PMID 11551790.

[53] Ye J., Osborne A.R., Groll M., Rapoport T.A. (2004). "Rec-A like motor ATPaseslessons from structures".Biochim. Biophys. Acta1659

(1): 118. doi:10.1016/j.bbabio.2004.06.003. PMID 15511523.

[54] Zaitseva J., Jenewein S., Jumpertz T., Holland I.B., Schmitt L. (2005). "H662 is the linchpin of ATP hydrolysis in the nucleotide-binding

domain of the ABC transporter HlyB".EMBO J.24 (11): 190110. doi:10.1038/sj.emboj.7600657. PMC 1142601. PMID 15889153.

[55] Maegley K.A., Admiraal S.J., Herschlag D. (1996). "Ras-catalyzed hydrolysis of GTP: a new perspective from model studies".Proc. Natl.

Acad. Sci. USA93 (16): 81606. doi:10.1073/pnas.93.16.8160. PMC 38640. PMID 8710841.

[56] Matte A, Tari LW, Delbaere LT (April 1998). "How do kinases transfer phosphoryl groups?". Structure6 (4): 4139.

doi:10.1016/S0969-2126(98)00043-4. PMID 9562560.

[57] Hollenstein K., Dawson R.J., Locher K.P. (2007). "Structure and mechanism of ABC transporter proteins". Curr. Opin. Struct. Biol.17 (4):

4128. doi:10.1016/j.sbi.2007.07.003. PMID 17723295.

[58] Higgins C.F., Linton K.J. (2004). "The ATP switch model for ABC transporters".Nat. Struct. Mol. Biol.11 (10): 918

926.doi:10.1038/nsmb836. PMID 15452563.

[59] Locher K.P. (2004). "Structure and mechanism of ABC Transporters". Curr. Opin. Struct. Biol.14 (4): 42643.

doi:10.1016/j.sbi.2004.06.005. PMID 15313236.

[60] Oldham M.L., Davidson A.L., Chen J. (2008). "Structural insights into ABC transporter mechanism". Curr. Opin. Struct. Biol.18 (6):

726733. doi:10.1016/j.sbi.2008.09.007. PMC 2643341. PMID 18948194.

[61] Chang G (2003). "Multidrug resistance ABC transporters".FEBS Lett.555 (1): 1025. doi:10.1016/S0014-5793(03)01085-8.

PMID 14630327.

[62] Senior A.E., Al-Shawi M.K., Urbatsch I.L. (1995). "The catalytic cycle of P-glycoprotein".FEBS Lett.377 (3): 2859.

doi:10.1016/0014-5793(95)01345-8. PMID 8549739.

[63] Martin C., Higgins C.F., Callaghan R. (2001). "The vinblastine binding site adopts high- and low-affinity conformations during a transport

cycle of P-glycoprotein".Biochemistry40 (51): 1573342. doi:10.1021/bi011211z. PMID 11747450.

[64] Manciu L. et al. (2003). "Intermediate structural states involved in MRP1-mediated drug transport. Role of glutathione".J. Biol. Chem.278

(5): 334756. doi:10.1074/jbc.M207963200. PMID 12424247.

[65] Kreimer D.I., Chai K.P., Ames G.F.-L. (2000). "Nonequivalence of the nucleotide-binding subunits of an ABC transporter, the histidine

permease, and conformational changes in the membrane complex".Biochemistry39 (46): 1418395. doi:10.1021/bi001066. PMID 11087367.

[66] Vigano C., Margolles A., Van Veen H.W., Konings W.N., Ruyssschaert J.-M. (2000). "Secondary and tertiary structure changes of

reconstituted LmrA induced by nucleotide binding or hydrolysis. A fourier transform attenuated total reflection infrared spectroscopy and

tryptophan fluorescence quenching analysis".J. Biol. Chem.275 (15): 109627. doi:10.1074/jbc.275.15.10962. PMID 10753896.

[67] Sonveaux N., Vigano C., Shapiro A.B., Ling V., Ruyssschaert J.-M. (1999). "Ligand-mediated tertiary structure changes of reconstituted

P-glycoprotein. A tryptophan fluorescence quenching analysis".J. Biol. Chem.274 (25): 1764954. doi:10.1074/jbc.274.25.17649.

PMID 10364203.

[68] Rosenberg M.F. et al. (2001). "Repacking of the transmembrane domains of P-glycoprotein during the transport ATPase cycle".EMBO J.

20 (20): 561525. doi:10.1093/emboj/20.20.5615. PMC 125677. PMID 11598005.

[69] Mcmurry L., Petrucci R.E., Jr , Levy S.B. (1980). "Active efflux of tetracycline encoded by four genetically different tetracycline resistance

determinants in Escherichia coli".Proc. Natl. Acad. Sci. USA77 (7): 39747. doi:10.1073/pnas.77.7.3974. PMC 349750. PMID 7001450.[70] Dawson R.J., Locher K.P. (2007). "Structure of the multidrug ABC transporter Sav1866 from Staphylococcus aureus in complex with

AMP-PNP".FEBS Lett.581 (5): 9358. doi:10.1016/j.febslet.2007.01.073. PMID 17303126.

[71] Velamakanni S., Yao Y., Gutmann D.A., van Veen H.W. (2008). "Multidrug transport by the ABC transporter Sav1866 from

Staphylococcus aureus".Biochemistry47 (35): 93009308. doi:10.1021/bi8006737. PMID 18690712.

[72] Reuter G., Janvilisri T., Venter H., Shahi S., Balakrishnan L., van Veen H.W. (2003). "The ATP binding cassette multidrug transporter

LmrA and lipid transporter MsbA have overlapping substrate specificities".J Biol Chem278 (37): 3519335198.

doi:10.1074/jbc.M306226200. PMID 12842882.

[73] Raetz C.R.H., Reynolds C.M., Trent M.S., Bishop R.E. (2007). "Lipid A modification systems in gram-negative bacteria".Annu. Rev.

Biochem76: 295329. doi:10.1146/annurev.biochem.76.010307.145803. PMC 2569861. PMID 17362200.

[74] http://www.rcsb.org/pdb/explore/explore.do?structureId=3b5w

[75] http://www.rcsb.org/pdb/explore/explore.do?structureId=3b5x

[76] http://www.rcsb.org/pdb/explore/explore.do?structureId=3b60

[77] Chang G., Roth C.B. (2001). "Structure of MsbA from E. coli: A homolog of the multidrug resistance ATP binding cassette (ABC)

transporters". Science293 (5536): 1793800. doi:10.1126/science.293.5536.1793. PMID 11546864. (Retracted, see Chang, G; Roth,
http://www.rcsb.org/pdb/explore/explore.do?structureId=3b60http://www.rcsb.org/pdb/explore/explore.do?structureId=3b5xhttp://www.rcsb.org/pdb/explore/explore.do?structureId=3b5w


19/21


CB; Reyes, CL; Pornillos, O; Chen, YJ; Chen, AP (2006). "Retraction". Science314 (5807): 1875.

doi:10.1126/science.314.5807.1875b. PMID 17185584.)

[78] Reyes C.L., Chang G. (2005). "Structure of the ABC transporter MsbA in complex with ADPvanadate and lipopolysaccharide". Science

308: 10281031. doi:10.1126/science.1107733. PMID 15890884. (Retracted, see Chang, G; Roth, CB; Reyes, CL; Pornillos,

O; Chen, YJ; Chen, AP (2006). "Retraction". Science314 (5807): 1875. doi:10.1126/science.314.5807.1875b.

PMID 17185584.)}}

[79] Buchaklian A.H., Funk A.L., Klug C.S. (2004). "Resting state conformation of the MsbA homodimer as studied by site-directed spin

labeling".Biochemistry43 (26): 86006. doi:10.1021/bi0497751. PMID 15222771.[80] Dong J., Yang G., Mchaourab H.S. (2005). "Structural basis of energy transduction in the transport cycle of MsbA". Science308 (5724):

10238. doi:10.1126/science.1106592. PMID 15890883.

[81] Borbat PP, Surendhran K, Bortolus M, Zou P, Freed JH, Mchaourab HS (October 2007). "Conformational motion of the ABC transporter

MsbA induced by ATP hydrolysis" (http://dx.plos.org/10.1371/journal.pbio.0050271).PLoS Biol.5 (10): e271.

doi:10.1371/journal.pbio.0050271. PMC 2001213. PMID 17927448. .

[82] Gutmann D.A., Ward A., Urbatsch I.L., Chang G., van Veen H.W. (2010). "Understanding polyspecificity of multidrug ABC transporters:

closing in on the gaps in ABCB1". Trends Biochem Sci35 (1): 3642. doi:10.1016/j.tibs.2009.07.009. PMID 19819701.

[83] Lage, L (2009). "ABC Transporters as Target for RNA Interference-mediated Reversal of Multidrug Resistance".ABC Transporters in

Microorganisms. Caister Academic Press. ISBN 978-1-904455-49-3.

[84] http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-8371

[85] Glavinas H, Krajcsi P, Cserepes J, Sarkadi B (Jan 2004). "The role of ABC transporters in drug resistance, metabolism and toxicity" (http://

www.

bentham-direct.

org/

pages/

content.php?CDD/

2004/

00000001/

00000001/

005AU.

SGM). Curr Drug Deliv1 (1): 27

42.doi:10.2174/1567201043480036. PMID 16305368. .

[86] Glavinas H, Mhn D, Jani M, Oosterhuis B, Herdi-Szab K, Krajcsi P (Jun 2008). "Utilization of membrane vesicle preparations to study

drug-ABC transporter interactions".Expert Opin Drug Metab Toxicol4 (6): 72132. doi:10.1517/17425255.4.6.721. PMID 18611113.

[87] Jeffrey P, Summerfield SG (Oct-Nov 2007). "Challenges for bloodbrain barrier (BBB) screening".Xenobiotica37 (1011): 113551.

doi:10.1080/00498250701570285. PMID 17968740.

[88] This entire volume is dedicated to various methods used: Nikaido, H; Hall, J (1998). "ABC Transporters: Biochemical, Cellular, and

Molecular Aspects" (http://www.sciencedirect.com/science?_ob=PublicationURL&

_tockey=#TOC#18066#1998#997079999#473639#FLA#& _cdi=18066& _pubType=BS& _auth=y& _version=1&_urlVersion=0&

_userid=10&md5=ada84c26bdb8a5e4fbc754825a3ae6a1).Methods in enzymology292: 3853. doi:10.1016/S0076-6879(98)92003-1. .

[89] Horio M, Gottesman MM, Pastan I (May 1988). "ATP-dependent transport of vinblastine in vesicles from human multidrug-resistant cells"

(http://www.pnas.org/cgi/pmidlookup?view=long&pmid=3368466). Proc Natl Acad Sci USA.85 (10): 35804.

doi:10.1073/pnas.85.10.3580. PMC 280257. PMID 3368466. .

[90] http://www.solvobiotech.com/technologies/vesicular-transport-assay

[91] http://nutrigene.4t.com/humanabc.htm

[92] Saier MH (Jun 2000). "A functional-phylogenetic classification system for transmembrane solute transporters" (http://mmbr.asm.org/cgi/

pmidlookup?view=long&pmid=10839820). Microbiol. Mol. Biol. Rev.64 (2): 354411. doi:10.1128/MMBR.64.2.354-411.2000.

PMC 98997. PMID 10839820. . TCDB (http://www.tcdb.org/tcdb/index.php?tc=3.A.1)

[93] The ABC Transporters of Human Physiology and Disease Website (http://www.worldscibooks.com/lifesci/7371. html)

1. Dean, Michael. The Human ATP-Binding Cassette (ABC) Transporter Superfamily (http://www.ncbi.nlm. nih.

gov/books/bv.fcgi?rid=mono_001.chapter.137). Bethesda (MD):National Library of Medicine (US), NCBI;

2002 November.

2. ABC Nomenclature Committee. ABC-Transporter Genes nomenclature scheme (http://www.gene.ucl.ac.uk/

nomenclature/genefamily/abc.html#table1), enacted October 22, 1999. Verified availability August 2, 2005.

3. Wain, H. M.; White, J. A.; Povey, S. (http://www.gene.ucl.ac.uk/nomenclature/genefamily/abcabs.htm)

1. Deeley, R. G. (http://meds.queensu.ca/qcri/deeley/ri_rgd.htm#ABC)

2. Matsson, P. ATP-Binding Cassette Efflux Transporters and Passive Membrane Permeability in Drug Absorption

and Disposition. Acta Universitatis Upsaliensis. (http://www.diva-portal.org/diva/

getDocument?urn_nbn_se_uu_diva-8371-1__fulltext.pdf)
http://www.diva-portal.org/diva/getDocument?urn_nbn_se_uu_diva-8371-1__fulltext.pdfhttp://www.diva-portal.org/diva/getDocument?urn_nbn_se_uu_diva-8371-1__fulltext.pdfhttp://meds.queensu.ca/qcri/deeley/ri_rgd.htm#ABChttp://www.gene.ucl.ac.uk/nomenclature/genefamily/abcabs.htmhttp://www.gene.ucl.ac.uk/nomenclature/genefamily/abc.html#table1http://www.gene.ucl.ac.uk/nomenclature/genefamily/abc.html#table1http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mono_001.chapter.137http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mono_001.chapter.137http://www.worldscibooks.com/lifesci/7371.htmlhttp://www.tcdb.org/tcdb/index.php?tc=3.A.1http://mmbr.asm.org/cgi/pmidlookup?view=long&pmid=10839820http://mmbr.asm.org/cgi/pmidlookup?view=long&pmid=10839820http://nutrigene.4t.com/humanabc.htmhttp://www.solvobiotech.com/technologies/vesicular-transport-assayhttp://www.pnas.org/cgi/pmidlookup?view=long&pmid=3368466http://en.wikipedia.org/w/index.php?title=Michael_M._Gottesmanhttp://www.sciencedirect.com/science?_ob=PublicationURL&_tockey=%23TOC%2318066%231998%23997079999%23473639%23FLA%23

abc transporters

Documents