Posttranslational Modification

Nucleus acetylation, phosphorylation

Lysosome mannose-6-phosphate labelled N-linked sugar

Mitochondria N-formyl acylation

Golgi N- and O-linked ologosaccharide, sulfation,

palimitoylation

ER N-linked oligosaccharide, GPI-anchor

Cytosol acetylation, methylation, phosphorylation,

Ribosome myristoylation

Plasma membrane N- and O-glycosylation, GPI-anchor

Extracellular fluid N- and O-glycosylation, acetylation,

phosphorylation

Extracellular matrix N- and O-glycosylation, phosphorylation,

hydroxylation

ModificationLocation

1. stability of protein

2. Biochemical activity (activity regulation)

3. Protein targeting (protein localization)

4. protein signaling (protein-protein interaction, cascade amplification)

Purpose of modification

Types

Folding of proteins Protein folding in cells probably

involves multiple pathways. Initially,

regions of secondary structure may form,

followed by folding into super secondary

structures. Large ensembles of folding

intermediates are rapidly brought to a

single native conformation.

Not all proteins fold spontaneously as

they are synthesized in the cell. Folding

for many proteins is facilitated by the

action of specialized proteins. Molecular

chaperones are proteins that interact with

partially folded or improperly folded

polypeptides, facilitating correct folding

pathways or providing microenvironments

in which folding can occur.

Proteolysis and Intein protein splicing

E.g. Insulin by protein splicing

Chemical modification1.Amino Group Modification:Amino group modification involves the addition of a functional group at the N terminus of the amino acid

Acetylation

N-α-acetyl transferase

Acetyaltion is an acylation

(introduction of an acyl group

to an organic compound)

process which involves the

substitution of an organic

group of acetic acid for an

active hydrogen atom at the N-

terminus.

Pyroglutamate Formation:

Pyroglutamate is formed through the

cyclization (ring formation in the chemical

compound) of glutamine.

It is commonly observed in antibodies that

contain glutamate or glutamine residues at their

N-termini.

The amino group and the glutamate or

glutamine condenses to form a five-member

ring called Pyroglutamate.

Methylation

Glycosylation• The addition of sugar residues to the protein backbone.

• Most extensive posttranslational modification.

• Carried out in the ER and Golgi apparatus prior to secretion or surface display.

• All mammalian cell surface proteins of glycoprotein.

• Most secreted proteins are glycoprotein (notable exceptions include insulin, growth hormone).

• N-Linked

• O-Linked

• Membrane anchor

Contd….

• The membrane bound proteins are prevalent in most Eukaryotic

systems and serve to regulate the release of molecules from cell

surfaces and exchange of membrane molecules.

• Specifically they play a critical role in the recognition of enzymatic

and antigenic molecules as well receptor mediated signal transduction

pathways.

• Proteins destined for anchorage onto a membrane surface are first

adhered at the carboxyterminus (c-terminal region) to a

phosphodeister linkage system.

• This is comprised of the adsorption of phospothanolamine to a core of

trimannosyl-non-acetylated Glucosamine. This Glucosamine structure

is then linked to phosphatidylinositol, which is then finally adhered to

the lipid bilayer via another phosphodeister linker. Solubilization of

the membrane bound protein is achieved via cleavage of the

phosphatidynisotil bond by Phospholipase C.

GPI Anchoring

Phosphorylation

Protein phosphorylation is a reversible post-transcriptional modification which plays an important role in signal transduction, protein function and localization.

The phosphorylation of serine , threonine and tyrosine residues is the most common in mammalian cells, whereas the phopshorylation of histidine and aspartate is rare in eukaryotes.

Amidation

The amide group for C-terminal

amidation is contributed by a

Glycine residue. This Glycine is

present in a precursor C-terminal

sequence that resembles XGXX,

Amidation neutralizes negative

charges on the C-terminus of the

polypeptide.

Peptidylglycine α-hydroxylating

monooxygenase and peptidyl α-

hydroxyglycine α-amidating lyase

are two enzymes associated with

amidation,

Sumoylation• Small Ubiquitin-related Modifier proteins, or SUMO proteins,

are small proteins that covalently attach to proteins in a process known as sumoylation.

• As its name suggests, SUMO proteins and ubiquitin are similar proteins sharing many characteristics including the sequence of enzymatic reactions directing either process

• unlike ubiquitin, SUMO proteins do not act as proteolyticsignals. SUMOylation has been associated with various cellular functions and processes including cell cycle regulation, DNA repair, chromosomal maintenance, modifying cytoplasmic signal transduction, nuclear import and subnuclear compartmentalization, DNA repair, transcription regulation, and stress response

Protein-protein interactions occurbetween two proteins that are similar in size. Theinterface between the two molecules tend to beflatter and smoother than those in protein-ligandinteractions. Protein-protein interactions areusually more rigid; the interfaces of theseinteractions do not have the ability to alter theirconformation in order to improve binding andease movement. Conformational changes arelimited by steric constraint and thus are said to berigid.

Molecular docking is the technique that is used to study whether amolecule binds to another and if so how does it really bind. The term“docking” is mostly related to protein molecule interactions.

Protein trafficking or site specific transport