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SIGNAL TRANSDUCTION& QUORUM SENSING

Oleh: Dr. Giyanto

• Cell-to-cell communication is absolutely essentialfor multicellular organisms.

• Cells must communicate to coordinate their activities.

• Communication between cells is also important formany unicellular organisms.

• Biologists have discovered some universalmechanisms of cellular regulation, involving thesame small set of cell-signaling mechanisms.

• Cells may receive a variety of signals, chemicalsignals, electromagnetic signals, and mechanicalsignals.

Introduction

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General Mechanism of Signal Transduction

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• Cell signaling has remained important in themicrobial world.

• Myxobacteria, soil-dwelling bacteria, use chemicalsignals to communicate nutrient availability.

• When food is scarce, cells secrete a signal to other cellsleading them to aggregate and form thick-walled spores.

• Plants and animals use hormones to signal atgreater distances.

• In animals, specialized endocrine cells releasehormones into the circulatory system, by which theythey travel to target cells inother parts of the body.

• In plants, hormones maytravel in vessels, but moreoften travel from cell tocell or by diffusion in air.

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• The process must involve three stages.

• In reception, a chemical signal binds to a cellularprotein, typically at the cell’s surface.

• In transduction, binding leads to a change in thereceptor that triggers a series of changes along a signal-transduction pathway.

• In response,the transducedsignal triggersa specificcellularactivity.

• A cell targeted by a particular chemical signal has areceptor protein that recognizes the signal molecule.

• Recognition occurs when the signal binds to a specific siteon the receptor because it is complementary in shape.

• When ligands (small molecules that bind specificallyto a larger molecule) attach to the receptor protein,the receptor typically undergoes a change in shape.

• This may activate the receptor so that it can interact withother molecules.

1. A signal molecule binds to a receptorprotein causing the protein to change shape

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• Most signal molecules are water-soluble and toolarge to pass through the plasma membrane.

• They influence cell activities by binding to receptorproteins on the plasma membrane.

• Binding leads to change in the shape or the receptor or toaggregation of receptors.

• These trigger changes in the intracellular environment.

• Three major types of receptors are G-protein-linkedreceptors, tyrosine-kinase receptors, and ion-channel receptors.

2. Most signal receptors are plasmamembrane proteins

• A G-protein-linked receptor consists of a receptorprotein associated with a G-protein on thecytoplasmic side.

• The receptor consists of seven alpha helices spanning themembrane.

• Effective signalmolecules includeyeast matingfactors,epinephrine,other hormones,andneurotransmitters.

G-Protein

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• The G protein acts as an on-off switch.

• If GDP is bound, the G protein is inactive.

• If ATP is bound, the G protein is active.

G-Protein

• The G-protein system cycles between on and off.

• When a G-protein-linked receptor is activated bybinding with an extracellular signal molecule, thereceptor binds to an inactive G protein in membrane.

• This leads the G protein to substitute GTP for GDP.

• The G protein then binds with another membraneprotein, often an enzyme, altering its activity andleading toa cellularresponse.

G-Protein

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• The G protein can also act as a GTPase enzyme andhydrolyzes the GTP, which activated it, to GDP.

• This change turns the G protein off.

• The whole system can be shut down quickly when theextracellular signal molecule is no longer present.

G-Protein

• G-protein receptor systems are extremelywidespread and diverse in their functions.

• In addition to functions already mentioned, they play animportant role during embryonic development andsensory systems.

• Similarities among G proteins and G-protein-linked receptors suggest that this signaling systemevolved very early.

• Several human diseases are the results of activities,including bacterial infections, that interfere withG-protein function.

G-Protein

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• The tyrosine-kinase receptor system is especiallyeffective when the cell needs to regulate andcoordinate a variety of activities and trigger severalsignal pathways at once.

• Extracellular growth factors often bind to tyrosine-kinase receptors.

• The cytoplasmic side of these receptors function asa tyrosine kinase, transferring a phosphate groupfrom ATP to tyrosine on a substrate protein.

Tyrosine-Kinase

• A individual tyrosine-kinase receptors consists ofseveral parts:

• an extracellular signal-binding sites,

• a single alpha helix spanning the membrane, and

• an intracellulartail with severaltyrosines.

Tyrosine-Kinase

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• When ligands bind to two receptors polypeptides,the polypeptides aggregate, forming a dimer.

• This activates the tyrosine-kinase section of both.

• These add phosphates to the tyrosine tails of theother polypeptide.

Tyrosine-Kinase

• The fully-activated receptor proteins activate avariety of specific relay proteins that bind tospecific phosphorylated tyrosine molecules.

• One tyrosine-kinase receptor dimer may activate ten ormore different intracellular proteins simultaneously.

• These activated relayproteins trigger manydifferent transductionpathways andresponses.

Tyrosine-Kinase

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• This allows or blocks ionflow, such as Na+ or Ca2+.

• Binding by a ligand to theextracellular side changes theprotein’s shape and opens thechannel.

• Ion flow changes theconcentration inside the cell.

• When the ligand dissociates,the channel closes.

Ligand-gated ion channels

• Ligand-gated ion channelsare protein pores that open orclose in response to achemical signal.

• Ligand-gated ion channels are very important inthe nervous system.

• Similar gated ion channels respond to electrical signals.

Ligand-gated ion channels

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• Testosterone, like otherhormones, travels through theblood and enters cellsthroughout the body.

• In the cytosol, they bind andactivate receptor proteins.

• These activated proteins enterthe nucleus and turn on genesthat control male sexcharacteristics.

• These activated proteins act as transcriptionfactors.

• Transcription factors control which genes are turned on- that is, which genes are transcribed into messengerRNA (mRNA).

• The mRNA molecules leave the nucleus and carryinformation that directs the synthesis (translation) ofspecific proteins at the ribosome.

• Other intracellular receptors are already in thenucleus and bind to the signal molecules there(e.g., estrogen receptors).

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• The transduction stage of signaling is usually amultistep pathway.

• These pathways often greatly amplify the signal.

• If some molecules in a pathway transmit a signal tomultiple molecules of the next component, the result canbe large numbers of activated molecules at the end of thepathway.

• A small number of signal molecules can produce alarge cellular response.

• Also, multistep pathways provide more opportunitiesfor coordination and regulation than do simplersystems.

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• Many signaling pathways involve small, nonprotein,water-soluble molecules or ions, called secondmessengers.

• These molecules rapidly diffuse throughout the cell.

• Second messengers participate in pathways initiatedby both G-protein-linked receptors and tyrosine-kinase receptors.

• Two of the most important are cyclic AMP and Ca2+.

3. Certain signal molecules and ions are keycomponents of signaling pathways (second

messengers)

• Other G-protein systems inhibit adenylyl cyclase.

• These use a different signal molecule to activate otherreceptors that activate inhibitory G proteins.

• Certain microbes cause disease by disrupting theG-protein signaling pathways.

• The cholera bacterium, Vibrio cholerae, colonizes thethe small intestine and produces a toxin that modifies aG protein that regulates salt and water secretion.

• The modified G protein is stuck in its active form,continuously stimulating productions of cAMP.

• This causes the intestinal cells to secrete large amountsof water and salts into the intestines, leading to profusediarrhea and death if untreated.

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Quorum Sensing

• A process that enables bacteria to communicate using secretedsignaling molecules called autoinducers

• This process enables a population of bacteria to regulate geneexpression collectively and therefore, control behavior on acommunity-wide scale.

• Cell-cell communication can occur within and between bacterialspecies, and between bacteria and their eukaryotic hosts.

Overview - How Cells Coordinate Behavior

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Quorum Sensing Systems:Gram negative class

• LuxI/R systems

• Autoinducers: acylated homoserine lactone

• LuxI-type enzymes synthesize acylated homoserine lactone(AHL) autoinducers by ligating a specific acyl moiety to thehomocysteine moiety of S-adenosylmethionine (SAM)

• LuxR-type proteins bind their cognate autoinducers and controltranscription of target genes.

Quorum Sensing Systems:Gram positive class

• Two-component systems involved

• Autoinducers: modified oligopeptides

• The signals are synthesized as precursor peptides, which aresubsequently processed and secreted

• Sensor histidine kinases detect the extracellular peptideautoinducers, autophosphorylate and transmit sensoryinformation via phosphorylation of a response regulator

• Response regulator changes gene expression

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Quorum Sensing Systems:The hybrid class

• This class was initially identified in Vibrio harveyi.

• It produces two disinct autoinducers, AI-1 and AI-2

• AI-1 is an acylated homoserine lactone (AHL), similar to Gramnegative class

• AI-2 has no resemblance to other inducers

• Both AI-1 and AI-2 signal transduction occurs by a two-component system, similar to Gram positive class. Thus, this isa hybrid class.

Quorum Sensing Systems

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Quorum Sensing Systems

Intraspecies vs interspeciescommunications

• AHL type autoinducers are for intraspeciescommunications

• AI-2 and its synthase, LuxS, are widespread,existing in many bacterial phyla. AI-2 issuggested to serve as an interspecies bacterialcommunication signal.

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Signal Discrimination

• Many bacteria possess multiple quorum-sensing systems, whichcan be organized in series or in parallel.

• V. harveyi responds to AI-1 and AI-2 in parallel.

• B. subtilis also uses parallel systems to respond to differentoligopeptide autoinducers.

• P. aeruginosa uses two LuxI/R systems acting in series toregulate overlapping groups of target genes.

The Vibrio harveyi quorum-sensing system

Bacillus subtilis competence andsporulation • Competence develops with density as

more cells lyse and DNA is available

• ComX (grey circles)

• ComA-P activates ComS which preventsdegradationof ComK which activatesgenes

• Sporulation when nutrients are depleted- results in shutdown of competencepathway

• CSF signal (white diamond)

• Low CSF inhibits RapC phosphatase ofComA-P

• High CSF inhibits comS and RapBphosphatase of Spo0A-P signal ofsporulation

Extracellular structures

• Capsule - adherent

• Slime layer

• like capsule but looselyadherent at edges

• Glycocalyx - polysaccharide

fibers visible only by EM;used in attachment

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Biofilms are sites of high density

Gene transfer

Antibioticresistance

Loss of flagella

• Biofilm - organized microbial system of layersof microbial cells embedded in apolysaccharide matrix of microbial originassociated with surfaces

• Aquatic environments - on algae, rocks, ships

• Medicine - on catheters, teeth, contact lenses, airconditioners

• Industry - pipes, plastics

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Biofilm Help Bacteria Established inSpecific Ecology