第十一章细胞通讯北大陈建国细胞生物学

Cellsignalingcanaffectvirtuallyeveryaspectofcellstructureandfunction:

Activationofenzymeactivity;--metabolism,apoptosis,immunology

Changeincytoskeletalorganization;--movement,immunology

Changeinionpermeability;--neuraltransmission,immunology

InitiationofDNAsynthesis;--cellcycle,cellgrowth

Activationorrepressionofgeneexpression.--celldifferentiation,developmentChapter11CellSignaling3.Signaltransductionmediatedbythereceptorsonthecellsurface2.Signaltransductionmediatedbythereceptorswithincells1.Overviewofintercellularsignaling4.Signalsthatoriginatefromcontactsbetweenthecellsurfaceandthesubstratum5.Convergence,divergence,andcrosstalkamongdifferentsignalingpathwaysManybacteria,forexample,respondtochemicalsignalsthataresecretedbytheirneighborsandincreaseinconcentrationwithincreasingpopulationdensity—tosuccessfullyinfectthehostQuorumsensing:Allowsbacteriatocoordinatetheirbehavior,includingtheirmotility,antibioticproduction,sporeformation,andsexualconjugationUnicellularorganisms:bacteria,archaea,protozoa,algaeandfungiAlthoughthesecellslargelyleadindependentlives,theycancommunicateandinfluenceoneanother’sbehavior.1.OverviewofintercellularsignalingYeastcellscommunicatewithoneanotherinpreparationformating:whenahaploidindividualisreadytomate,itsecretesapeptidematingfactor

thatsignalscellsoftheoppositematingtypetostopproliferatingandpreparetomate.Thesubsequentfusionoftwohaploidcellsofoppositematingtypeproducesadiploidcell,whichcanthenundergomeiosisandgeneratinghaploidcellswithnewassortmentsofgenes.----Tohelpaspeciessurviveinanunpredictedenvironment.UnicellularorganismsCellscommunicatebymeansofhundredsofkindsofsignalmolecules:proteins,smallpeptides,aminoacids,nucleotides,steroids,retinoids(类视黄醇),fattyacidderivatives,anddissolvedgasessuchasnitricoxide(NO)andcarbonmonoxide(CO).1)Secretedsignal-dependentsignaling:mostofthesignalmoleculesarereleasedintotheextracellularspacebyexocytosisfromthesignalingcell.2)Contact-dependentsignaling:transmembraneproteins,importantduringdevelopmentandinimmuneresponses.3)GapJunctions:

allowneighboringcellstoshareinformation.Multicellularanimals1)Secretedsignalmolecules-dependentcommunicationEndocrinesignaling:cellssecretesignalmolecules,calledhormones,intothebloodstream,whichcarriesthemoleculesfarandwide,allowingthemtoactontargetcellsthatmaylieanywhereinthebody.Paracrinesignaling:affectsonlycellsinthelocalenvironmentofthesignalingcell.Usually,thesignalingandtargetcellsinparacrinesignalingareofdifferentcelltypes.Forparacrinesignalstoactonlylocally,thesecretedmoleculesmustnotbeallowedtodiffusetoofar;sotheyareoftenrapidlytakenupbyneighboringtargetcells,destroyedbyextracellularenzymes,orimmobilizedbytheextracellularmatrix.Autocrinesignaling:cellsproducesignalsthattheythemselvesrespondto.Cancercells,forexample,oftenusethisstrategytostimulatetheirownsurvivalandproliferationEndocrine,paracrine,andautocrinesignalingdifferaccordingtothedistanceoverwhichthesignalingmoleculeacts.Inendocrinesignaling,signalingmoleculesarereleasedbyacellandactontargetcellsatadistance.Inanimals,thesignalingmoleculeiscarriedtotargetcellsbythebloodorotherextracellularfluids.Inparacrinesignaling,thesignalingmoleculesarereleasedandaffectonlytargetcellsincloseproximity.Inautocrinesignaling,thecellthatreleasesthesignalingmoleculeisalsothetargetcell.Growthhormoneisanexampleofendocrinesignalingbecausethegrowthhormoneissynthesizedinthepituitary,locatedatthebaseofthebrain,andtravelstotheliverviatheblood.1)Secretedsignalmolecules-dependentcommunicationManysignalmoleculesremainboundtothesurfaceofthesignalingcellandinfluenceonlycellsthatcontactit.Suchcontact-dependentsignalingisespeciallyimportantduringdevelopmentandinimmuneresponses.Contact-dependentsignalingduringdevelopmentcansometimesoperateoverrelativelylargedistances,wherethecommunicatingcellsextendlongprocessestomakecontactwithoneanother.2)Contact-dependentcommunication3)GapJunction-dependentcommunicationGapjunctionsarenarrowwater-filledchannelsthatdirectlyconnectthecytoplasmsofadjacentepithelialcells,&someothercelltypes.Thechannelsallowtheexchangeofinorganicionsandothersmallwatersolublemolecules,butnotofmacromoleculessuchasproteinsornucleicacids.Thus,cellsconnectedbygapjunctionscancommunicatewitheachotherdirectly.Inthisway,gapjunctionsprovideforthemost

intimatecellcommunication.Receptorproteinsarepresentintargetcells.Bindingofextracellularsignalingmoleculestocell-surfacereceptorstriggersaconformationalchangeinthereceptor,whichinturnleadstointracellularsignal-transductionpathwaysthatultimatelymodulatecellularmetabolism,function,orgeneexpression.Extracellularsignalsareinitiatedbyreceptors

(1)synthesisand(2)releaseofthesignalingmoleculebythesignalingcell;(3)transportofthesignaltothetargetcell;(4)bindingofthesignalbyaspecificreceptorproteinleadingtoaconformationalchange;(5)initiationofoneormoreintracellularsignal-transductionpathwaysbytheactivatedreceptor;(6)specificchangesincellularfunction,metabolism,ordevelopment;(7)feedbackregulationusuallyinvolvingdeactivationofthereceptor;(8)removalofthesignalingmoleculewhichtogetherterminatesthecellularresponse.Extracellularsignalingusuallyinvolves8stepsThesesignalmoleculesworkincombinationstoregulatethebehaviorofthecell.GeneralprinciplesofcellsignalingAnindividualcelloftenrequiresmultiplesignalstosurvive(bluearrows)andadditionalsignalstogrowanddivide(redarrow)ordifferentiate(greenarrows).Ifdeprivedofappropriatesurvivalsignals,acellwillundergoaformofcellsuicideknownasapoptosis.Theactualsituationisevenmorecomplex.(1)EachcellisprogrammedtorespondtospecificcombinationsofexreaceluularsignalmoleculesThecellintegratesdifferentinformationandmountsanappropriateandcomprehensiveresponse.Signal-integrationismediatedbyimportantmoleculesthatultimatelymodulatecellularmetabolism,function,orgeneexpression.ExtracellularsignalsAandBactivatedifferentintracellularsignalingpathways,eachofwhichleadstothephosphorylationofproteinYbutatdifferentsitesontheprotein.ProteinYisactivatedonlywhenbothofthesesitesarephosphorylated,andthereforeitbecomesactiveonlywhensignalsAandBaresimultaneouslypresent.(2)DifferentcellsresponddifferentlytothesamemoleculeVariousresponsesinducedbyneurotransmitteracetylcholineMusclerelaxation(3)Fourfeaturesofcellsignaling(4)ThefateofdevelopingcellsdependsonthepositioninMorphogenGradientsThesamesignalactingonthesamecelltypecanhavequalitativelydifferenteffectsdependingonthesignal’sconcentration.suchdifferentresponsesofacelltodifferentconcentrationsofthesamesignalmoleculearecrucialinanimaldevelopment,whencellsarebecomingdifferentfromoneanother.Theextracellularsignalmoleculeinthesecasesduringdevelopmentiscalledamorphogen,itdiffusesoutfromalocalizedcellularsource,generatingaconcentrationgradient.Therespondingcellsadoptdifferentcellfatesinaccordancewiththeirpositioninthegradient:thosecellsclosesttothesignalingcenterfollowonepathwayofdevelopment,whereasthoseslightlyfurtherawayfollowanother,andsoon.Signalmolecules&Receptors(1)Signalmolecules:(2)Receptorsincludetwoclasses:

心房排钠肽：排钠、排水，血管舒张TLR2/1TriacyllipopeptidesPam3-CysTLR2/6DiacyllipopeptidesMALP-2LTAZymosanTLR3TLR4/MD2/CD14LPSFlagellinTLR5TLR7/8TLR9UropathogenicbacteriaTLR11dsRNAssRNACpGDNATLRreceptorsTIRLRRMyD88Adaptorsub-groupTIRAP/MalTRAMDDTRIF/LPS2(3)Functionofthereceptors:Receptor-mediatedendocytosisSignaltransduction:1.Theactivationofreceptormakesforthecascades;2.Thedesensitizationofreceptormakesforclosedownofthecascades;3.Thedown-regulationofreceptormakesforthedepressedreaction.

Relayproteinssimplypassthemessagetothenextsignalingcomponentinthechain.Adaptor

proteins

linkonesignalingproteinstoanother.Amplifier

proteins

(enzymesorionchannels)signalingcascade.Transducer

proteins

convertthesignalintoadifferentform.Bifurcation

proteinsspreadthesignalfromonesignalingpathwaytoanother.Integrator

proteins

receivesignalsfromtwoormoresignalingpathwaysandintegratethembeforerelayingasignalonward.Messengerproteinscarrythesignalfromoneparttoanother(fromcytosoltonucleus).Latentgeneregulatory

proteins

areactivatedatthecellsurfacebyactivatedreceptorsandthenmigratetothenucleustostimulategenetranscription.IL-1012510301h2h4hIRAKIP:IL-1RIP:IRAK30”IL-1RIRAKTRAF6IRAKTRAF6Howdowestudythesignaltransduction?TAK1TAK1TAB2TAB2TAB1TAB1TAK1MKK6025103060IP:TAK1IL-1(min)025103060P-100S-100IB:Kinaseassay:PhosphorylationThephosphorylationandactivationofsignalproteinsJiangZF.etal2002,MolecularandCellularBiology,22:7158TwotypesofintracellularsignalingcomplexesMostactivatedcell-surfacereceptorsrelaysignalsviasmallmolecules(secondmessengers)andanetworkofintracellularsignalingproteins.Twotypesofintracellularsignalingcomplexes,enhancethespeed,efficiency,andspecificityoftheresponse.S-100P-100TRAF6IRAKTAB2TAK105100510IP:TAB2IL-1(min)DeterminationofproteincomplexindifferentcellularcompartmentsThisresultindicatedthatTAB2-TRAF6interactedatPM(P-100)&translocatedtocytosol(S-100)afterwards.IRAKTRAF6TAB1TAB2TAK1MyD88IL-1ComplexIIL-1RIRAKTRAF6IL-1RAcPIRAK4ComplexIITAB1TAB2TAK1TRAF6ComplexIIIInteractionsbetweenintracellularsignalingproteinsaremediatedbymodularbindingdomains(磷酸肌醇)(4)Fourmajorsignalingpathways心房排钠肽受体：排钠、排水，血管舒张TwomolecularswitchesforsignalingPhosphorylationanddephosphorylationviaproteinkinasesandphosphatases.Therebystimulatingorinhibitingtheactivities.GTPbindingproteinisinducedtoexchangeitsboundGDPforGTP,whichactivatestheprotein;theproteintheninactivatesitselfbyhydrolyzingitsboundGTPtoGDP.TheNobelPrizeinPhysiologyorMedicine1992“Fortheirdiscoveriesconcerningreversibleproteinphosphorylationasabiologicalregulatorymechanism”Signaltransductionpathwaysconsistprimarilyofproteinkinasesandproteinphosphatases.ThealterationsintheconformationofsignalingproteinsareusuallyaccomplishedbyproteinkinasesandproteinphosphatasesThehumangenomeencodesasmanyas2000differentproteinkinasesandmorethan300differentproteinphosphatases.TheNobelPrizeinPhysiologyorMedicine1994“FortheirdiscoveriesofG-proteinsandtheroleoftheseproteinsinsignaltransductionincells”G-proteinactivationandinactivationcycle(NIH.1970s)TheaccessoryproteinsforcyclingofG-proteinsGuaninenucleotide-exchangefactors(GEFs):

AninactiveGproteinisconvertedtotheactiveformwhentheboundGDPisreplacedwithaGTP.GEFsareproteinsthatbindtoaninactiveGproteinandstimulatedissociationoftheboundGDP.TherebyGEFsactivatesG-proteinGuaninenucleotide-dissociationinhibitors(GDIs):GDIsareproteinsthatinhibitthereleaseofaboundGDPfromaG-protein,thusmaintainingG-proteininactive,GDP-boundstate.GTPase-activatingproteins(GAPs):MostGproteinspossesssomecapabilitytohydrolyzeaGTP,butthiscapabilityisgreatlyacceleratedbyinteractionwithspecificGAPs.TherebyGAPsinactivateG-proteinbystimulatingittohydrolyzeitsboundGTP.3.Signaltransductionmediatedbythereceptorsonthecellsurface2.Signaltransductionmediatedbythereceptorswithincells1.Overviewofintercellularsignaling4.Signalsthatoriginatefromcontactsbetweenthecellsurfaceandthesubstratum5.Convergence,divergence,andcrosstalkamongdifferentsignalingpathways2.SignaltransductionmediatedbythereceptorswithincellsVarioussmallhydrophobicsignalmoleculesdiffusedirectlyacrosstheplasmamembraneoftargetcellsandbindtointracellularreceptorsthataregeneregulatoryproteins.Thesesignalmoleculesincludesteroidhormones,thyroidhormones,retinoids,andvitaminD.Theybindtotheirrespectiveintracellularreceptorproteinsandaltertheabilityoftheseproteinstocontrolthetranscriptionofspecificgenes.1)TheNuclearReceptorsAllligandsfornuclearreceptorsaresmallandhydrophobic!皮质（甾）醇雌二醇睾酮甲状腺素Nuclearreceptorsareallstructurallyrelated,beingpartoftheverylargenuclearreceptorsuperfamily.ManyfamilymembershavebeenidentifiedbyDNAsequencingonly,andtheirligandisnotyetknown;theseproteinsarethereforereferredtoasorphannuclearreceptors.Morethanhalfofthe48nuclearreceptorsencodedinthehumangenomeareorphans.B.ThesignaltriggeredbyNONitricOxideGasSignalsbyDirectlyRegulatingtheActivityofSpecificProteinsInsidetheTargetCellTwodiscovers:Duringthe1980s,bacteria-killingactivityofmacrophagesincellculturedependsonthepresenceofarginineinthemedium.arg.isasubstratefornitricoxidesynthasesNO(importantbiologicalmolecule).

Ithasbeenknownformanyyearsthatacetylcholinedilatebloodvesselsbycausingtheirsmoothmusclestorelax.In1980,R.Furchgottconcludedthatbloodvesselsaredilatedbecausetheendothelialcellsproduceasignalmoleculethatmakessmoothmusclecellsrelax.In1986workbyFurchgottandparallelworkbyLouisIgnarroidentifiedNOasthesignalthatcauserelaxationofthevascularsmoothmuscle.1998,ReceivedNobelPrize(R.Furchgott,L.Ignarro,F.Murad)TheactionofNitricoxideonbloodvesselsc-GMPPDEGMPViagraThemechanismbywhichacetylcholinestimulationoftheendothelialcellsleadstosmoothmusclerelaxationalsoexplainsthemechanismofactionofthechemicalnitroglycerin.Thedrugsildenafil,soldunderthetradenameViagra,isaninhibitorofac-GMP-specificphosphodiesterasethatnormallycatalyzesthebreakdownofcGMP.Thecarbonmonoxide(CO)actsasacellularmessengertostimulatetheproductionofcGMPbystimulatingG-cyclase.C.IntracellularReceptorsTakeuchi&Akira,2010,CellTLRsTLR3,7/8,9DNA/RNAdsRNAMyd88/TRIFMAVSNF

BIRF3/7CytokinesInflammationInflammasomeCaspase1IL-1/IL-18NLRsdsDNA1)

TLRs2)

RLRs3)

cGAS4)

NLRs5)

CLRs(C-typeLectin)Sting/MITAERISRLRCLRSYKPKCδInnateImmunityExtracellularInfectionIntracellularInfectionThirumala-DeviKanneganti,2010,NatureReviewsImmunologyIntracellularreceptorsforinfectionordangerDanger-associatedsignalsInnateimmunitytocytosolicNucleicAcidsfrommicrobialNF

BIRF3/7CARDRLRsHelicaseCARDCARDMAVSRNATBK1IKKiDNAIKKα/βIFI16DDX41cGASATP+GTPERSTINGMit3.Signaltransductionmediatedbythereceptorsonthecellsurface2.Signaltransductionmediatedbythereceptorswithincells(Nuclear,NO/CO,cytosolic)1.Overviewofintercellularsignaling4.Signalsthatoriginatefromcontactsbetweenthecellsurfaceandthesubstratum5.Convergence,divergence,andcrosstalkamongdifferentsignalingpathways1)SignalsmediatedbytheionchannelreceptorsInachemicalsynapse,electricalactivityinthepresynapticneuronisconverted(viatheactivationofvoltage-gatedchannels)intothereleaseofachemicalcalledaneurotransmitterthatbindstoreceptors(transmitter-gatedchannels)locatedintheplasmamembraneofthepostsynapticcell.Theneurotransmitterinitiatesanelectricalresponsethatexcitesthepostsynapticneuron.Mostofthechannelproteinsareionchannels,includingthreetypes,withionchannelsthattheycanbeopenedandclosedMajorelementsinchemicalsynaptictransmissionThesynapseandsynapticvesiclecycleThreestatesoftheacetylcholinereceptor2)SignalsmediatedbyGprotein-linkedreceptorsGPCRsuperfamily:

C.elegansgenome:19,000genes,encodesapproximately1000differentGPCRs.700GPCRsinhumans,1000GPCRsinmiceconcernedwiththesenseofsmellalone!

GPCRsformthelargestfamilyofcell-surfacereceptors,andtheymediatemostresponsestosignalsfromtheexternalworld,aswellassignalsfromothercells,includinghormones,neurotransmitters,andlocalmediators.Oursensesofsight,smell,andtaste(withthepossibleexceptionofsourtaste)dependonthem.Ligands:hormones,neurotransmitters,chemoattractants,odorantsandtastants,photons.ThestructureofGprotein-linkedreceptorsSeventransmembraneα

helix;C-terminal:Ser-andThr-rich,sitesofphosphorylationmakeforthedesensitizationofGPCRs.Theprimaryligandbindstotheextracellularportionofthereceptor.ThestructureofGprotein-linkedreceptorsDespitethediversityoftheligandsthatactivatethem,allGPCRshaveasimilarstructure,consistingofasinglepeptidechainthatthreadsbackandforthacrossthelipidbilayer7times.TheyalluseGproteinstorelaythesignal.ThestructureandactivationofGproteins促肾上腺皮质激素）(肾上腺素)（前列腺素E1）

GPCRsthatactbyincreasingcyclicAMParecoupledtoastimulatoryGprotein(Gs),whichactivatesadenylylcyclaseandtherebyincreasescyclicAMPconcentration.AnotherGprotein,calledinhibitoryGprotein(Gi),inhibitsadenylylcyclase.Familiesaredeterminedbyaminoacidsequencerelatednessoftheasubunits.Onlyselectedexamplesareincluded.About20asubunitsandatleast6bsubunitsand11gsubunitshavebeendescribedinhumans.FourMajorFamiliesofGProteinsActivationofGproteinbyanactivatedGPCRBindingofanextracellularsignaltoaGPCRchangestheconformationofthereceptor,whichinturnalterstheconformationoftheGprotein.ThealterationoftheasubunitoftheGproteinallowsittoexchangeitsGDPforGTP,activatingboththeαsubunitandtheβ/γcomplex,bothofwhichcanregulatetheactivityoftargetproteinsintheplasmamembrane.Thereceptorstaysactivewhiletheexternalsignalmoleculeisboundtoit,anditcanthereforecatalyzetheactivationofmanymoleculesofGprotein,whichdissociatefromthereceptoronceactivated.二酰基甘油三磷酸肌醇磷酯酰肌醇Phosphatidylinositol(PIP2)IntracellularSecondMessengersInareactioncatalyzedbytheenzymeadenylyl

cyclase,cyclicAMP(cAMP)issynthesized

fromATPthroughacyclizationreactionthatremovestwophosphategroupsaspyrophosphate;cAMPisunstableinthecellbecauseitishydrolyzedbyphosphodiesterasestoform5’-AMP,asindicated.ThesynthesisanddegradationofcyclicAMP(cAMP)ThesynthesisanddegradationofcyclicGMP(cGMP)cAMPcGMPAswithcAMP,acontinuousrapidsynthesis(byguanylylcyclase)andrapiddegradation(bycGMPphosphodiesterase)controlstheconcentrationofcGMPinthecytosol.c-di-GMPc-di-AMPc-GAMPs(c-GMP-AMP)Cyclicdi-nucleotidesNewSecondMessengersBacteriaBacteria,EukaryotesThestructureofcyclicGMP-AMPsc-di-GMPcGAMPc-di-GMPhasbeenimplicatedinbiofilmformation,bacterialdispersion,motility,cellcycleregulationanddifferentiationAllactivateinnateimmunity,leadingtotheproductionofpro-inflammatorycytokines

RelayofthesignalfromG-proteintoeffectorCyclicAMPsignalingpathway（促肾上腺皮质激素）（血液中的复合胺）（生长激素抑制素）（黄体激素）(肾上腺素)(卵泡刺激素)(肾上腺素)Theresponsebyalivertoglucagonorepinephrine②Toinhibitglycogensynthesis③Toenhanceglycogendegradation①ToregulategeneexpressionGlucoseActivationofcAMP-dependentproteinkinase(PKA)bycAMPIP3-Ca2+pathwayandDG-PKCpathwayManyGPCRsexerttheireffectsmainlyviaGproteinsthatactivatetheplasmamembrane-boundenzymephospholipaseC-β(PLCβ).Thephospholipaseactsonaphosphorylatedphosphatidylinositol4,5-bisphosphate[PI(4,5)P2,orPIP2],whichispresentintheinnerhalfoftheplasmamembranelipidbilayer.ReceptorsthatactivatethisinositolphospholipidsignalingpathwaymainlydosoviaaGproteincalledGq,whichactivatesPLCβinthesamewaythatGsactivatesadenylylcyclase.TheactivatedphospholipasecleavesthePIP2togeneratetwoproducts:inositol1,4,5-trisphosphate(IP3)anddiacylglycerol(DAG).Atthisstep,thesignalingpathwaysplitsintotwobranches.PLCβactsonPIP2presentintheinnerhalfoftheplasmamembrane,cleavesPIP2togeneratetwoproducts:IP3andDAG,thussplitsthesignalintotwobranches.DAGPhospholipaseCresultsintheelevationofintracellularCa+

Signals

GPLRGPPLCIP3andDAG(twinsignals).IP3IP3receptor(Ca2+channelatthesurfaceofsER)ElevationofcytosolicCa2+;DAG&Ca2+

activatesPKCtophosphorylateSerandThrontargetproteins.Calciumbindstocalcium-bindingproteins(CaM)whichaffectsotherproteins.PKCStructureofcalmodulin,acytosolicproteinof148aminoacidsthatbindCa2+CaMisfounduniversallyinplants,animals,andeukaryoticmicroorganisms.Ca2+/CaMdependentproteinkinase(CaM-kinase)mediatemanyoftheactionsofCa2+inanimalcells.Thefunctionsofcalcium-CaM:

start-upembryodevelopmentafterthefecundation.leadtocontractofmusclecells;leadtosecretionofendocrineandnervecells.Calciumcanaffectanumberofdifferenttypesofcellulareffectors,includingPKC.Dependingonthecelltype,calciumionscanactivateorinhibitvariousenzymeandtransportsystems,changetheionicpermeabilityofmembranes,inducemembranefusion,oraltercytoskeletalstructureandfunction.钙调磷酸酶bG-protein-linkedreceptordesensitizationdependsonreceptorphosphorylationbyPKA,PKC,CaMK2orG-protein-linkedreceptorkinases(GRKs)Thetargetcellscanbecomedesensitizedtoasignalmoleculebyfiveways.

Threegeneralwaysofthedesensitization:1.Receptorinactivationbyalteration;2.Receptorsequestrationbyinternalization;3.Receptordown-regulationbydestroyinginLs.

Sequestration;down-regulation;inactivation;inactivation;inhibitoryproteinThemechanismsthatshutoffasignalareasimportantasthemechanismsthatturnitonADP-riboseADP-ribosetransferaseactivityGTPaseactivitylostToxinsproducedbybacteriathatcausecholeraaffectingGsαNature2011Gprotein–coupledreceptors由β2肾上腺素所激活的G蛋白偶联受体与Gs蛋白复合体的晶体结构(PDB

3SN6)。Blue部分显示的是受体，Orange是Gα亚基，Yellow的是Gβ亚基，而Cyan的是Gγ亚基。可以看到Gα亚基的C端处于一个由第五和第六跨膜螺旋之间的膜内环向外移动所产生的空穴之中。Gprotein–coupledreceptorsItisremarkablethatabouthalfofallknowndrugsworkthroughGPCRsorthesignalingpathwaysGPCRsactivate.Ofthe700genesinthehumangenomethatencodeGPCRs,about150encodeorphanreceptors,forwhichtheligandisunknown.Manyofthemarelikelytargetsfornewdrugsthatremaintobediscovered.ManymorethingsneedtolearnaboutGPCR3)Receptortyrosinekinase(RTK)andRTK-RassignalingpathwayRTKarethesecondmajortypeofcell-surfacereceptors

(over50kindshavebeenidentified,thatareinvolvedprimarilyinthecontrolofcellgrowthanddifferentiationratherthanmetabolism.)

SignalingligandsofRTKs:1.Nervegrowthfactor(NGF)2.Platelet-derivedgrowthfactor(PDGF)3.Fibroblastgrowthfactor(FGF)4.Epidermalgrowthfactor(EGF)5.insulinandinsulin-likeGF(IGF-1)6.Ephrins(Eph):criticalinembryodevelopment&tumorogenesis7.vascularendothelialfactor(VEGF)Sixclassesofenzyme-linkedreceptorshavethusfarbeenidentified:Receptortyrosinekinase(RTK);Tyrosine-kinase-associatedreceptor;Receptorliketyrosinephosphatases;Receptorserine/threoninekinase;Receptorguanylylcyclases;Histidine-kinase-associatedreceptorStructureandactivationofreceptortyrosinekinasesRTKactivitystimulatedbycross-phosphorylation.PhosphorylatedTyrosineServeasdockingsitesforproteinwithSH2domains

(Srchomologyregion).OtherproteinmodulessuchasSH3bindstoproline-richmotifsinintracellularproteins.dimerizationStepsinactivationofRasbyRTKsPhosphotyrosinesofRTKactasbindingsitesforaspecificSH2proteincalledGRB2(Growthfactorreceptorbindingproteininmammalian).GRB2isnotaproteinwithcatalyticactivity,butonethatfunctionssolelyasanadaptermoleculethatlinksotherproteinsintoacomplex.Sos(sonofsevenless)isaguaninenucleotideexchangefactorforRas(Ras-GEF)WhenaligandbindstotheRTKandrecruitstheGrb2-Sostotheinnersurfaceofthemembrane,theSosproteinbindstoRascausingGDP/GTPexchange,thusactivatingRas.RTK-RassignalingpathwayLigand-RTK,recruitsGrb2-Sos,theSosbindstoRas,functionsasaguaninenucleotideexchangefactor(GEF),causingGDP/GTPexchange,thusactivatesRas.

GTP-GDPcyclingofRasproteinRasgenemutationsisfoundin30%ofallhumantumors.Ras(21Kda)consistsofasinglesmallsubunit.RashasaveryweakGTPaseactivityandwouldremainintheactive,GTP-boundstatefor30mins.Inthecell,RasactivityisregulatedbyGAPthatstimulatetheRasGTPaseabout105-fold.(Sos)(Sos)(Ras-GAP1)MAPkinase=mitogen-activatedproteinkinase;MAP-KKK=Raf(Ser/Thr-PK)14MAPKKKs,7MAPKKs,and13MAPKshavebeenidentifiedinmammals.FunctionsofMAPkinasecascadeyeastmatingpheromones;Fruitfliesdifferentiationofthephotoreceptors;Floweringplantsdefenseagainstpathogens.mammalscellproliferation.Ras-activatedMAP-kinaseserine/threonineMEK(Raf)ERKInsulinreceptorInsulinreceptorutilizesIRS,insteadofGrb2-Sos,totransmitthesignal.IRSs:insulinreceptorsubstratesPhosphorylatedIRSact