考研资料：北京大学细胞生物学第五章物质的跨膜运输(下)

B.NitricoxidecouplesGprotein-linkedreceptorstimulationinendothelialcellstorelaxationofsmoothmusclecellsinbloodvesselsIthasbeenknownformanyyearsthatacetylcholinedilatebloodvesselsbycausingtheirsmoothmusclestorelax.In1980,Furchgottconcludedthatbloodvesselsaredilatedbecausetheendothelialcellsproduceasignalmoleculethatmakessmoothmusclecellsrelax.In1986workbyFurchgottandparallelworkbyLouisIgnarroidentifiedNOasthesignalthatcauserelaxationofthevascularsmoothmuscle.1998,ReceivedNobelPrizeTheactionofNitricoxideonbloodvesselsThemechanismbywhichacetylcholinestimulationoftheendothelialcellsleadstosmoothmusclerelaxationalsoexplainsthemechanismofactionofthechemicalnitroglycerin.Thedrugsildenafil,soldunderthetradenameViagra,isaninhibitorofacyclicGMP-specificphosphodiesterasethatnormallycatalyzesthebreakdownofcyclicGMP.Thecarbonmonoxide(CO)actsasacellularmessengertostimulatetheproductionofcGMPbystimulatingG-cyclase.3.SignaltransductionmediatedbythereceptorsonthecellsurfaceA.MediatedbytheIon-LinkedReceptorswhichconvertchemicalsignalsintoelectricalones4or6-helixtransmembranereceptorB.SignaltransductionmediatedbyGprotein-linkedreceptorsThestructureofGprotein-linkedreceptors:Seven-helixtransmembrane;C-terminal:Ser-andThr-rich---thesitesofphosphorylationmakeforthedesensitizationofGPLR.Figure15-18TwomajorpathwaysbywhichG-protein-linkedcell-surfacereceptorsgeneratesmallintracellularmediators.Inbothcasesthebindingofanextracellularligandalterstheconformationofthecytoplasmicdomainofthereceptor,causingittobindtoaGproteinthatactivates(orinactivates)aplasmamembraneenzyme.InthecyclicAMP(cAMP)pathwaytheenzymedirectlyproducescyclicAMP.IntheCa2+pathwaytheenzymeproducesasolublemediator(inositoltrisphosphate)thatreleasesCa2+fromtheendoplasmicreticulum.ThestructureandactivationofGproteinsFigure15-23AcurrentmodelofhowGscouplesreceptoractivationtoadenylylcyclaseactivation.Aslongastheextracellularsignalingligandremainsbound,thereceptorproteincancontinuetoactivatemoleculesofGsprotein,therebyamplifyingtheresponse.Moreimportant,analphascanremainactiveandcontinuetostimulateacyclasemoleculeformanysecondsafterthesignalingliganddissociatesfromthereceptor,providingevengreateramplification.C.CyclicAMPsignalingpathwayG-proteinactivationandinactivationcycleTheactivationofproteinkinaseAbycyclicAMPsSecondmessengers(cAMP),aneffector,amplifytheresponsetoasingleextracellularligandbycAMPtotriggerareactioncascade.ThecascadestartswiththebindingofcAMPtocAMP-dependentproteinkinaseA.PKAinhibitsglycogensynthaseandactivatesphosphorylasekinase.DoubleMessengersystemFigure15-32TwointracellularpathwaysbywhichactivatedC-kinasecanactivatethetranscriptionofspecificgenes.Inone(redarrows)C-kinaseactivatesaphosphorylationcascadethatleadstothephosphorylationofapivotalproteinkinasecalledMAP-kinase,whichinturnphosphorylatesandactivatesthegeneregulatoryproteinElk-1.Elk-1isboundtoashortDNAsequenceinassociationwithanotherDNA-bindingprotein.Intheotherpathway(greenarrows)C-kinaseactivationleadstothephosphorylationofIk-B,whichreleasesthegeneregulatoryproteinNF-kBsothatitcanmigrateintothenucleusandactivatethetranscriptionofspecificgenes.Themechanismsthatshutoffasignalareasimportantasthemechanismsthatturniton.Choleraiscausedbyabacteriumthatmultipliesintheintestine,whereitproducesaproteincalledcholeratoxin.ThisentersthecellsliningtheintestineandmodifiestheαsubunitofGproteinsothatitcannolongerhydrolyzeitsboundGTP.Thealteredαsubunitthusremainsintheactivestateindefinitely,continuingtotransmitasignaltoitstargetproteins:OutflowofNa+andwaterintothegut.D.ThepathwaythroughphospholipaseCresultsinariseinintracellularCa+Cytolasmiccalciumlevelsaredeterminedbyeventswithinamembrane.IP3-Ca2+pathwayandDG-PKCpathwayElevationofcytosolicCa2+viatheIPsignalingpathwaySignalsGPLRGPPLCIP3andDAG(twinsignals).IP3IP3receptor(Ca2+channel,locatedatthesurfaceofsER)ElevationofcytosolicCa2+;DAGactivatesPKCtophosphoralateSerandThrontargetproteins.Calciumbindstocalcium-bindingproteins(CaM)whichaffectsotherproteins.Structureofcalmodulin,acytosolicproteinof148aminoacidsthatbindCa2+ionsRegulatingcalciumconcentrationsinplantcellsCytosoliccalciumchangesinresponsetoseveralstimuli,includinglight,pressure,gravity,andhormones.Calciumsignalingaidsindecreasingturgorpressureinguardcells.Ca2+/CaMdep.proteinkinase(CaM-kinase)mediatemanyoftheactionsofCa2+inanimalcells.Thefunctionsofincreasethelevelsofcytosoliccalcium-CaM:start-upembryodevelopmentafterthefecundation.excitatingcontractofmusclecells;excitatingsecretionofendocrineandnervecells.G-protein-linkedreceptordesensitizationdependsonreceptorphosphorylationbyPKA,PKC,CaMK2orG-protein-linkedreceptorkinases(GRKs)Thetargetcellscanbecomedesensitizedtoasignalmoleculebyfiveways.Threegeneralwaysofthedesensitization:1.Receptorinactivationbyalteration;2.Receptorsequestrationbyinternalization;3.Receptordown-regulationbydestroyinginLs.Sequestration;down-regulation;inactivation;inactivation;inhibitoryproteinE.Receptortyrosinekinase(RTK)andRTK-RassignalingpathwayRTKa