Chapter 06 Protein Sorting

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1、Chapter6ProteinSorting1.RoadMapofProteinTrafficA.Proteinsareimportedintoorganellesbythreemechanisms.¾GatedTransport:Transportthroughnuclearpores.¾TransmembraneTransport:ERlumen,Mitochondrion,Chloroplast,Peroxisome.¾VesicularTransport:ER‐Golgi‐Plasmamem

2、brane,Lysosome,Endosome.B.SortingSignal¾Aminoacidsequencethatdirectsthedeliveryofaproteintoaspecificlocationoutsidethecytosol.¾SignalSequence:Mostproteinsortingsignalsresideinastrechofaminoacidsequence,typically15‐60residueslong.¾SignalPatch:Sortingsig

3、nalscomposedofmultipleinternalaminoacidsequencesthatformaspecificthree‐dimensionalarrangementofatomsontheprotein’ssurface.SignalpeptidesandSignalpatchesTwowaysthatasortingsignalcanbebuiltintoaprotein.(A)Thesignalresidesinasinglediscretestretchofaminoac

4、idsequence,calledasignalpeptide,thatisexposedinthefoldedprotein.Signalpeptidesoftenoccurattheendofthepolypeptidechain,buttheycanalsobelocatedelsewhere.(B)Asignalpatchcanbeformedbythejuxtapositionofaminoacidsfromregionsthatarephysicallyseparatedbeforeth

5、eproteinfolds;alternatively,separatepatchesonthesurfaceofthefoldedproteinthatarespacedafixeddistanceapartcouldformthesignal.C.SignalSequence¾SignalsequencesareaftenfoundattheN‐terminusandremovedbysignalpeptidasesfromthefinishedproteinsoncethesortingpro

6、cessiscomplete.SignalSequencescanalsobefoundelsewhere.¾Signalsequencesarebothnecessaryandsufficientforproteintargeting.¾Signalsequencesarerecognizedbysortingreceptorsthatguideproteinstotheirappropriatedestinations.SomeTypicalSignalSequences2.TransportB

7、etweentheNucleusandtheCytosol¾Passivetransport—passivelydiffuseò3000-4000NPCs/mammaliancell.Toimportabout106histone/3mins.(DNA-sythesizingcell)=100histone/min/NPCòEachNPCcontainsoneormoreopenaqueouschannels:9nmindiameterand15nmlong.òTheeffectivesizeoft

8、hesechannelshasbeendeterminedbyinjectingvarioussizesofcolloidalgoldparticlesandexaminedbyelectronmicroscopy.<10nmindiameter<60kDglobularprotein¾ActivetransportthroughNPCsGatedTransportBidirectionaltrafficbetweenthenucleusandthecytosol.T