细胞生物学试卷：Chapter 06 Protein Sorting

Chapter06ProteinSortingSignalingmolecule:Generaltermforanyextracellularorintracellularmoleculeinvolvedinmediatingtheresponseofacelltoitsexternalenvironmentortoothercells.Signal-recognitionparticle(SRP):AcytosolicribonucleoproteinparticlethatbindstotheERsignalsequenceinanascentsecretoryproteinanddeliversthenascentchain/ribosomecomplextotheERmembranewheresynthesisoftheproteinandtranslocationintotheERarecompleted．Signalsequence:Arelativelyshortaminoacidsequencewithinaproteinthatdirectstheproteintoaspecificlocationwithinthecell；alsocalledsignalpeptideanduptake-targetingsequence．Signaltransduction:Conversionofasignalfromonephysicalorchemicalformintoanother．Incellbiologycommonlyreferstothesequentialprocessinitiatedbybindingofanextracellularsignaltoareceptorandculminatinginoneormorespecificcellularresponses.Aproteinthatisnormallysecretedisgeneticallyengineeredbyalteringthegenethatencodesit.ThepartofthegeneencodingtheKDELsequenceisremovedfromtheC-terminusandtwostop-transfersequencesareinsertedinthemiddleofitsgene.Whenthealteredgeneisreinsertedintothecells,whathappens?Theproteinisnolongerfoundinthecisternalspace.Instead,itisfoundinthemembranesincethestop-transfersequencespreventitfrompassingcompletelythroughthechannelintothecisternae.ItwillalsotravelviathebiosyntheticpathwaytothecellmembraneifitlackstheKKXXsequencethatrestrictssuchproteinstotheERmembranes.17.Aproteinisisolatedandthesequenceofitsoligosaccharidechainsdetermined.ThechainsarefoundtobeO-linkedoligosaccharides.Wherewerethesugarsaddedtotheproteinchain?O-linkedoligosaccharidesareassembledtotallywithintheGolgiapparatus,thereforethesugarsinthisproteinwereaddedtotheproteinintheGolgiapparatus.18.AmoleculecalledaequorinfluoresceswhenitbindsCa2+ions.Ifyouareobservinganaequorin-injectedcellthatnormallysecretescertainproteins,whatshouldyouobserveinthatcelljustpriortoitssecretionoftheseproteins?Youshouldseethecellfluoresce,sincethereleaseofCa2+ionstriggerssecretion.Someoftheseionswillbindtoaequorincausingittofluoresce.19.AnorganellewhoseinteriorexhibitsalowpHisidentified.Itisirregularlyshapedandcontainsacidphosphatase.Whatisitmostlikelytobe?Alysosome.20.PlantcellsareexposedtoaninhibitorofH+-ATPaseinthetonoplastmembrane.Whatwilltheimmediateeffectbe?SincetheenzymepumpsH+ionsintothevacuole,disablingitwillbelikelytoraisethepHinsidethevacuole.Whichofthefollowingsequencesismostlikelytobepartofasignalpeptideofasecretoryprotein?RefertothediagramofaminoacidRgroupsinyourtextforhelp. a.-lys-arg-glu-gly-phe-gly-lys-glu- b.-gly-asp-met-glu-asp-gly-lys-arg- c.-arg-leu-asp-lys-phe-glu-arg-pro- d.-leu-gly-met-leu-phe-met-leu-phe- e.-arg-arg-lys-asp-glu-met-gly-asp-10.Radioactivephospholipidprecursorsareexposedtogrowingcellsinabriefpulse.Afterachasewithnonlabeledphospholipidprecursors,itisclearthatthelabelappearsfirstovertheendoplasmicreticulum.Sometimelater,thelabelappearsinthecellmembraneafterbeingseenintheGolgiapparatusandcytoplasmicvesicles.Towhatconclusiondothedataleadyou?Membranelipidsappeartobesynthesizedintheendoplasmicreticulum,afterwhichtheymovetotheGolgiapparatusandthentovesicles,whichcontributethephospholipidstothemembraneuponexocytosis.11.AsasecretoryproteinistransportedthroughtheRERmembrane,ablockofsugarsistransferredfromalipidmoleculeembeddedinthemembranetoanaminoacidontheprotein.TheblockofsugarsisidentifiedasanN-linkedoligosaccharide.Whichaminoacidonthesecretoryproteinismostlikelytoserveastheacceptorfortheoligosaccharide?Anasparagineresidueoftheproteinismostlikelytoserveasthepointofattachment.12.Thecomponentsofnucleotidesugarsthatdonatesugarstogrowingoligosaccharidechainshavespecificrelationshipstoeachother.Aparticularsugarisdonatedtothegrowingchainbyonlyonenucleotide.Whatiswrongwiththefollowinglistofnucleotidesugarsbelow:CMP-sialicacid,GDP-mannose,GDP-sialicacid,GDP-fucose,UDP-galactose,UDP-N-acetylglucosamine?Therearetwonucleotidesugarsinthelistthatdonatesialicacid.Thereshouldandcanbeonlyone.13.Acellisgivenradioactiveprecursorsoftheoutersugarsonthecoreoligosaccharidechainthatistransferredtoasecretoryproteinasitenterstheendoplasmicreticulum.Inpulsechaseexperiments,exposedsilvergrainsappearovertheendoplasmicreticulumearlyon,butlatertherearenoexposedsilvergrainsseenovertheendoplasmicreticulumortheGolgiapparatuswheretheproteinsareknowntohavegonenext.Whatdoesthisexperimentsuggest?TheterminalsugarsareaddedtotheproteinuponenteringtheER,buttheyareremovedbeforetheproteinreachestheGolgiapparatus,probablyintheER.14.YouhaveisolatedfromacellvesiclesthatareshowntobederivedfromtheGolgiapparatus.Theytakeupalargeamountofosmiumtetroxidewhenstainedandcontainlittlenucleotidediphosphataseactivity.Thesevesiclesalsocontainafewproteinstypicaloftheendoplasmicreticulum.IsitpossibletodeterminewhereintheGolgibodythesevesiclesoriginated?Explainyouranswer.SincetheGolgiapparatusisnotuniformincomposition,suchadeterminationshouldbepossible.CisGolgimembranestypicallytakeuposmiumtetroxidebetterthanotherpartsoftheGolgi,butshouldcontainlittleornonucleotidediphosphatase,sincethisenzymeisnormallyfoundatthetransface.ThepresenceofafewERproteinsalsosuggeststhatthevesiclesareprobablyfromthecisGolgisincesomeERproteinscanbetakentherefromtheER.Theywill,however,berecycledbacktotheERbeforetheygettothetransface.15.Ascientistisstudyingthemovementofvesiclesbetweendifferentcellularcompartments.Shestartsbytreatingthecellswithanumberofknowninhibitorsofcellulara