08 Yeast Transport

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1、8YeastTransportTransportprocessesinalivingcellcanbedividedintoseveralaspects:(i)importandexportofmacromolecules,preferablyproteinsorpeptides,mediatedbyintracellularsortingandtrafficking;(ii)importandexportofsmallmolecules,wherebynutrients(sugars,aminoacids,phosphate,vitamins)havetobeimp

2、orted,while(iii)essentialionsinmostcaseswillbesuppliedtothecellbyexchange;(iv)toxiccompoundshavetobeexportedfromthecelltopreventdeleteriousreactions.8.1InternalTransportandImport/ExportofProteinsVariousmembrane-delimitedcompartmentsintheyeastcytoplasmplaykeyrolesinthetrafficofproteinsbo

3、thintoandoutofthecell.Further,transportofproteinsbetweenmembranecompartmentswhichinvolvesvesiclesisaprocesscommontoalleukaryotesandyeasthasservedasanimportantmodelforourunderstandingofsuchprocesses(Figure8-1).Figure8-1:Schemeofyeastintracellularproteintrafficking.Figure8-2:Principleofme

4、mbranefusion.KeycomponentsinvolvedinvesiclefusionwithtargetmembranesaretheSNAREs(solubleNSFattachmentproteinreceptors)(Figure8-2).ComplementarySNAREsarepresentonthedonorandacceptormembranespriortofusionandaredistinguishedbyashortdomain,the‘SNAREmotif’,withconservedsequencefeaturesandapr

5、opensitytoformcoiled-coils.AprerequisiteforefficientbilayerfusionisthatfourSNAREmotifshavetoformabundleinaparallelα-helicalcoil,therebybringingthemembranesintoclosecontact.ThistransSNAREcomplexisreferredtoasaSNAREpin,wherebytheSNAREsaredistinguishedastargetmembranet-SNAREsorvesiclemembr

6、anev-SNAREs.Contactswithinthecoiled-coiloftheSNAREpinarebroughtaboutbyhydrophobicinteractions,withtheexceptionofan‘ioniclayer’inthemiddleofthebundle.Aseachhelixtypicallydisplayseitheraglutamine(Q)oranarginine(R)residueafurtherclassificationofSNAREshasbeendevised:threetypesofQ-SNAREsinte

7、ractwithoneR-SNARE(Table8-1).SNAREshavebeenidentifiedontheendoplasmicreticulum(ER),Golgimembrane,vacuole/lysosomeandplasmamembrane(PM),andonthevesiclesthatcanbegeneratedfromeachofthesemembranes(Figure8-3).MostSNAREs(20outof24inyeast)aretail-anchoredproteins(Table8-1).FourSNAREs