前体蛋白与叶绿体外表面结合.ppt

structureandfunctionofproteintransportmachineriesinchloroplasts,左冰峰杨洋06/11/7,chloroplasts,Thechloroplastsisanognanelleofprokaryoticoriginthatissituatedinaneukaryoticcellularenvironment.,chloroplasts,Sixsubcompartments:AninnerenvelopemembraneAnouterenvelopemembraneIntermembranespaceStromaThylakiodmembraneThylakiodlumen,proteintransportmechanisms,Thetwostypes:Importofnuclear-encodedproteinsintothechloroplaststromathroughenvelopemembraneproteintransportintothylakiodlumenacrossthethylakoidmembrane,Importofproteinsintothechloroplaststroma,Process:.前体蛋白与叶绿体外表面结合:前体蛋白N端的转运肽(transitpeptides)和叶绿体外表面蛋白转运复合体上受体的相互作用;需少量ATP.转运的早期阶段:在GTP或低浓度的ATP存在下,前体蛋白与蛋白转运复合体(translocationcomplexes)紧密结合.转运末期:前体蛋白穿膜转运进入叶绿体基质,在分子伴侣(cytosolicchaperone)的协助下加工成为成熟的蛋白.,transitpeptides(转运肽),Generalfeatures:ahighcontentinhydroxylatedresidues;analmostcompletelackofacidicresidues;anoverallhydrophilicnature.anumberoftransitpeptidesphosphorylationinthecytosolcouldsupporttheirassociationwithguidancecomplexes.,transitpeptides(转运肽),Structure:transitpeptidecontainsthreeparts:N端含非极性氨基酸残基参与前体蛋白与受体的结合。中间段富含羟基和带正电荷的氨基酸残基参与前体蛋白的精确定位。C端形成双极性的折叠与转运后转运肽准确切割有关。,transitpeptides(转运肽),Thegeneralfounction:targettheproteintochloroplastitprovidesan“address”thatlocatizesthepolypeptidetooneofthesubcompartments.Afterreachingtheorganellestroma,theyareremovedbyastromalprocessingpeptidase(SPP),cytosolicchaperone,14-3-3proteinsandHsp70在蛋白转运前,对前体蛋白进行解折叠,使其从杂的空间结构变成线型结构,以便于进行穿膜输;进入叶绿体的蛋白质,又在监护蛋白的作用下重新恢复其空间构象并实施其功能.,translocationcomplexes,theyareoligomericproteincomplexes.theyaredividedintoToccomplexandTiccomplex.Itisinitiatedbyenergy-independentbindingoftheprecursorproteintosurfaceexposedreceptorsfollowedbyGTP-andATPdependentpartialtranslocationacrosstheouterenvelopemembrane.,translocationcomplexes,Toccomplexs:theToccomplexwhichwasfirstisolatedfrompeachloroplastsconsistsofthreeproteinsthataredesignatedaccordingtotheirmolecularweightasToc34,Toc75,andtoc159.,Importofproteinsintothechloroplaststroma,Toc34andToc159arereceptorproteinsthatareresponsiblefortheinitialbindingoftheprecursorproteins,bindingandhydrolysisofGTP,showinghighestaffinityforprecursorsintheGTP-form.Toc159isembeddedwithintheouterenvelopewithaCOOH-terminalmembraneanchordomain(M-domain),whileboththeNH2-terminalacidicdomain(A-domain)andthecentralGTPbindingdomain(G-domain)areexposedtothecytosol.,Importofproteinsintothechloroplaststroma,Toc34islikewiseembeddedwithinthemembranewithaCOOH-terminalanchor.Remarkably,itshydrophiliccytosolicdomainshowssignificanthomologytotheG-domainofToc159extendingbeyondtheG-motifsconservedinallGTP-bindingproteins.Toc34istheinitialreceptorthattransferstheprecursorproteintoToc159byaheterodimerizationstep.Toc159wasfurthermoreshowntooperateasGTP-drivenmotorinthetranslocationprocess,Importofproteinsintothechloroplaststroma,Toc75isapolytopicmembraneproteinwithpresumably16amphiphilicb-sheetswhichformacation-selectiveionchannelwithinthemembrane.Toc75interactswithbothToc34andToc159andpossessesalowaffinityprecursorbindingsitewhichstronglys