前體蛋白與葉綠體外表面結(jié)合

前體蛋白與葉綠體外表面結(jié)合第1頁/共16頁chloroplastsSixsubcompartments:AninnerenvelopemembraneAnouterenvelopemembraneIntermembranespaceStromaThylakiodmembraneThylakiodlumen第2頁/共16頁proteintransportmechanismsThetwostypes:⑴Importofnuclear-encodedproteinsintothechloroplaststromathroughenvelopemembrane⑵proteintransportintothylakiodlumenacrossthethylakoidmembrane第3頁/共16頁第4頁/共16頁ImportofproteinsintothechloroplaststromaProcess:Ⅰ.前體蛋白與葉綠體外表面結(jié)合:前體蛋白N端的轉(zhuǎn)運肽(transitpeptides)和葉綠體外表面蛋白轉(zhuǎn)運復(fù)合體上受體的相互作用;需少量ATP.Ⅱ.轉(zhuǎn)運的早期階段:在GTP或低濃度的ATP存在下,前體蛋白與蛋白轉(zhuǎn)運復(fù)合體(translocationcomplexes)緊密結(jié)合.Ⅲ.轉(zhuǎn)運末期:前體蛋白穿膜轉(zhuǎn)運進入葉綠體基質(zhì),在分子伴侶(cytosolicchaperone)的協(xié)助下加工成為成熟的蛋白.第5頁/共16頁transitpeptides(轉(zhuǎn)運肽)Generalfeatures:ahighcontentinhydroxylatedresidues;analmostcompletelackofacidicresidues;anoverallhydrophilicnature.

anumberoftransitpeptidesphosphorylationinthecytosolcouldsupporttheirassociationwithguidancecomplexes.第6頁/共16頁transitpeptides(轉(zhuǎn)運肽)Structure:transitpeptidecontainsthreeparts:⑴N端含非極性氨基酸殘基——參與前體蛋白與受體的結(jié)合。⑵中間段富含羥基和帶正電荷的氨基酸殘基——參與前體蛋白的精確定位。⑶C端形成雙極性的β折疊——與轉(zhuǎn)運后轉(zhuǎn)運肽準確切割有關(guān)。第7頁/共16頁transitpeptides(轉(zhuǎn)運肽)Thegeneralfounction:

⑴targettheproteintochloroplast

⑵itprovidesan“address”thatlocatizesthepolypeptidetooneofthesubcompartments.☆A(yù)fterreachingtheorganellestroma,theyareremovedbyastromalprocessingpeptidase(SPP)第8頁/共16頁cytosolicchaperone14-3-3proteinsandHsp70在蛋白轉(zhuǎn)運前,對前體蛋白進行解折疊,使其從雜的空間結(jié)構(gòu)變成線型結(jié)構(gòu),以便于進行穿膜輸;進入葉綠體的蛋白質(zhì),又在監(jiān)護蛋白的作用下重新恢復(fù)其空間構(gòu)象并實施其功能.第9頁/共16頁translocationcomplexes◆theyareoligomericproteincomplexes.◆theyaredividedintoToccomplexandTiccomplex.◆Itisinitiatedbyenergy-independentbindingoftheprecursorproteintosurfaceexposedreceptorsfollowedbyGTP-andATPdependentpartialtranslocationacrosstheouterenvelopemembrane.第10頁/共16頁translocationcomplexesToccomplexs:

theToccomplexwhichwasfirstisolatedfrompeachloroplastsconsistsofthreeproteinsthataredesignatedaccordingtotheirmolecularweightasToc34,Toc75,andtoc159.第11頁/共16頁ImportofproteinsintothechloroplaststromaToc34andToc159arereceptorproteinsthatareresponsiblefortheinitialbindingoftheprecursorproteins,bindingandhydrolysisofGTP,showinghighestaffinityforprecursorsintheGTP-form.Toc159isembeddedwithintheouterenvelopewithaCOOH-terminalmembraneanchordomain(M-domain),whileboththeNH2-terminalacidicdomain(A-domain)andthecentralGTPbindingdomain(G-domain)areexposedtothecytosol.第12頁/共16頁ImportofproteinsintothechloroplaststromaToc34islikewiseembeddedwithinthemembranewithaCOOH-terminalanchor.Remarkably,itshydrophiliccytosolicdomainshowssignificanthomologytotheG-domainofToc159extendingbeyondtheG-motifsconservedinallGTP-bindingproteins.Toc34istheinitialreceptorthattransferstheprecursorproteintoToc159byaheterodimerizationstep.Toc159wasfurthermoreshowntooperateasGTP-drivenmotorinthetranslocationprocess第13頁/共16頁ImportofproteinsintothechloroplaststromaToc75isapolytopicmembraneproteinwithpresumably16amphiphilicb-sheetswhichformacation-selectiveionchannelwithinthemembrane.Toc75interactswithbothToc34andToc159andpossessesalowaffinityprecursorbindingsitewhichstronglysuggeststhatitprovidesthemajorproteintra