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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,structure and function of,protein transport machineries in chloroplasts,左冰峰 杨洋,06/11/7,chloroplasts,The,chloroplasts is an ognanelle of prokaryotic,origin that is situated in an eukaryotic cellular environment,.,chloroplasts,Six subcompartments:,An inner envelope membrane,An outer envelope membrane,Intermembrane space,Stroma,Thylakiod membrane,Thylakiod lumen,protein transport mechanisms,The two stypes:,Import of nuclear-encoded proteins into the chloroplast stroma through envelope membrane,protein transport into thylakiod lumen,across the thylakoid membrane,Import of proteins into the chloroplast stroma,Process:,.前体蛋白与叶绿体外表面结合:前体蛋白N端的,转运肽,(,transit peptides,)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,(,translocation complexes,)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,(,cytosolic chaperone,),的协助下加工成为成熟的蛋白.,transit peptides,(转运肽),General features,:a high content in,hydroxylated residues;an almost complete,lack of acidic residues;an overall hydrophilic,nature.,a number of transit peptides,phosphorylation,in the cytosol could support their association,with guidance complexes.,transit peptides,(转运肽),Structure:,transit peptide contains three parts:,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。,transit peptides,(转运肽),The general founction,:,target the protein to chloroplast,it provides an“address”that locatizes the polypeptide to one of the subcompartments.,After reaching the organelle stroma,they are removed by a,stromal processing peptidase,(SPP),cytosolic chaperone,14-3-3 proteins and Hsp70,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.,translocation complexes,they are oligomeric protein complexes.,they are divided into,Toc complex,and,Tic complex,.,It is initiated by energy-independent binding of the precursor protein to surface exposed receptors followed by GTP-andATP dependent partialtranslocation across the outer envelope membrane.,translocation complexes,Toc complexs:,the Toc complex which was first isolated,from pea chloroplasts consists of three,proteins that are designated according to,their molecular weight as Toc34,Toc75,and toc159,.,Import of proteins into the chloroplast stroma,Toc34 and Toc159 are,receptor proteins,that,are responsible for the initial binding of the,precursor proteins,binding and,hydrolysis of,GTP,showing,highest affinity,for precursors in,the GTP-form.,Toc159,is embedded within the outer,envelope with a COOH-terminal membrane,anchor domain(,M-domain,),while both the,NH2-terminal acidic domain(,A-domain,)and,the centralGTP binding domain(,G-domain,),are exposed to the cytosol.,Import of proteins into the chloroplast stroma,Toc34,is likewise embedded within the membrane,with a COOH-terminal anchor.Remarkably,its,hydrophilic cytosolic domain shows significant,homology to the G-domain of Toc159 extending,beyond the G-motifs conserved in all GTP-binding,proteins.,Toc34 is the initial receptor that transfers the,precursor protein to Toc159 by a,heterodimerization,step,.Toc159 was furthermore shown to,operate as,GTP-driven motor,in the translocation process,Import of proteins into the chloroplast stroma,Toc75,is a,polytopic membrane protein,with presumably 16 amphiphilic b-sheets which form a cation-selective ion channel within the membrane.,Toc75 interacts with both Toc34 and Toc159 and possesses a,lowaffinity,precursor binding site which strongly suggests that it provides the major protein,translocation channel,in the outer envelope membrane.,Import of proteins into the chloroplast stroma,Toc64,Toc12,Toc132,Toc90,Toc33,Toc120,
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