Rial Daniela V, Ottado Jorgelina, Ceccarelli Eduardo A
Molecular Biology Division, Rosario Institute of Molecular & Cellular Biology, Faculty of Biochemical and Pharmaceutical Sciences, National University of Rosario, Suipacha 531, S2002LRK Rosario, Argentina.
J Biol Chem. 2003 Nov 21;278(47):46473-81. doi: 10.1074/jbc.M306684200. Epub 2003 Sep 10.
Protein import into chloroplasts is postulated to occur with the involvement of molecular chaperones. We have determined that the transit peptide of ferredoxin-NADP(H) reductase precursor binds preferentially to an Hsp70 from chloroplast stroma. To investigate the role of Hsp70 molecular chaperones in chloroplast protein import, we analyzed the import into pea chloroplasts of preproteins with decreased Hsp70 binding affinity in their transit peptides. Our results indicate that the precursor with the lowest affinity for Hsp70 molecular chaperones in its transit peptide was imported to chloroplasts with similar apparent Km as the wild type precursor and a 2-fold increase in Vmax. Thus, a strong interaction between chloroplast stromal Hsp70 and the transit peptide seems not to be essential for protein import. These results indicate that in chloroplasts the main unfolding force during protein import may be applied by molecular chaperones other than Hsp70s. Although stromal Hsp70s undoubtedly participate in chloroplast biogenesis, the role of these molecular chaperones in chloroplast protein translocation differs from the one proposed in the mechanisms postulated up to date.
据推测，蛋白质进入叶绿体的过程需要分子伴侣的参与。我们已经确定，铁氧还蛋白-NADP（H）还原酶前体的转运肽优先与叶绿体基质中的一种Hsp70结合。为了研究Hsp70分子伴侣在叶绿体蛋白质导入中的作用，我们分析了转运肽中与Hsp70结合亲和力降低的前体蛋白导入豌豆叶绿体的情况。我们的结果表明，其转运肽对Hsp70分子伴侣亲和力最低的前体蛋白被导入叶绿体时，其表观Km与野生型前体相似，Vmax增加了2倍。因此，叶绿体基质Hsp70与转运肽之间的强相互作用似乎对蛋白质导入并非必不可少。这些结果表明，在叶绿体中，蛋白质导入过程中的主要解折叠力可能由Hsp70以外的分子伴侣施加。尽管基质Hsp70无疑参与叶绿体的生物发生，但这些分子伴侣在叶绿体蛋白质转运中的作用与迄今为止所假设的机制中提出的作用不同。
