Ostersetzer O, Adam Z
Department of Agricultural Botany, Hebrew University of Jerusalem, Rehovot, Israel.
Plant Cell. 1997 Jun;9(6):957-65. doi: 10.1105/tpc.9.6.957.
Unassembled subunits of the cytochrome b6f complex as well as components of other unassembled chloroplastic complexes are rapidly degraded within the organelle. However, the mechanisms involved in these proteolytic processes are obscure. When the Rieske FeS protein (RISP) is imported into isolated chloroplasts in vitro, some of the protein does not property assemble with the cytochrome complex, as determined by its sensitivity to exogenous protease. When assayed in intact, lysed, or fractionated chloroplasts, the imported RISP was found to be sensitive to endogenous proteases as well. The activity responsible for degradation of the unassembled protein was localized to the thylakoid membrane and characterized as a metalloprotease requiring zinc ions for its activity. The degradation rate was stimulated by light, but no involvement of ATP or redox control was observed. Instead, when the RISP that was attached to thylakoid membranes was first illuminated on ice, degradation proceeded in either light or darkness at equal rates suggesting a light-induced conformational change making the protein prone to degradation. Antibodies raised against native FtsH, a bacterial, membrane-bound, ATP-dependent, zinc-stimulated protease, effectively inhibited degradation of the unassembled RISP, suggesting a role for the chloroplastic FtsH in this process.
细胞色素b6f复合体的未组装亚基以及其他未组装的叶绿体复合体的组分在细胞器内会迅速降解。然而，这些蛋白水解过程所涉及的机制尚不清楚。当 Rieske FeS 蛋白（RISP）在体外导入分离的叶绿体时，根据其对外源蛋白酶的敏感性判断，一些蛋白不能与细胞色素复合体正常组装。在完整、裂解或分级分离的叶绿体中进行检测时，发现导入的 RISP 对内源蛋白酶也敏感。负责降解未组装蛋白的活性定位于类囊体膜，其特征为一种需要锌离子激活的金属蛋白酶。降解速率受光照刺激，但未观察到ATP或氧化还原调控的参与。相反，当附着在类囊体膜上的RISP先在冰上光照处理后，无论在光照还是黑暗条件下，降解都以相同速率进行，这表明光照诱导的构象变化使该蛋白易于降解。针对天然FtsH（一种细菌的、膜结合的、ATP依赖的、锌激活的蛋白酶）产生的抗体能有效抑制未组装RISP的降解，表明叶绿体FtsH在这一过程中发挥作用。
