Doron Lior, Goloubinoff Pierre, Shapira Michal
Department of Life Sciences, Ben-Gurion University of the Negev, Beer Sheva, Israel.
Department of Plant Molecular Biology, Faculty of Biology and Medicine, University of Lausanne, Lausanne, Switzerland.
Front Mol Biosci. 2018 Feb 15;5:2. doi: 10.3389/fmolb.2018.00002. eCollection 2018.
Photosynthesis is performed by large complexes, composed of subunits encoded by the nuclear and chloroplast genomes. Assembly is assisted by general and target-specific chaperones, but their mode of action is yet unclear. We formerly showed that ZnJ2 is an algal chaperone resembling BSD2 from land plants. In algae, it co-migrates with the transcript on chloroplast polysomes, suggesting it contributes to the synthesis of RbcL (Doron et al., 2014). ZnJ2 contains four CXXCXGXG motifs, comprising a canonical domain typical also of DnaJ-type I (DNAJA). It contributes to the binding of protein substrates to DnaK and promotes an independent oxidoreductase activity (Mattoo et al., 2014). To examine whether ZnJ2 has oxidoreductase activity, we used the RNaseA assay, which measures the oxidation-dependent reactivation of reduced-denatured RNaseA. Although ZnJ2 assisted the native refolding of reduced-denatured RNaseA, its activity was restricted to an oxidizing environment. Thus, ZnJ2 did not carry the exclusive responsibility for the formation of disulfide bridges, but contributed to the stabilization of its target polypeptides, until they reached their native state. A ZnJ2 cysteine deficient mutant maintained a similar holding chaperone activity as the wild-type and did not induce the formation of disulfide bonds. ZnJ2 is devoid of a J-domain. It thus does not belong to the J-domain co-chaperones that target protein substrates to DnaK. As expected, , its aggregation-prevention activity was not synergic to the ATP-fueled action of DnaK/DnaJ/GrpE in assisting the native refolding of denatured malate dehydrogenase, nor did it show an independent refolding activity. A phylogenetic analysis showed that ZnJ2 and BSD2 from land plants, are two different proteins belonging to a larger group containing a cysteine-rich domain, that also includes the DNAJAs. Members of this family are apparently involved in specific assembly of photosynthetic complexes in the chloroplast.
光合作用由大型复合体完成，这些复合体由核基因组和叶绿体基因组编码的亚基组成。组装过程由通用和靶向特异性伴侣蛋白协助，但它们的作用方式尚不清楚。我们之前表明，ZnJ2是一种藻类伴侣蛋白，类似于陆生植物的BSD2。在藻类中，它与叶绿体多聚核糖体上的转录本共同迁移，表明它有助于RbcL的合成（多龙等人，2014年）。ZnJ2包含四个CXXCXGXG基序，构成了典型的I型DnaJ（DNAJA）结构域。它有助于蛋白质底物与DnaK的结合，并促进独立的氧化还原酶活性（马图等人，2014年）。为了研究ZnJ2是否具有氧化还原酶活性，我们使用了RNaseA测定法，该方法测量还原变性RNaseA的氧化依赖性再活化。虽然ZnJ2协助还原变性RNaseA的天然重折叠，但其活性仅限于氧化环境。因此，ZnJ2并非二硫键形成的唯一负责者，但有助于其靶多肽的稳定，直至它们达到天然状态。一个ZnJ2半胱氨酸缺陷突变体保持了与野生型相似的保持伴侣活性，并且不诱导二硫键的形成。ZnJ2没有J结构域。因此，它不属于将蛋白质底物靶向DnaK的J结构域共伴侣蛋白。正如预期的那样，其防止聚集的活性与DnaK/DnaJ/GrpE以ATP为燃料协助变性苹果酸脱氢酶天然重折叠的作用没有协同作用，也没有显示出独立的重折叠活性。系统发育分析表明，来自陆生植物的ZnJ2和BSD2是两种不同的蛋白质，属于一个更大的包含富含半胱氨酸结构域的组，该组还包括DNAJA。这个家族的成员显然参与叶绿体中光合复合体的特定组装。
