Angiosperms have three PsbQ-like (PQL) proteins in addition to the PsbQ subunit of the oxygen-evolving complex of photosystem (PS) II. Previous studies have shown that two PQL proteins, PnsL2 and PnsL3, are subunits of the chloroplast NADH dehydrogenase-like (NDH) complex involved in the PSI cyclic electron flow. In addition, another PsbQ homolog, PQL3, is also required for the NDH activity; however, molecular function of PQL3 has not been elucidated. Herein, we show that PQL3 is an assembly factor, particularly for the accumulation of subcomplex B (SubB) of the chloroplast NDH. In the pql3 mutant of Arabidopsis thaliana, the amounts of NDH subunits in SubB, PnsB1 and PsnB4, were decreased, causing severe reduction in the NDH-Photosystem I supercomplex. Analysis using blue-native polyacrylamide gel electrophoresis suggested that the incorporation of PnsL3 into SubB was affected in the pql3 mutant. Unlike other PsbQ homologs, PQL3 was weakly associated with thylakoid membranes and was only partially protected from thermolysin digestion. Consistent with the function as an assembly factor, PQL3 accumulated independently in other NDH mutants, such as pnsl1-3. Furthermore, PQL3 accumulated in young leaves in a manner similar to the accumulation of CRR3, an assembly factor for SubB. These results suggest that PQL3 has developed a distinct function as an assembly factor for the NDH complex during evolution of the PsbQ protein family in angiosperms.

中文翻译：

---

PsbQ样蛋白3充当拟南芥中叶绿体NADH脱氢酶样复合物的装配因子。

除了光系统（PS）II的析氧复合物的PsbQ亚基外，被子植物还具有三种PsbQ类（PQL）蛋白。先前的研究表明，两个PQL蛋白PnsL2和PnsL3是参与PSI循环电子流的叶绿体NADH脱氢酶样（NDH）复合物的亚基。此外，NDH活性还需要另一个PsbQ同源物PQL3。然而，PQL3的分子功能尚未阐明。在这里，我们显示PQL3是一个组装因子，特别是对于叶绿体NDH的亚复合物B（SubB）的积累。在拟南芥的pql3突变体中，SubB，PnsB1和PsnB4中NDH亚基的数量减少，导致NDH-Photosystem I超复合物的严重减少。使用蓝色原生聚丙烯酰胺凝胶电泳进行的分析表明，在pql3突变体中，PnsL3掺入SubB受到影响。与其他PsbQ同源物不同，PQL3与类囊体膜弱相关，并且仅部分保护其免受嗜热菌素消化。与作为装配因子的功能一致，PQL3在其他NDH突变体（例如pnsl1-3）中独立积累。此外，PQL3在幼叶中的积累方式与CRR3（SubB的组装因子）的积累相似。这些结果表明，PQL3在被子植物中PsbQ蛋白家族的进化过程中已作为NDH复合物的装配因子发挥了独特的功能。PQL3与类囊体膜弱相关，并且仅被部分保护免受嗜热菌素消化。与作为装配因子的功能一致，PQL3在其他NDH突变体（例如pnsl1-3）中独立积累。此外，PQL3在幼叶中的积累方式与CRR3（SubB的组装因子）的积累相似。这些结果表明，PQL3在被子植物中PsbQ蛋白家族的进化过程中已作为NDH复合物的装配因子发挥了独特的功能。PQL3与类囊体膜弱相关，并且仅被部分保护免受嗜热菌素消化。与作为装配因子的功能一致，PQL3在其他NDH突变体（例如pnsl1-3）中独立积累。此外，PQL3在幼叶中的积累方式与CRR3（SubB的组装因子）的积累相似。这些结果表明，PQL3在被子植物中PsbQ蛋白家族的进化过程中已作为NDH复合物的装配因子发挥了独特的功能。