Wang Xiaomin, Zhong Jiahui, Li Bilang, Yang Shuo, Wang Peiyi, Shao Jingxia, Meng Jingjing, Liu Xiayan, Yu Fei, Qi Yafei
State Key Laboratory for Crop Stress Resistance and High-Efficiency Production and College of Life Sciences, Northwest A&F University, Yangling, Shaanxi, 712100, People's Republic of China.
Plant J. 2025 Jul;123(2):e70369. doi: 10.1111/tpj.70369.
The 90-kDa heat shock protein (HSP90) is a central component of the chaperone system for protein homeostasis (proteostasis). In Arabidopsis, AtHSP90.5 is the sole chloroplast-localized HSP90 family member, yet its role in chloroplast proteostasis remains poorly characterized. Here, we identify and characterize the pale green arabidopsis 5 mutant, pga5-1, which exhibits defective chloroplast development and impaired accumulation of photosynthetic protein complexes. Genetic analysis revealed that pga5-1 is a hypomorphic allele of AtHSP90.5, harboring a missense mutation (G646E) localized closely to the substrate-binding site. Biochemical studies demonstrated that AtHSP90.5 interacts with AtFtsH12, and the ATPase activity of AtHSP90.5 is essential for the oligomerization of AtFtsH12 complexes. Strikingly, the mutation of the conserved residue (E106A) for the ATPase activity of AtHSP90.5 can rescue the embryonic lethality of AtHSP90.5 null mutants, yielding albino seedlings with non-photosynthetic plastids, and partially complement pga5-1. Furthermore, we show that AtHSP90.5 associates with subunits of light-harvesting antenna complexes, including LhcB1, LhcB2, and LhcA1, and is epistatic to PGA4/cpSRP54 in regulating the accumulation of a chimeric chloroplast marker protein, LhcB2-GFP. Collectively, our findings establish a critical role for AtHSP90.5 in maintaining photosynthesis protein complexes and uncover a previously unknown functional link between AtHSP90.5 and AtFtsH12 in chloroplast protein translocation.
90千道尔顿热休克蛋白（HSP90）是蛋白质稳态（蛋白平衡）伴侣系统的核心组成部分。在拟南芥中，AtHSP90.5是叶绿体定位的HSP90家族唯一成员，但其在叶绿体蛋白平衡中的作用仍不清楚。在这里，我们鉴定并表征了浅绿色拟南芥5突变体pga5-1，它表现出叶绿体发育缺陷和光合蛋白复合体积累受损。遗传分析表明，pga5-1是AtHSP90.5的一个低表达等位基因，带有一个错义突变（G646E），该突变位于底物结合位点附近。生化研究表明，AtHSP90.5与AtFtsH12相互作用，AtHSP90.5的ATP酶活性对于AtFtsH12复合体的寡聚化至关重要。令人惊讶的是，AtHSP90.5的ATP酶活性保守残基（E106A）突变可以挽救AtHSP90.5缺失突变体的胚胎致死性，产生具有非光合质体的白化苗，并部分互补pga5-1。此外，我们表明AtHSP90.5与捕光天线复合体的亚基相关联，包括LhcB1、LhcB2和LhcA1，并且在调节嵌合叶绿体标记蛋白LhcB2-GFP的积累方面对PGA4/cpSRP54呈上位性。总的来说，我们的发现确立了AtHSP90.5在维持光合作用蛋白复合体中的关键作用，并揭示了AtHSP90.5与AtFtsH12在叶绿体蛋白转运中以前未知的功能联系。
