Institute for Protein Research, Osaka University, Suita 565-0871, Japan.
Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan.
Proc Natl Acad Sci U S A. 2022 Aug 23;119(34):e2208277119. doi: 10.1073/pnas.2208277119. Epub 2022 Aug 15.
Chloroplast protein import is mediated by translocons named TOC and TIC on the outer and inner envelope membranes, respectively. Translocon constituents are conserved among green lineages, including plants and green algae. However, it remains unclear whether Rhodophyta (red algae) share common chloroplast protein import mechanisms with the green lineages. We show that in the rhodophyte , plastome-encoded Tic20 localized to the chloroplast envelope and was transiently associated with preproteins during import, suggesting its conserved function as a TIC constituent. Besides plastome-encoded FtsH and several chaperones, a class of GTP (guanosine 5'-triphosphate)-binding proteins distinct from the Toc34/159 GTPase family associated transiently with preproteins. This class of proteins resides mainly in the cytosol and shows sequence similarities with Sey1/RHD3, required for endoplasmic reticulum membrane fusion, and with the periplastid-localized import factor PPP1, previously identified in the Apicomplexa and diatoms. These GTP-binding proteins, named plastid targeting factor for protein import 1 (PTF1) to PTF3, may act as plastid targeting factors in Rhodophyta.
叶绿体蛋白的输入是由分别位于外膜和内膜上的称为 TOC 和 TIC 的易位子介导的。易位子的组成在绿色谱系中是保守的，包括植物和绿藻。然而，红藻（红藻）是否与绿色谱系共享共同的叶绿体蛋白输入机制仍不清楚。我们表明，在红藻中，质体编码的 Tic20 定位于叶绿体被膜，并在输入过程中与前体蛋白短暂相关，表明其作为 TIC 成分的保守功能。除了质体编码的 FtsH 和几种伴侣蛋白外，一类与Toc34/159 GTPase 家族不同的 GTP（鸟苷 5'-三磷酸）结合蛋白也与前体蛋白短暂相关。这一类蛋白质主要位于细胞质中，与内质网膜融合所必需的 Sey1/RHD3 以及先前在肉足虫和硅藻中发现的质体定位导入因子 PPP1 具有序列相似性。这些 GTP 结合蛋白，命名为蛋白导入的质体靶向因子 1（PTF1）至 PTF3，可能在红藻中作为质体靶向因子发挥作用。
