Departments of Molecular Biology and Plant Biology, University of Geneva, Switzerland.
FEBS J. 2022 Nov;289(22):6908-6918. doi: 10.1111/febs.16464. Epub 2022 May 9.
Most chloroplast proteins are nucleus-encoded, translated on cytoplasmic ribosomes as precursor proteins, and imported into chloroplasts through TOC and TIC, the translocons of the outer and inner chloroplast envelope membranes. While the composition of the TOC complex is well established, there is still some controversy about the importance of a recently identified TIC complex consisting of Tic20, Tic214, Tic100, and Tic56. TOC and TIC form a supercomplex with a protein channel at the junction of the outer and inner envelope membranes through which preproteins are pulled into the stroma by the ATP-powered Ycf2 complex consisting of several FtsH-like ATPases and/or by chloroplast Hsp proteins. Several components of the TOC/TIC system are moonlighting proteins with additional roles in chloroplast gene expression and metabolism. Chaperones and co-chaperones, associated with TOC and TIC on the cytoplasmic and stromal side of the chloroplast envelope, participate in the unfolding and folding of the precursor proteins and act together with the ubiquitin-proteasome system in protein quality control. Chloroplast protein import is also intimately linked with retrograde signaling, revealing altogether an unsuspected complexity in the regulation of this process.
大多数质体蛋白是由核编码的，在细胞质核糖体上作为前体蛋白翻译，然后通过TOC 和 TIC（叶绿体外膜和内膜的转位体）导入叶绿体。虽然 TOC 复合物的组成已经得到很好的确立，但对于最近发现的由 Tic20、Tic214、Tic100 和 Tic56 组成的 TIC 复合物的重要性仍存在一些争议。TOC 和 TIC 通过一个位于外膜和内膜交界处的蛋白通道形成一个超级复合物，前体蛋白通过由几个 FtsH 样 ATP 酶和/或叶绿体 HSP 蛋白组成的、由 ATP 驱动的 Ycf2 复合物被拉入基质中。TOC/TIC 系统的几个组件是具有叶绿体基因表达和代谢等额外功能的兼职蛋白。伴侣蛋白和共伴侣蛋白与质体膜的细胞质和基质侧的 TOC 和 TIC 相关，参与前体蛋白的展开和折叠，并与泛素-蛋白酶体系统一起参与蛋白质质量控制。叶绿体蛋白的输入与逆行信号密切相关，这揭示了这个过程的调节具有出人意料的复杂性。
