In photosynthetic eukaryotes, thousands of proteins are translated in the cytosol and imported into the chloroplast through the concerted action of two translocons—termed TOC and TIC—located in the outer and inner membranes of the chloroplast envelope, respectively. The degree to which the molecular composition of the TOC and TIC complexes is conserved over phylogenetic distances has remained controversial. Here, we combine transcriptomic, biochemical, and genetic tools in the green alga Chlamydomonas (Chlamydomonas reinhardtii) to demonstrate that, despite a lack of evident sequence conservation for some of its components, the algal TIC complex mirrors the molecular composition of a TIC complex from Arabidopsis thaliana. The Chlamydomonas TIC complex contains three nuclear-encoded subunits, Tic20, Tic56, and Tic100, and one chloroplast-encoded subunit, Tic214, and interacts with the TOC complex, as well as with several uncharacterized proteins to form a stable supercomplex (TIC-TOC), indicating that protein import across both envelope membranes is mechanistically coupled. Expression of the nuclear and chloroplast genes encoding both known and uncharacterized TIC-TOC components is highly coordinated, suggesting that a mechanism for regulating its biogenesis across compartmental boundaries must exist. Conditional repression of Tic214, the only chloroplast-encoded subunit in the TIC-TOC complex, impairs the import of chloroplast proteins with essential roles in chloroplast ribosome biogenesis and protein folding and induces a pleiotropic stress response, including several proteins involved in the chloroplast unfolded protein response. These findings underscore the functional importance of the TIC-TOC supercomplex in maintaining chloroplast proteostasis.

在光合真核生物中，数千种蛋白质在细胞质中翻译，并通过分别位于叶绿体包膜外膜和内膜的两个易位子（称为 TOC 和 TIC）的协同作用输入叶绿体。TOC 和 TIC 复合物的分子组成在系统发育距离上的保守程度仍然存在争议。在这里，我们结合了绿藻衣藻（Chlamydomonas Reinhardtii）的转录组学、生化和遗传工具来证明，尽管其某些成分缺乏明显的序列保守性，但藻类 TIC 复合体反映了来自莱茵衣藻的 TIC 复合体的分子组成。拟南芥。衣藻 TIC 复合物包含三个核编码亚基 Tic20、Tic56 和 Tic100，以及一个叶绿体编码亚基 Tic214，并与 TOC 复合物以及几种未表征的蛋白质相互作用，形成稳定的超复合物 (TIC-TOC) ），表明跨两个包膜的蛋白质输入是机械耦合的。编码已知和未表征的 TIC-TOC 成分的核和叶绿体基因的表达是高度协调的，这表明必须存在一种跨区室边界调节其生物发生的机制。Tic214（TIC-TOC复合物中唯一的叶绿体编码亚基）的条件抑制会损害在叶绿体核糖体生物发生和蛋白质折叠中起重要作用的叶绿体蛋白的输入，并诱导多效性应激反应，包括参与叶绿体未折叠蛋白的几种蛋白回复。这些发现强调了 TIC-TOC 超级复合物在维持叶绿体蛋白质稳态中的功能重要性。