Prof. Zhen Yan’s research team at Westlake published an article online on Cell (https://doi.org/10.1016/j.cell.2022.10.030) entitled "Structure of a TOC-TIC supercomplex spanning two chloroplast envelope membranes".

Chloroplast is a fundamental organelle essential for plant photosynthesis and photoautotrophic growth. It was suggested that chloroplasts originated from a monophyletic endosymbiotic event, during which a cyanobacterium was engulfed by a eukaryotic cell more than a billion years ago. During evolution, most of the endosymbiont genes were transferred to the host nuclear genome and only about 100 genes were retained in the chloroplast. An estimated 2000-3000 proteins need to be transported into the chloroplast upon synthesis in the cytosol as precursor proteins (preproteins), which contain N-terminal transit peptides that direct them to the chloroplast.

The translocon at the outer chloroplast membrane (TOC) and translocon at the inner chloroplast membrane (TIC) complexes are the preprotein translocon machineries that mediate preprotein import across the outer envelope membrane (OEM) and inner envelope membrane (IEM) of the chloroplast. TOC and TIC thus play indispensable roles in chloroplast biogenesis and homeostasis. Defects of TOC and TIC components lead to severe phenotypes such as embryonic lethality or albinism in plants.

Biochemical, genetic, and physiological data in pea and Arabidopsis have enabled the identification of many components of TOC and TIC in the past three decades, but their exact molecular identities and assembly remain unclear. The lack of a high-resolution structure of the intact TOC-TIC supercomplex hinders the molecular understanding of this fundamental protein translocation machinery.

In this article published recently, they reported a cryo-electron microscopy structure of TOC-TIC supercomplex from Chlamydomonas, containing a total of 14 identified components. The preprotein conducting pore of TOC is a hybrid β-barrel co-assembled by Toc120 and Toc75, while the translocation path of TIC is formed by transmembrane helices from Tic20 and YlmG, rather than a classic model of Tic110. A rigid intermembrane space (IMS) scaffold bridges two chloroplast membranes, one on which a large hydrophilic cleft connects TOC and TIC, together forming a pathway for preprotein translocation. Their study provides structural insights into the TOC-TIC supercomplex composition, assembly, and preprotein translocation mechanism, and lays a foundation to interpret the evolutionary conservation and diversity of this fundamental translocon machinery. This study, and future studies on the TOC-TIC supercomplex from other plant organisms, will shed light on the potential implications of chloroplast biology for food security and climate action.