Structure of a TOC-TIC supercomplex spanning two chloroplast envelope membranes

The TOC and TIC complexes are essential translocons that facilitate the import of the nuclear genome-en-coded preproteins across the two envelope membranes of chloroplast, but their exact molecular identities and assembly remain unclear. Here, we report a cryoelectron microscopy structure of TOC-TIC supercom-plex from Chlamydomonas, containing a total of 14 identified components. The preprotein-conducting pore of TOC is a hybrid b-barrel co-assembled by Toc120 and Toc75, while the potential 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, and a large hydro-philic cleft on the IMS scaffold connects TOC and TIC, forming a pathway for preprotein translocation. Our 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.

Automated summary

This study reports the structure of the TOC-TIC supercomplex from Chlamydomonas, providing insights into its composition, assembly, and preprotein translocation mechanism. The findings lay a foundation for interpreting the evolutionary conservation and diversity of this fundamental translocon machinery.