Toc75

Toc75, short for Translocon at the Outer Chloroplast membrane 75, is a β-barrel protein that forms the channel component of the TOC complex responsible for importing nucleus-encoded proteins into chloroplasts. It is located in the outer envelope of chloroplasts and is encoded in the nuclear genome; the mature protein is targeted to the chloroplast outer membrane and integrated as part of the translocon.

Function: Toc75 acts as the main pore that allows unfolded preproteins bearing transit peptides to cross the outer membrane. It works in cooperation with receptor components such as Toc159 and Toc34 and with the TIC complex at the inner envelope to energize and complete transit into the organelle. The TOC–TIC import pathway is essential for chloroplast biogenesis, affecting photosynthetic and housekeeping protein import. Loss of Toc75 function typically halts plastid development and can cause seedling lethality or pale phenotypes.

Structure and diversity: Toc75 is an integral outer-membrane protein with a beta-barrel transmembrane domain forming the channel. The protein often contains additional cytosolic-facing regions that may mediate interactions with receptors or auxiliary factors. Several Toc75-like paralogs exist in plants, some with tissue- or development-specific expression, and related proteins such as OEP80 are studied as potential related channels.

Genetics and evolution: Toc75 genes are conserved across photosynthetic eukaryotes, reflecting an ancient origin of the chloroplast protein import machinery stemming from cyanobacterial endosymbiosis. Research continues to elucidate the precise assembly, regulation, and interactions within the TOC complex.