Uncovering the protein translocon at the chloroplast inner envelope membrane.

Chloroplasts require protein translocons at the outer and inner envelope membranes, termed TOC and TIC, respectively, to import thousands of cytoplasmically synthesized preproteins. However, the molecular identity of the TIC translocon remains controversial. Tic20 forms a 1-megadalton complex at the inner membrane and directly interacts with translocating preproteins.

In vivo studies on the roles of two closely related Arabidopsis Tic20 proteins, AtTic20-I and AtTic20-IV.

Protein translocation across the inner envelope of plastids is mediated by the TIC (translocon at the inner envelope membrane of chloroplasts) protein translocation machinery. Tic20 has been shown to function as a central component of TIC machinery. The Arabidopsis genome encodes four Tic20 homologous proteins, AtTic20-I, AtTic20-II, AtTIC20-IV and AtTic20-V, among which only AtTic20-I has been extensively characterized and demonstrated to be essential for protein import into chloroplasts.

A 1-megadalton translocation complex containing Tic20 and Tic21 mediates chloroplast protein import at the inner envelope membrane.

Chloroplast protein import is mediated by two hetero-oligomeric protein complexes, the Tic and Toc translocons, which are located in the inner and outer envelope membranes. At the inner membrane, many Tic components have been identified and characterized, but it remains unclear how these Tic proteins are organized to form a protein-conducting channel or whether a stable Tic core complex that binds translocating preproteins exists. Here, we report the identification of a 1-megadalton (MD) translocation complex as an intermediate during protein translocation across the inner membrane in Arabidopsis thaliana and pea (Pisum sativum).