Tat substrates open the portal

Aldridge et al. suggest how chloroplast proteins could induce assembly of the pore complex that transports them across the thylakoid membrane.

The twin-arginine translocase (Tat) complex transports folded proteins across the thylakoid membranes of chloroplasts and the plasma membrane of bacteria. The chloroplast Tat complex consists of the multipass transmembrane scaffold cpTatC and the single-pass proteins Hcf106 and Tha4, the latter of which is thought to oligomerize and form the translocase pore. To avoid excessive leakage across the thylakoid membrane, however, the complex only forms a functional pore in the presence of a substrate protein bearing a twin-arginine–containing signal peptide. Aldridge et al. examined how the Tat components interact with each other and the substrate protein to initiate pore assembly.

The signal peptide twin-arginine motif binds to a region of cpTatC exposed to the chloroplast stroma, but Aldridge et al. found that a hydrophobic portion of the peptide contacts a different site on cpTatC, located near the thylakoid lumen. The signal peptide isn’t long enough to contact both sites on a single cpTatC molecule but could, the researchers suggest, insert between two cpTatC subunits and contact one site on each.

The cpTatC proteins would usually cup together like two halves of a coconut shell but could open up after the signal peptide’s insertion, changing cpTatC’s interactions with the other Tat components. Indeed, in the presence of a substrate protein, the pore-forming subunit Tha4 contacted regions on the concave face of cpTatC that it couldn’t contact in the absence of a signal peptide. This could facilitate Tha4’s oligomerization and pore formation, a proposition that now needs to be studied in more detail.