Proteins enter chloroplasts through a four-channeled translocon.

Unlike organelles with single membranes, chloroplasts must import proteins through both an outer and an inner membrane. Schleiff et al., reporting on page 541, have now isolated and characterized a protein complex from the chloroplast outer membrane, revealing a translocon with an unusual structure.

Previous work identified four proteins important for translocation across the outer chloroplast membrane, but the architecture of the translocon remained unknown. In the new work, the authors purified a core complex containing the translocon proteins Toc75, Toc34, and a fragment of Toc159, and found that this complex can recognize and translocate chloroplast precursor proteins. The absence of Toc64 in the functional complex suggests that this protein only transiently associates with the core translocon. Electron microscopy and image analysis reveals that the core complex has a toroidal shape, with a finger-like domain dividing a central cavity into four curved channels.

The complex is able to bind to four precursor proteins at a time, though the curved shape of the channels suggests that they might not be able to translocate simultaneously. The structure of the complex is consistent with an earlier model, in which Toc34 is the initial receptor for precursor proteins while Toc159 drives translocation through the pore. ▪