Toc159 stuffs the preprotein through the channel in proteoliposomes.

Schleiff/NAS

In the first successful reconstitution of a chloroplast transport system, only three proteins are required for GTP-dependent translocation of preproteins into proteoliposomes, according to Enrico Schleiff, Marko Jelic, and Jürgen Soll (Maximilian Universität München, Munich, Germany). Based on this in vitro system, the authors propose that the preprotein first binds to Toc34 in a GTP-dependent manner and is then passed off to Toc159, which pushes the polypeptide through the Toc75 channel itself, also in a GTP-dependent process.

“We were rather surprised that such a minimal system could actually translocate a protein,” says Schleiff. It was previously thought that molecular chaperones associated with the Tic complex generated the force to drive the protein through the channel.

Combining the new results with structural information, which shows the ratio of Toc34, Toc75, and Toc159 in an assembled channel is 4:4:1, Schleiff and colleagues hypothesize that Toc159 acts as a dynamic component of the system. They envision that it drives the preprotein through the narrow channel with repeated up and down strokes, in a motion that resembles a sewing machine needle. The movement required for such a stroke is not very large, says Schleiff; with a channel height of 10 nm, it would only require the movement of ∼60 amino acids, assuming a helical region. “The receptor Toc159 has 1,500 amino acids, so a movement of 60 amino acids is easy to achieve without disturbing the entire structure.” ▪References:

Schleiff, E, et al. 2003. Proc. Natl. Acad. Sci. USA. 10.1073/pnas.0730860100.

Schleiff, E., et al.
2003
.
J. Cell Biol.
160
:
541
–551.