The Department of Biomedicine

BBB seminar: Tom Rapoport

Mechanisms of protein translocation across membranes

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Tom Rapoport
Department of Cell Biology, Harvard Medical School, Boston, MA, USA

Many proteins in bacteria are transported during their biosynthesis across or are integrated into the plasma membrane, a process that is similar to protein translocation across endoplasmic reticulum (ER) membrane in eukaryotes. Transport occurs through a protein-conducting channel that is formed from a conserved heterotrimeric membrane protein complex (SecY or Sec61 complex). The channel associates with different partners in different translocation pathways. In bacteria, the SecY channel can associate with the translating ribosome (co-translational translocation) or with the cytoplasmic ATPase SecA (post-translational translocation).

The crystal structure of an archaeal SecY complex shows the architecture of the closed channel. The structure suggests that a single SecY copy forms the translocation pore. This prediction was confirmed for SecA-mediated translocation by disulfide bridge crosslinking experiments, which show that both the signal sequence and the mature region of a translocating polypeptide chain are contained in the same SecY copy. However, translocation appears to be mediated by oligomers of the SecY complex; nucleotide-binding fold 1 (NBF1) of SecA interacts with a non-translocating SecY copy, while other domains of SecA “push” the polypeptide chain through a neighboring SecY copy.

We have recently determined the crystal structure of SecA bound to the SecY complex at 4.5Å resolution. The structure shows one copy of SecA in its transitions state of ATP hydrolysis bound to one copy of SecY complex. The latter corresponds to the translocating SecY copy. Both SecA and SecY undergo major conformational changes upon interaction. The structure suggests mechanisms for how SecA moves polypeptides through the SecY channel. In addition, the data reveal that SecA binding opens the lateral gate of SecY for signal sequence intercalation, and induces plug movement. These results indicate that the channel is opened for translocation in two distinct steps, induced by SecA and signal sequence binding.

Host : Jaakko Saraste <jaakko.saraste@biomed.uib.no>, Department of Biomedicine