New results in this issue (page 219) from Cannon et al. indicate that a translocating peptide passes through the center of a single SecY protein complex.
The idea is a paradigm shift for the translocation field. The general consensus has been that four copies of SecY (Sec61 in eukaryotes) oligomerize to form the translocon pore, since one ribosome associates with four copies of Sec61. But SecY crystallized as a monomer with only hydrophobic residues on the external sides of the transmembrane region. Since the channel is known to have a hydrophilic interior, something was amiss.
Cannon et al. now use cross-linking studies to show that the translocating peptide contacts residues in the center of a single SecY. Residues on its external face did not cross-link with the passing peptide, as would be expected if oligomers formed the translocon.
Combined with the X-ray structure of SecY, the new data suggest that the translocon resembles an hourglass. The passing peptide contacts mainly residues at the “waist,” which lies at the middle of the lipid bilayer. Fewer contact points mean less friction and thus a lower energy needed to move the chain along. Now researchers will want to determine why the ribosome gathers four SecY/Sec61 complexes. Maybe the oligomers recruit other important translocation proteins, but this remains unproven.