Figure 5.

Model for the co- and post-translational targeting of substrates to the Sec complex. Ssb1 would bind to the ribosome-nascent chain complex and then associate through Sec72 with the Sec complex (consisting of the Sec61 channel, the Sec62-Sec63 complex and the Sec71-Sec72 complex). Ssa1 would bind to completed polypeptide chains and mediate their post-translational interaction with the Sec complex. In both cases, translocation would occur through the Sec complex that is dedicated to post-translational translocation and does not require ribosome-Sec61 interaction. It should be noted that, in the case of co-translational targeting, translocation through the Sec complex could commence before the polypeptide is completed.