FIGURE 5. Different stages of translocation.
From the following article:
Protein translocation across the eukaryotic endoplasmic reticulum and bacterial plasma membranes
Tom A. Rapoport
Nature 450, 663-669(29 November 2007)
doi:10.1038/nature06384
a, Translocation of a secretory protein. The red line indicates the hydrophobic region of a signal sequence. Depicted is the co-translational mode of translocation, but similar schemes can be envisioned for the other modes. For simplicity, only the translocating Sec61/SecY copy is shown. b, Translocation of membrane proteins. When a hydrophobic transmembrane sequence (in red) has emerged from the ribosome, it can bind reversibly in several conformations. If the hydrophobic sequence is long and the N terminus is not retained in the cytosol, it can flip across the membrane (upper panel). If the N terminus is retained in the cytosol and the polypeptide chain is further elongated by the translating ribosome (indicated by the loop between the ribosome and channel), the C terminus can translocate across the membrane (lower panel).