Fig. 4: Proposed models of the outer membrane translocation process of the GspDα secretin (a) and the GspDβ secretin (b).
From: Membrane translocation process revealed by in situ structures of type II secretion system secretins
a Firstly, GspDα forms multimers on the inner membrane, and adopts an unstable and swinging conformation, changing between conformation A (light blue) and conformation B (dark blue). The multimer could dissociate from the inner membrane and enter the periplasm (light blue cartoon model), but it could not pass through the peptidoglycan and insert into the outer membrane. When the peptidoglycan pore size is enlarged by D-methionine, the multimer translocates to the outer membrane, where it exists in a more stable conformation (green). b GspDβ first forms multimers on the inner membrane (yellow); then the pilotin GspS (pink) approaches and binds the GspDβ monomer, transporting it to the outer membrane. On the outer membrane, GspDβ assembles into a multimer again and GspS stays associated, forming a GspDβ–GspS multimer complex (light red). IM inner membrane, PG peptidoglycan, OM outer membrane, D-Met D-methionine.
