Fig. 6.

Implications of uOMP expansion in the periplasm. uOMPs are posttranslationally secreted through the Sec translocon (magenta), N- to C-terminally. (Step A) The emerging uOMP N terminus in the periplasm may be recognized by SurA. (Step B) After complete translocation into the periplasm, one or more SurA protomers bind specific segments on uOMP clients, solubilizing the uOMP in expanded conformations nearly the width of the periplasm. The expanded uOMP may also be able to form heterocomplexes with other chaperones in the OMP biogenesis pathway: Skp (Step C) and FkpA (Step D). The size of uOMPs bound to SurA is approximately double the size of uOMPs bound to Skp. Unfolded OmpA₁₇₁ bound to Skp is shown as a gray surface representation in the Skp trimer located proximal to the translocon. (Step E) The extended, unbound C-terminal region of the SurA-bound uOMP is positioned to encounter the BAM complex, which recognizes the OMP β-signal and catalyzes uOMP folding into the outer membrane.