Fig. 8. Putative model for sorting of β-barrel precursors through the lateral gate of Sam50.

β-Barrel precursors are translocated from the intermembrane space (IMS) side into the lumen of Sam50. The C-terminal β-signal of the precursor specifically binds to β-strand 1 of Sam50 by replacing the endogenous β-signal of Sam50 (β-strand 16). This induces an opening of the lateral gate of Sam50 between β-strands 1 and 16. Further strands of the precursor are inserted into the lateral gate in β-hairpin-like structures. Loop 6 of Sam50 promotes transfer of the precursor into the lateral gate. The full-length precursor is released from the lateral gate into the lipid phase of the outer membrane (OM). Thinning of the membrane in proximity to the lateral gate facilitates membrane insertion of the β-barrel protein.