The Bam (Omp85) complex is involved in secretion of the autotransporter haemoglobin protease

Autotransporters are large virulence factors secreted by Gram-negative bacteria. They are synthesized with a C-terminal domain that forms a beta-barrel pore in the outer membrane implicated in translocation of the upstream 'passenger' domain across the outer membrane. However, recent structural data suggest that the diameter of the beta-barrel pore is not sufficient to allow the passage of partly folded structures observed for several autotransporters. Here, we have used a stalled translocation intermediate of the autotransporter Hbp to identify components involved in insertion and translocation of the protein across the outer membrane. At this intermediate stage the beta-domain was not inserted and folded as an integral beta-barrel in the outer membrane whereas part of the passenger was surface exposed. The intermediate was copurified with the periplasmic chaperone SurA and subunits of the Barn (Omp85) complex that catalyse the insertion and assembly of outer-membrane proteins. The data suggest a critical role for this general machinery in the translocation of autotransporters across the outer membrane.