b -domain. In this report, we provide genetic, biochemical and structural evidence demonstrating that a region within the BrkA passenger (Glu 601 -Ala 692 ) is necessary for folding the passenger. This region is not required for surface display in the outer membrane protease OmpT-deficient Escherichia coli strain UT5600. However, a BrkA mutant protein bearing a deletion in this region is susceptible to digestion when expressed in E. coli strains expressing OmpT suggesting that the region is required to maintain a stable structure. The instability of the deletion mutant can be rescued by surface expressing Glu 601 -Ala 692 in trans suggesting that this region is acting as an intramolecular chaperone to effect folding of the passenger concurrent with or following translocation across the outer membrane.