The Escherichia coli outer membrane protein OmpA acquires secondary structure prior to its integration into the membrane

Almost all proteins that reside in the outer membrane (OM) of Gram-negative bacteria contain a membrane-spanning segment that folds into a unique beta barrel structure and inserts into the membrane by an unknown mechanism. To obtain further insight into outer membrane protein (OMP) biogenesis, we revisited the surprising observation reported over 20 years ago that the Escherichia coli OmpA beta barrel can be assembled into a native structure in vivo when it is expressed as two noncovalently linked fragments. Here, we show that disulfide bonds between beta strand 4 in the N-terminal fragment and beta strand 5 in the C-terminal fragment can form in the periplasmic space and greatly increase the efficiency of assembly of "split" OmpA, but only if the cysteine residues are engineered in perfect register (i.e., they are aligned in the fully folded beta barrel). In contrast, we observed only weak disulfide bonding between beta strand 1 in the N-terminal fragment and beta strand 8 in the C-terminal fragment that would form a closed or circularly permutated beta barrel. Our results not only demonstrate that beta barrels begin to fold into a beta-sheet-like structure before they are integrated into the OM but also help to discriminate among the different models of OMP biogenesis that have been proposed.