Bacterial Outer Membrane Proteins Are Targeted to the Bam Complex by Two Parallel Mechanisms

Membrane proteins that are integrated into the outer membrane of Gram-negative bacteria typically contain a unique "beta barrel" structure that serves as a membrane spanning segment. A conserved "beta signal" motif is located at the C ter- minus of the p barrel of many outer membrane proteins (OMPs), but the function of this sequence is unclear. We found that mutations in the beta signal slightly delayed the assembly of three model Escherichia coli OMPs by reducing their affinity for the barrel assembly machinery (Bam) complex, a heterooligomer that catalyzes beta barrel insertion, and led to the degradation of a fraction of the protein in the periplasm. Interestingly, the absence of the periplasmic chaperone SurA amplified the effect of the mutations and caused the complete degradation of the mutant proteins. In contrast, the absence of another periplasmic chaperone (Skp) suppressed the effect of the mutations and considerably enhanced the efficiency of assembly. Our results reveal the existence of two parallel OMP targeting mechanisms that rely on a cis-acting peptide (the beta signal) and a trans-acting factor (SurA), respectively. Our results also challenge the long-standing view that periplasmic chaperones are redundant and provide evidence that they have specialized functions. IMPORTANCE Proteins that are embedded in the outer membrane of Gram-negative bacteria (OMPs) play an important role in protecting the cell from harmful chemicals. OMPs share a common architecture and often contain a conserved sequence motif (beta motif) of unknown function. Although OMPs are escorted to the outer membrane by proteins called chaperones, the exact function of the chaperones is also unclear. Here, we show that the beta motif and the chaperone SurA both target OMPs to the beta barrel insertion machinery in the outer membrane. In contrast, the chaperone Skp delivers unintegrated OMPs to protein degradation complexes. Our results challenge the long-standing view that chaperones are functionally redundant and strongly suggest that they have specialized roles in OMP targeting and quality control.