Dual recognition of multiple signals in bacterial outer membrane proteins enhances assembly and maintains membrane integrity

Abstract

Outer membrane proteins (OMPs) are essential components of the outer membrane of Gram-negative bacteria. In terms of protein targeting and assembly, the current dogma holds that a “β-signal” imprinted in the final β-strand of the OMP engages the β-barrel assembly machinery (BAM complex) to initiate membrane insertion and assembly of the OMP into the outer membrane. Here, we reveal an additional rule, that signals equivalent to the β-signal are repeated in other, internal β-strands within bacterial OMPs. The internal signal is needed to promote the efficiency of the assembly reaction of these OMPs. BamD, an essential subunit of the BAM complex, recognizes the internal signal and the β-signal, arranging several β-strands for rapid OMP assembly. The internal signal-BamD ordering system is not essential for bacterial viability but is necessary to retain the integrity of the outer membrane against antibiotics and other environmental insults. Bacterial outer membrane proteins are recognized and bound by BamD at specific signals located in multiple β-strands at the C-terminus of these proteins.