Virtually all integral outer membrane proteins (OMPs) produced by Gram-negative bacteria contain a unique ‘β barrel’ structure that serves as a membrane spanning domain. The universal barrel assembly machine (BAM) catalyzes OMP assembly (folding and membrane insertion) in vivo, and purified Escherichia coli BAM that is reconstituted into proteoliposomes catalyzes OMP assembly in vitro. Here we show that BAM also catalyzes the assembly of OMPs into outer membrane fractions (‘native OMs’) that are purified by optimized conventional methods. Interestingly, we found that OMP assembly was moderately impaired when native OMs were isolated from a mlaA- strain that is deficient in maintaining OM lipid homeostasis but was strongly reduced when native OMs were isolated from a pldA- strain that is deficient in a parallel pathway. We also found that the mlaA and pldA deletions altered the OM phospholipid profile to different degrees that correlated with the degree to which the mutations impaired OMP assembly. Taken together, our results provide direct evidence that the mla and pldA pathways play distinct roles in maintaining OM homeostasis and strongly suggest that OM phospholipids play a more significant role in OMP biogenesis than previously appreciated.
