How do outer membrane proteins (OMPs) make it from the interior of the cell, through the periplasm, and into the outer membrane? In Pseudomonas aeruginosa, Bam and Lol proteins are big helpers that get some, but not all, OMPs where they need to go. The authors of a study in mBio explain this spotty service may be due to individual differences in the membrane proteins. In short, some folks need the help more than others.
By depleting cells of certain components of either targeting pathway, Bam or Lol, Hoang et al. demonstrated which targeting pathway works for which OMP. Hits to the Bam pathway affected the ability of a membrane protein with a β-barrel structure to get to the outer membrane (including OprF, pictured at left - click on the image to view animation**). Knocking down the Lol pathway, on the other hand, affected the movement of all lipoproteins to the outer membrane. But neither pathway seemed necessary to get the protein XcpQ to the outer membrane.
The results indicate that intrinsic properties of OMPs determine which localization factors OMPs rely on and how much help they need, say the authors. Proteins with strong β-barrel structural features all seem to rely on some form of the OM translocation system, but non- β-barrel proteins may be targeted into the OM by an alternative machinery. The authors think the transit through the periplasm is critical: structural features of OMPs determine their tendency to misfold during periplasmic transit, and thereby influence the extent to which those proteins rely on the Bam machinery to help them through. β-barrel proteins could be more prone to misfolding during periplasmic transit and insertion into the outer membrane, and are thus more reliant on Bam machinery.
(**Thanks to R.E.W. Hancock Laboratory at the University of British Columbia and to Fiona Brinkmann, who created the animation.)