Absence of YhdP,TamB, and YdbH leads to defects in glycerophospholipid transport and cell morphology in Gram-negative bacteria

The outer membrane (OM) of Gram-negative bacteria provides the cell with a formidable barrier that excludes external threats. The two major constituents of this asymmetric barrier are lipopolysaccharide (LPS) found in the outer leaflet, and glycerophospholipids (GPLs) in the inner leaflet. Maintaining the asymmetric nature and balance of LPS to GPLs in the OM is critical for bacterial viability. The biosynthetic pathways of LPS and GPLs are well characterized, but unlike LPS transport, how GPLs are translocated to the OM remains enigmatic. Understanding this aspect of cell envelope biology could provide a foundation for new antibacterial therapies. Here, we report that YhdP and its homologues, TamB and YdbH, members of the “AsmA-like” family, are critical for OM integrity and necessary for proper GPL transport to the OM. The absence of the two largest AsmA-like proteins (YhdP and TamB) leads to cell lysis and antibiotic sensitivity, phenotypes that are rescued by reducing LPS synthesis. We also find that yhdP, tamB double mutants shed excess LPS through outer membrane vesicles, presumably to maintain OM homeostasis when normal anterograde GPL transport is disrupted. Moreover, a yhdP, tamB, ydbH triple mutant is synthetically lethal, but if GPL transport is partially restored by overexpression of YhdP, the cell shape adjusts to accommodate increased membrane content as the cell accumulates GPLs in the IM. Our results therefore suggest a model in which “AsmA-like” proteins transport GPLs to the OM, and when hindered, changes in cell shape and shedding of excess LPS aids in maintaining OM asymmetry.