A distinctive feature of the Gram-negative bacterial cell envelope is the asymmetric outer membrane (OM), where lipopolysaccharides and phospholipids (PLs) reside in the outer and inner leaflets, respectively. This unique lipid asymmetry renders the OM impermeable to external insults, including antibiotics and bile salts. In , the complex comprising osmoporin OmpC and the OM lipoprotein MlaA is believed to maintain lipid asymmetry by removing mislocalized PLs from the outer leaflet of the OM. How this complex performs this function is unknown. Here, we defined the molecular architecture of the OmpC-MlaA complex to gain insights into its role in PL transport. Using photo-cross-linking and molecular dynamics simulations, we established that MlaA interacts extensively with OmpC and is located entirely within the lipid bilayer. In addition, MlaA forms a hydrophilic channel, likely enabling PL translocation across the OM. We further showed that flexibility in a hairpin loop adjacent to the channel is critical in modulating MlaA activity. Finally, we demonstrated that OmpC plays a functional role in maintaining OM lipid asymmetry together with MlaA. Our work offers glimpses into how the OmpC-MlaA complex transports PLs across the OM and has important implications for future antibacterial drug development.
The outer membrane (OM) is a formidable permeability barrier that protects Gram-negative bacteria from detergents and antibiotics. It possesses exquisite lipid asymmetry, requiring the placement and retention of lipopolysaccharides (LPS) in the outer leaflet, and phospholipids (PLs) in the inner leaflet. To establish OM lipid asymmetry, LPS are transported from the inner membrane (IM) directly to the outer leaflet of the OM. In contrast, mechanisms for PL trafficking across the cell envelope are much less understood. In this review, we summarize and discuss recent advances in our understanding of PL transport, making parallel comparisons to well-established pathways for OM lipoprotein (Lol) and LPS (Lpt). Insights into putative PL transport systems highlight possible connections back to the ‘Bayer bridges’, adhesion zones between the IM and the OM that had been observed more than 50 years ago, and proposed as passages for export of OM components, including LPS and PLs.
The outer membrane (OM) of Gram-negative bacteria is an asymmetric lipid bilayer with outer leaflet lipopolysaccharides (LPS) exposed to extracellular milieu and inner leaflet phospholipids (PLs) facing the periplasm. This unique lipid asymmetry is the key to its innate drug resistance, rendering the OM impermeable to external insults, including antibiotics and bile salts. To maintain this OM barrier, the OmpC-Mla system removes mislocalized PLs from the OM outer leaflet, and transports them back to the inner membrane (IM); in the first step, the OM OmpC-MlaA complex transfers PLs to the periplasmic chaperone MlaC. This process likely occurs via a hydrophilic channel in MlaA, yet mechanistic details have remained elusive. Here, we obtain a molecular view of the architecture of the MlaA-MlaC transient complex by mapping the interaction surfaces between MlaA and MlaC in Escherichia coli. We show that electrostatic interactions are important for MlaC recruitment, and that MlaC eventually binds MlaA at the periplasmic face in a manner that juxtaposes the MlaA channel and the MlaC lipid binding cavity. We further provide evidence for conformational changes in the MlaA channel that correlate with functional states of MlaA, as well as interactions with MlaC binding and OM porins. Our work offers novel insights into the molecular mechanism for lipid transfer by the OmpC-MlaA complex for overall retrograde transport of PLs to the IM to maintain OM lipid asymmetry.
20A distinctive feature of the Gram- (which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.
A distinctive feature of the Gram‐negative bacterial cell envelope is the presence of an asymmetric outer membrane (OM), where lipopolysaccharides (LPS) and phospholipids (PLs) reside in the outer and inner leaflets, respectively. This unique lipid asymmetry renders the OM impermeable to external insults, thus allowing survival of bacteria in harsh environments. In Escherichia coli, the OmpC‐Mla system is responsible for maintenance of OM lipid asymmetry. Osmoporin OmpC and the OM lipoprotein MlaA form a complex proposed to remove PLs from the outer leaflet of the OM. How this complex is organized in the OM to perform this function is not known. In this report, we define the molecular architecture of the OmpC‐MlaA complex to gain insights into its function in PL transport. We show that MlaA sits entirely within the OM lipid bilayer, and interacts with the OmpC trimer at its dimeric interfaces. Molecular dynamics simulations reveal a membrane‐spanning hydrophilic channel within MlaA, suggesting a path for PL translocation across the OM. We demonstrate that a hydrophobic hairpin loop adjacent to this putative channel is critical for modulating the activity of MlaA; restricting flexibility of this structure significantly perturbs the function of the complex. Finally, we establish that OmpC plays an active role in maintaining OM lipid asymmetry together with MlaA. Our work provides a glimpse into the molecular mechanism of how the OmpC‐MlaA complex may extract PLs from the outer leaflet of the OM, and highlights key features that could be exploited in the development of future antimicrobial drugs.Support or Funding InformationNational University of Singapore Graduate School for Integrative Sciences and Engineering scholarship (to J.Y.).Singapore Ministry of Education Academic Research Fund Tier 3 grant (MOE2012‐T3‐1‐008) grant to (P.J.B.).Singapore Ministry of Education Academic Research Fund Tier 1 and Tier 2 (MOE2013‐T2‐1‐148) grants (to S.‐S.C.).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.
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