RT Journal Article SR Electronic T1 Molecular basis for the maintenance of lipid asymmetry in the outer membrane of Escherichia coli JF bioRxiv FD Cold Spring Harbor Laboratory SP 192500 DO 10.1101/192500 A1 Yeow, Jiang A1 Tan, Kang Wei A1 Holdbrook, Daniel A. A1 Chong, Zhi-Soon A1 Marzinek, Jan K. A1 Bond, Peter J. A1 Chng, Shu-Sin YR 2018 UL http://biorxiv.org/content/early/2018/02/08/192500.abstract AB A distinctive feature of the Gram-negative bacterial cell envelope is the 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. In Escherichia coli, the OmpC-MlaA complex is believed to maintain lipid asymmetry by removing mislocalized PLs from the outer leaflet of the OM. How it performs this function is unknown. Here, we define the molecular architecture of the OmpC-MlaA complex to gain insights into its role in PL transport. We establish that MlaA sits entirely within the bilayer in complex with OmpC and provides a hydrophilic channel possibly for PL translocation across the OM. Furthermore, we show that flexibility in a hairpin loop adjacent to the channel modulates MlaA activity. Finally, we demonstrate that OmpC plays an active 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 antimicrobial drug development.