UNDERSTANDING THE MOLECULAR MECHANISM OF THE OMPC-MLA SYSTEM

Abstract

The outer membrane (OM) of Gram-negative bacteria is asymmetric, comprising lipopolysaccharides in the outer leaflet and phospholipids (PLs) in the inner leaflet. This confers permeability barrier function against external insults. To maintain OM lipid asymmetry, the OmpC-Mla system is believed to remove aberrantly localized PLs from the OM and transport them to the inner membrane (IM). Mechanistic details, including transport directionality and protein interactions to mediate PL transport across the periplasm is unknown. In this study, we mapped the interaction surfaces of MlaC and its interaction partners using site-specific in vivo photo-crosslinking. In addition, we developed a sensitive radioactivity-based point-to-point in vitro lipid transfer assay between the periplasmic chaperone MlaC and IM MlaFEDB complex. Our work uncovers a model where PLs reversibly partition between two lipid binding sites in MlaC and MlaFEDB, and ATP binding and/or hydrolysis shift this equilibrium to ultimately drive retrograde PL transport by the OmpC-Mla system.