LplT‐Aas system: The primary mechanism for lysophospholipid remodeling in E. coli

Abstract

Lysophospholipids (LPLs) are metabolic intermediates in phospholipid turnover and are derived from multiple endogenous and exogenous sources in bacterial membrane envelope. Distinct from their diacyl counterparts, these detergent‐like lipid molecules alter the local membrane properties and their accumulation may lead to severe membrane disruption and eventually cell lysis. LplT and Aas form a novel lysophospholipid remodeling system found exclusively in Gram‐negative bacteria. LplT is a member of Major Facilitator Superfamily and facilitates energy‐independent lipid flipping across the bacterial inner membrane. Aas catalyzes acyl transfer to LPL from acyl‐ACP on the cytoplasmic surface. The biological function and molecular mechanism of this lipid remodeling system remain largely unknown. We found that LplT‐Aas effectively facilitate remodeling of all three major bacterial phospholipids, PE, PG and cardiolipin, in E.coli. LplT imports lyso‐PE, lyso‐PG into the cells, which are subsequently acylated by the action of Aas to form their respective diacyl forms. We also identified a novel cardiolipin hydrolysis reaction by venom phospholipase A2 to form diacyl cardiolipin progressing to the completely deacylated headgroup. These two distinct cardiolipin derivatives are both translocated by LplT and then remodeled by Aas to form a triacyl form of cardiolipin. These results reveal the first cardiolipin remodeling mechanism in bacteria. LplT cannot transport lyso‐PC or lyso‐PA, suggesting its primary role in phospholipid repairing mechanism in the bacterial membrane envelope. To understand this novel LPL transport mechanism, we found that the substrate fatty acid chain is not required for LplT transport and its substrate binding was not inhibited by either orthophosphate or glycerol 3‐phosphate, suggesting that either a glycerol or ethanolamine headgroup is the chemical determinant for substrate recognition. Membrane phospholipids could not serve as a competitive inhibitor in vitro. This strict selectivity for lyso lipids allows LplT‐Aas to fulfill their efficient lipid repairing function in a membrane environment. Based on a structural model, we propose a “sideway sliding” mechanism to explain how a conserved membrane‐embedded α‐helical interface excludes membrane from the LplT binding site to facilitate efficient flipping of LPL across the cell membrane. Very recently, we found that LplT‐Aas play a potent role in maintaining membrane stability and integrity in the bacterial envelope and may be involved in the bacterial resistance against secretory phospholipase A2‐mediated host immune attack. These data demonstrate the first evidence for the biological function of this novel LPL remodeling system in E.coli.Support or Funding InformationNIH grant R01GM098572