Structure and lipid dynamics in the maintenance of lipid asymmetry inner membrane complex of A. baumannii

Daniel Mann,Syma Khalid,Samuel I Miller,Julien R C Bergeron,Frank Dimaio,Kamolrat Somboon,Svetomir B Tzokov,Junping Fan,Andrew Muenks,Daniel P Farrell

doi:10.1038/s42003-021-02318-4

Abstract

Multi-resistant bacteria are a major threat in modern medicine. The gram-negative coccobacillus Acinetobacter baumannii currently leads the WHO list of pathogens in critical need for new therapeutic development. The maintenance of lipid asymmetry (MLA) protein complex is one of the core machineries that transport lipids from/to the outer membrane in gram-negative bacteria. It also contributes to broad-range antibiotic resistance in several pathogens, most prominently in A. baumannii. Nonetheless, the molecular details of its role in lipid transport has remained largely elusive. Here, we report the cryo-EM maps of the core MLA complex, MlaBDEF, from the pathogen A. baumannii, in the apo-, ATP- and ADP-bound states, revealing multiple lipid binding sites in the cytosolic and periplasmic side of the complex. Molecular dynamics simulations suggest their potential trajectory across the membrane. Collectively with the recently-reported structures of the E. coli orthologue, this data also allows us to propose a molecular mechanism of lipid transport by the MLA system.

Highlights

Multi-resistant bacteria are a major threat in modern medicine
This lipid gradient, depicting the first and most important permeation barrier, is maintained by several machineries, including YebT, PqiB and the multicomponent maintenance of lipid asymmetry (MLA) system[3,4], which consists of MlaA present in the outer membrane, the shuttle MlaC in the periplasmic space and the MlaBDEF ABC
Molecular dynamics simulations of the outer membrane MlaA, and periplasmic MlaC proteins have previously been reported in the literature[6,7,11], but none of the MlaBDEF inner membrane complex

Summary

Introduction

Multi-resistant bacteria are a major threat in modern medicine. The gram-negative coccobacillus Acinetobacter baumannii currently leads the WHO list of pathogens in critical need for new therapeutic development. The two membranes in Gram-negative bacteria have distinct lipid compositions: the inner membrane consists of glycerophospholipids, with both leaflets having similar compositions, while the outer membrane is asymmetric, with an outer leaflet of lipopolysaccharides and an inner leaflet of glycerophospholipids[2] (Fig. 1A) This lipid gradient, depicting the first and most important permeation barrier, is maintained by several machineries, including YebT, PqiB and the multicomponent maintenance of lipid asymmetry (MLA) system[3,4], which consists of MlaA present in the outer membrane, the shuttle MlaC in the periplasmic space and the MlaBDEF ABC transporter system in the inner membrane (Fig. 1A). We performed molecular dynamics simulations to gain insights into the dynamics of lipids within their observed binding sites This provides important insights into the mechanism of lipid transport by the MLA system, and about the characterization of membrane proteins in detergent

Methods

Results

Conclusion