Molecular Dynamics of the MtrE Efflux Gate from N. Gonorrhoeae

Abstract

Neisseria gonorrhoeae is a pathogenic Gram-negative bacterium, causing gonorrhea in humans. Highly resistant N. gonorrhoeae strains, unresponsive to all existing antibiotics have recently been reported. There is thus an urgent need to find novel ways of overcoming resistance in this organism [1]. Active extrusion of antibiotics mediated by tripartite multidrug efflux pumps is amongst the most effective defense mechanisms in Gram-negative bacteria. In N. gonorrhoeae, the MtrCDE efflux complex spans both bacterial membranes, forming a continuous path from the cytoplasm to the external medium. Loss of this pump has been shown to restore antibiotic effectiveness in highly resistant strains [2]. We focus on passage through the outer membrane protein, MtrE, which has recently been structurally resolved in an open state [3]. Two internal aspartate rings can bind large cations, which block the gate, and have been proposed to form the selectivity gate of MtrE [4]. Our atomistic molecular dynamics simulations of MtrE show fast partial closure at the periplasmic entrance at standard pH. Protonation of the internal ring cause substantial fluctuations in the gate area, leading to states more widely open than the crystal structure. Protonation of glutamate residues facing the putative MtrC binding region [4] prevent closure. Altogether, our data suggest that protonation of charged residues, located both at the internal and the external surface of the periplasmic tip, has a crucial role on the regulation of MtrE gating, and possibly on MtrC recruitment. We also present the conductance and ion selectivity of the open and partially closed states predicted by CompEl simulations.[1] Ohnishi et al., Antimicrobial Agents and Chemotherapy, 55:3538-45 (2011).[2] Golparian et al., Antimicrobial Agents and Chemotherapy, 58:3556-59 (2014).[3] Lei et al., PLOS ONE, e97475 (2014).[4] Janganan et al., Molecular Microbiology, 88:590-602 (2013).