Theoretical insights into rotary mechanism of MotAB in the bacterial flagellar motor

Abstract

Many bacteria enable locomotion by rotating their flagellum. It has been suggested that this rotation is realized by the rotary motion of the stator unit, MotAB, which is driven by proton transfer across the membrane. Recent cryo-electron microscopy studies have revealed a 5:2 MotAB configuration, in which a MotB dimer is encircled by a ring-shaped MotA pentamer. While the structure implicates the rotary motion of the MotA wheel around the MotB axle, the molecular mechanisms of rotary motion and how they are coupled with proton transfer across the membrane remain elusive. In this study, we built a structure-based computational model for Campylobacter jejuni MotAB, conducted comprehensive protonation state-dependent molecular dynamics simulations, and revealed a plausible proton-transfer coupled rotation pathway. The model assumes rotation-dependent proton transfer, in which proton uptake from the periplasmic side to the conserved aspartic acid in MotB is followed by proton hopping to the MotA proton-carrying site, followed by proton export to the cytoplasm. We suggest that, by maintaining two of the proton-carrying sites of MotA in the deprotonated state, the MotA pentamer robustly rotates by [~]36{degrees} per proton transfer across the membrane. Our results provide a structure-based mechanistic model of the rotary motion of MotAB in bacterial flagellar motors and provide insights into various ion-driven rotary molecular motors. Significance StatementThis study aims to elucidate the mechanism by which bacteria move by rotating their flagella. The driving force for flagellar rotation is predicted to be driven by protons passing through the transmembrane protein MotAB, but the actual rotation mechanism has not yet been elucidated. Using advanced computational modeling and molecular dynamics simulations, we have elucidated the detailed processes by which proton translocation achieves the rotation of the bacterial flagellar motor. This work not only sheds light on the fundamental mechanisms of bacterial motility but also provides a framework for understanding similar ion-driven rotation mechanisms in other biological systems, potentially paving the way for new bioinspired technologies.