Stator Dynamics Depending on Sodium Concentration in Sodium-Driven Bacterial Flagellar Motors

Abstract

Bacterial flagellar motor (BFM) is a large membrane-spanning molecular rotary machine for swimming motility. Torque is generated by the interaction between the rotor and multiple stator units powered by ion-motive force (IMF). The number of bound stator units is dynamically changed in response to the external load and the IMF. However, the detailed dynamics of stator unit exchange process remains unclear. Here, we directly measured the speed changes of sodium-driven chimeric BFMs under fast perfusion of different sodium concentration conditions using computer-controlled, high-throughput microfluidic devices. We found the sodium-driven chimeric BFMs maintained constant speed over a wide range of sodium concentrations by adjusting stator units in compensation to the sodium-motive force (SMF) changes. The BFM has the maximum number of stator units and is most stable at 5 mM sodium concentration rather than higher sodium concentration. Upon rapid exchange from high to low sodium concentration, the number of functional stator units shows a rapidly excessive reduction and then resurrection that is different from predictions of simple absorption model. This may imply the existence of a metastable hidden state of the stator unit during the sudden loss of sodium ions.

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“…The flagellar motor consists of a rotor and a stator; the latter delivers torque to the former to induce rotation ( Berg, 2004 ). Torque is generated by multiple stator units that may associate and dissociate from the motor as a function of several factors ( Muramoto et al, 1994 ; Fung and Berg, 1995 ; Berg, 2003 ; Leake et al, 2006 ; Blair et al, 2008 ; Paulick et al, 2009 ; Lin et al, 2021 ). To generate torque, the stator units typically utilize the proton-motive force, although alternate sources of ion-motive force also may be used ( Manson et al, 1977 ; Yorimitsu and Homma, 2001 ; Wilhelms et al, 2009 ; Terahara et al, 2012 ; Minamino and Imada, 2015 ; Imazawa et al, 2016 ).…”