Abstract
Vibrio cholerae is motile by means of its single polar flagellum which is driven by the sodium-motive force. In the motor driving rotation of the flagellar filament, a stator complex consisting of subunits PomA and PomB converts the electrochemical sodium ion gradient into torque. Charged or polar residues within the membrane part of PomB could act as ligands for Na+, or stabilize a hydrogen bond network by interacting with water within the putative channel between PomA and PomB. By analyzing a large data set of individual tracks of swimming cells, we show that S26 located within the transmembrane helix of PomB is required to promote very fast swimming of V. cholerae. Loss of hypermotility was observed with the S26T variant of PomB at pH 7.0, but fast swimming was restored by decreasing the H+ concentration of the external medium. Our study identifies S26 as a second important residue besides D23 in the PomB channel. It is proposed that S26, together with D23 located in close proximity, is important to perturb the hydration shell of Na+ before its passage through a constriction within the stator channel.
Highlights
It is essential for bacteria to be able to move, either away from a repellent or towards an attractive environment [1,2]
The bacterial flagellum consists of three distinct parts: a hollow filament with a length of 15–20 μm composed of flagellin; the hook, which connects the filament to the motor complex; and the membrane embedded motor complex consisting of a rotor and a stator (Fig 1)
The PomA4PomB2 complex of the polar flagellum of V. cholerae provides at least one pathway for the transport of Na+ from the periplasm to the cytoplasm along the electrochemical Na+ gradient. This so-called stator complex is an essential part of the motor, since the flux of Na+ through PomA4PomB2 is a prerequisite for rotation of flagellum [6,52,59,68] (Fig 1)
Summary
It is essential for bacteria to be able to move, either away from a repellent or towards an attractive environment [1,2]. The flagellum is driven either by a proton- or a sodium-motive force [6,7,8,9], but the mechanism converting ion flux into torque still remains unclear. The stator complex defines the specificity to either protons or sodium ions, with four MotA and two MotB subunits forming a H+-dependent MotA4MotB2 stator [10,11,12,13,14], and the homologous PomA and PomB subunits forming a Na+dependent PomA4PomB2 stator [15,16,17,18,19] (Fig 1). Despite considerable effort [20,21,22,23], critical amino acid residue(s) conveying H+ or Na+ specificity in the stator could not yet be identified, PLOS ONE | DOI:10.1371/journal.pone.0123518 April 15, 2015
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