Putative channel components for the fast-rotating sodium-driven flagellar motor of a marine bacterium

Abstract

The polar flagellum of Vibrio alginolyticus rotates remarkably fast (up to 1,700 revolutions per second) by using a motor driven by sodium ions. Two genes, motX and motY, for the sodium-driven flagellar motor have been identified in marine bacteria, Vibrio parahaemolyticus and V. alginolyticus. They have no similarity to the genes for proton-driven motors, motA and motB, whose products constitute a proton channel. MotX was proposed to be a component of a sodium channel. Here we identified additional sodium motor genes, pomA and pomB, in V. alginolyticus. Unexpectedly, PomA and PomB have similarities to MotA and MotB, respectively, especially in the predicted transmembrane regions. These results suggest that PomA and PomB may be sodium-conducting channel components of the sodium-driven motor and that the motor part consists of the products of at least four genes, pomA, pomB, motX, and motY. Furthermore, swimming speed was controlled by the expression level of the pomA gene, suggesting that newly synthesized PomA proteins, which are components of a force-generating unit, were successively integrated into the defective motor complexes. These findings imply that Na ؉ -driven flagellar motors may have similar structure and function as proton-driven motors, but with some interesting differences as well, and it is possible to compare and study the coupling mechanisms of the sodium and proton ion flux for the force generation.

Intermolecular Cross-linking between the Periplasmic Loop3–4 Regions of PomA, a Component of the Na+-driven Flagellar Motor of Vibrio alginolyticus

“…Of these, PomA and PomB are similar to MotA and MotB, respectively. It has been inferred that PomA and PomB also form an ion channel (8). This inference was supported by the observation that mutations conferring resistance to phenamil, which is a known Na ϩ -channel inhibitor and specifically and strongly inhibits the Na ϩ -driven motor, mapped to pomA and pomB (14,16,17).…”

“…From the predicted topology of PomA, the N-and C-terminal regions and the large loop between transmembrane segments 2 and 3 are located in the cytoplasm. Loop 1-2 and loop [3][4] , between transmembrane segments 1 and 2 and segments 3 and 4, respectively, are thought to be exposed to the periplasmic space (8). When the loops were reacted with biotin maleimide, the labeling pattern was different; substitutions in loop 3-4 were biotinylated consistently with the membrane topology, whereas none of residues in loop 1-2 was labeled (20).…”

Functional Reconstitution of the Na+-driven Polar Flagellar Motor Component of Vibrio alginolyticus

“…The specific sodium channel blockers, amiloride and phenamil, are powerful tools for studying the mechanism of energy conversion in this system (24,25). Four proteins essential for torque generation, PomA, PomB, MotX, and MotY, were recently identified in the polar flagellar motor of Vibrio alginolyticus (26,27). PomA and PomB are homologous to MotA and MotB and contain four transmembrane segments and one transmembrane segment, respectively.…”

A conserved residue, PomB-F22, in the transmembrane segment of the flagellar stator complex, has a critical role in conducting ions and generating torque

“…PomA and PomB (MotA and MotB) are membrane proteins and form an A 4 : B 2 heterohexamer Sato & Homma, 2000a, b;Yorimitsu et al, 2004). PomA (MotA) has four transmembrane (TM) segments, whereas PomB (MotB) has only a single TM segment (Asai et al, 1997;Chun & Parkinson, 1988;Zhou et al, 1995). The C-terminal half of PomB (MotB) associates with the peptidoglycan layer and at least 11 PomA/B (MotA/B) complexes assemble around the rotor (Kojima et al, 2009;Leake et al, 2006;Reid et al, 2006).…”

“…MotA and MotB are stator proteins in the H + -driven motor of E. coli and Salmonella, while PomA and PomB, which are orthologues of MotA and MotB, are found in the Na + -driven motor of Vibrio spp. (Asai et al, 1997;Dean et al, 1984;Stader et al, 1986). Only the Na + -driven motor of Vibrio spp.…”