Roles of two Na⁺ channels, MotPS and Na_vBP, in motility, chemotaxis and pH homeostasis of alkaliphilic Bacillus pseudofirmus OF4

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In alkaliphilic Bacillus, Na⁺-dependent pH homeostasis involves Na⁺/H⁺ antiport activities as well as Na⁺ re-entry routes. The MotPS flagellar stator proteins that are homologous to MotAB are required for the Na⁺-dependent motility of alkaliphilic B. pseudofirmus OF4. Purified and reconstituted MotPS support amiloride analogue-sensitive Na⁺ flux. Mutants lacking functional MotPS display no deficit in pH homeostasis in pH shift experiments conducted with sub-optimal added [Na⁺] in the absence of solutes whose uptake is coupled to Na⁺ re-entry. By contrast, a role was evident for the recently described Na_vBP (voltage-dependent) Na⁺ channel at sub-optimal [Na⁺]. Na_vBP mutants exhibit a significant loss of pH homeostasis capacity. Na_vBP has an additional role in chemotaxis; mutants in Na_vBP exhibited inverse chemotaxis as did the wild type alkaliphile when treated with the Na_vBP channel inhibitor nifedipine.