NavBP, found in alkaliphilic Bacillus pseudofirmus OF4, is a member of the bacterial voltage-gated Na+ channel superfamily. The alkaliphile requires NavBP for normal chemotaxis responses and for optimal pH homeostasis during a shift to alkaline conditions at suboptimally low Na+ concentrations. We hypothesized that interaction of NavBP with one or more other proteins in vivo, specifically methyl-accepting chemotaxis proteins (MCPs), is involved in activation of the channel under the pH conditions that exist in the extremophile and could underpin its role in chemotaxis; MCPs transduce chemotactic signals and generally localize to cell poles of rod-shaped cells. Here, immunofluorescence microscopy and fluorescent protein fusion studies showed that an alkaliphile protein (designated McpX) that cross-reacts with antibodies raised against Bacillus subtilis McpB co-localizes with NavBP at the cell poles of B. pseudofirmus OF4. In a mutant in which NavBP-encoding ncbA is deleted, the content of McpX was close to the wild-type level but McpX was significantly delocalized. A mutant of B. pseudofirmus OF4 was constructed in which cheAW expression was disrupted to assess whether this mutation impaired polar localization of McpX, as expected from studies in Escherichia coli and Salmonella, and, if so, whether NavBP would be similarly affected. Polar localization of both McpX and NavBP was decreased in the cheAW mutant. The results suggest interactions between McpX and NavBP that affect their co-localization. The inverse chemotaxis phenotype of ncbA mutants may result in part from MCP delocalization.
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