The FlgN chaperone activates the Na(+)-driven engine of the Salmonella flagellar protein export apparatus

The bacterial flagellar protein export machinery consists of a transmembrane export gate complex and a cytoplasmic ATPase complex. The gate complex has two intrinsic and distinct H(+)-driven and Na(+)-driven engines to drive the export of flagellar structural proteins. Salmonella wild-type cells preferentially use the H(+)-driven engine under a variety of environmental conditions. To address how the Na(+)-driven engine is activated, we analyzed the fliJ(Δ13–24) fliH(Δ96–97) mutant and found that the interaction of the FlgN chaperone with FlhA activates the Na(+)-driven engine when the ATPase complex becomes non-functional. A similar activation can be observed with either of two single-residue substitutions in FlhA. Thus, it is likely that the FlgN-FlhA interaction generates a conformational change in FlhA that allows it to function as a Na(+) channel. We propose that this type of activation would be useful for flagellar construction under conditions in which the proton motive force is severely restricted.