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(Delta 13-24) fliH(Delta 96-97) 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. Minamino et al. report that the bacterial FlgN chaperone acts as a switch to activate a backup mechanism for H+-coupled flagellar protein export by interacting with FlhA(C) to activate the Na+-driven export engine. The proposed mechanism helps to explain how bacteria can maintain flagellar protein export when the ATPase complex export machinery becomes non-functional.

Automated summary

The bacterial flagellar protein export machinery consists of an export gate complex and ATPase complex, which utilize H+-driven and Na+-driven engines to export flagellar proteins. The FlgN chaperone can activate the Na+-driven engine when the ATPase complex is non-functional, suggesting a backup mechanism to maintain flagellar function. This proposed mechanism helps to explain how bacteria can construct flagella under conditions of restricted proton motive force.