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

Abstract

The bacterial flagellar protein export machinery promotes H+-coupled translocation of flagellar proteins to the cell exterior. When the cytoplasmic ATPase complex does not function, the transmembrane export gate complex opens its Na+ channel and continues protein transport. However, it remains unknown how. Here we report that the FlgN chaperone acts as a switch to activate a backup export mechanism for the ATPase complex by activating the Na+-driven engine. Impaired interaction of FlhA with the FliJ subunit of the ATPase complex increased Na+-dependence of flagellar protein export. Deletion of FlgN inhibited protein export in the absence of the ATPase complex but not in its presence. Gain-of-function mutations in FlhA restored not only the FlgN defect but also the FliJ defect. We propose that the interaction of FlgN with FlhA opens the Na+ channel in the export engine, thereby maintaining the protein export activity in the absence of the active ATPase complex.

Membrane voltage-dependent activation mechanism of the bacterial flagellar protein export apparatus

“…Flagellar protein export by the ΔHI B* mutant shows a clear dependence on the external Na + concentration over an external pH range of 6.0-8.0 (17,18). Consistently, the secretion level of FlgD increased considerably when 100 mM NaCl was present in the medium (SI Appendix, Fig.…”

“…We chose the hook-capping protein FlgD as a representative export substrate because the level of FlgD secretion by this mutant strain is even higher than the wild-type level (20). The transmembrane export gate complex of the ΔHI B* strain prefers Na + over H + as the coupling ion in an external pH range of 6.0-8.0 (17,18). Therefore, the ΔHI B* cells were grown in T-broth at external pH values of 7.0, 7.5, 8.0, or 8.5 in the absence of external Na + to examine the Δψ dependence of protein export.…”

“…A gain-of-function mutation, flhA(T490M), located in FlhA C (Fig. 4A), overcomes the FliJ defect to a considerable degree (18,26). Therefore, we investigated whether this gain-of-function mutation affects the Δψ dependence of flagellar protein export by the ΔHI B* mutant.…”

“…The flagellar protein export channel is intrinsically a dual-fuel engine that utilizes both H + and Na + as the coupling ion to drive flagellar protein export, and FlhA acts as a transmembrane ion channel to conduct both H + and Na + (17,18). In the wild-type protein export apparatus, neither the ΔpH nor ΔpNa component is essential; the Δψ component is sufficient for flagellar protein export (13,19).…”

“…ATPase complex becomes nonfunctional, the FlgN chaperone activates the Na + -driven export engine of the export gate complex over a wide range of external pH, allowing the export gate complex to drive Na + -coupled protein export (17,18). The transmembrane domain of FlhA (FlhA TM ) acts as a transmembrane ion channel for the transit of both H + and Na + across the cytoplasmic membrane (17).…”