Membrane voltage-dependent activation of the flagellar protein export engine

Abstract

AbstractIon motive force (IMF) consists of the electric potential difference (ΔΨ) and the ion concentration difference (ΔpI) across the cytoplasmic membrane. The flagellar protein export machinery is an ion/protein antiporter utilizing IMF to drive ion-coupled protein export, but it remains unknown how. Here, we report a ΔΨ-dependent activation mechanism of the transmembrane export gate complex. Depletions of both H+ and Na+ gradients nearly diminished flagellar protein export in the absence of the cytoplasmic ATPase complex, but an increase in ΔΨ by an upward shift of external pH from 7.5 to 8.5 dramatically recovered it. An increase in the cytoplasmic level of export substrates and gain-of-function mutations in FlhA enhanced protein export at external pH 7.5 in the absence of Na+ in a similar manner to ΔΨ increase. We propose that the export gate complex has a voltage-gated mechanism to activate the ion/protein antiporter of the flagellar protein export engine.