Membrane voltage-dependent activation of the flagellar protein export engine

Abstract

Ion motive force (IMF) consists of the electric potential difference ({Delta}{Psi}) and the ion concentration difference ({Delta}pI) 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 {Delta}{Psi}-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 {Delta}{Psi} 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 {Delta}{Psi} 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.