Abstract
The bacterial flagellar type III export apparatus consists of a cytoplasmic ATPase complex and a transmembrane export gate complex, which are powered by ATP and proton motive force (PMF) across the cytoplasmic membrane, respectively, and transports flagellar component proteins from the cytoplasm to the distal end of the growing flagellar structure where their assembly occurs (Minamino, 2014). The export gate complex can utilize sodium motive force in addition to PMF when the cytoplasmic ATPase complex does not work properly. A transmembrane export gate protein FlhA acts as a dual ion channel to conduct both H+ and Na+ ( Minamino et al., 2016 ). Here, we describe how to measure the intracellular Na+ concentrations in living Escherichia coli cells using a sodium-sensitive fluorescent dye, CoroNa Green ( Minamino et al., 2016 ). Fluorescence intensity measurements of CoroNa Green by epi-fluorescence microscopy allows us to measure the intracellular Na+ concentration quantitatively.