Prokaryotic voltage-gated sodium channels (Na V s) are homotetramers and are thought to inactivate through a single mechanism, named C-type inactivation. Here we report the voltage dependence and inactivation rate of the NaChBac channel from Bacillus halodurans, the first identified prokaryotic Na V , as well as of three new homologues cloned from Bacillus licheniformis (Na V BacL), Shewanella putrefaciens (Na V SheP), and Roseobacter denitrificans (Na V RosD). We found that, although activated by a lower membrane potential, Na V BacL inactivates as slowly as NaChBac. Na V SheP and Na V RosD inactivate faster than NaChBac. Mutational analysis of helix S6 showed that residues corresponding to the "glycine hinge" and "PXP motif" in voltage-gated potassium channels are not obligatory for channel gating in these prokaryotic Na V s, but mutations in the regions changed the inactivation rates. Mutation of the region corresponding to the glycine hinge in Na V BacL (A214G), Na V SheP (A216G), and NaChBac (G219A) accelerated inactivation in these channels, whereas mutation of glycine to alanine in the lower part of helix S6 in NaChBac (G229A), Na V BacL (G224A), and Na V RosD (G217A) reduced the inactivation rate. These results imply that activation gating in prokaryotic Na V s does not require gating motifs and that the residues of helix S6 affect C-type inactivation rates in these channels.Voltage-gated sodium channels (Na V s) 3 generate the rapid upstroke of action potentials in nerve cell axons (1). In mammalian Na V s, the channel is formed by the ␣-subunit, which comprises four repeats of six-transmembrane segments, with each repeat consisting of 300 -400 amino acids. The ␣-subunit carries several glycosylation sites and co-assembles with auxiliary subunits to form the native channel (2, 3). The only structural information on Na V s available to date is a density map of the Na V from the electric organ of the electric eel determined by cryoelectron microscopy (4). Due to its limited resolution of 19 Å, the density map did not provide insights into the gating or sodium selectivity.The first prokaryotic Na V , NaChBac, was cloned from Bacillus halodurans (5). Subsequently, three more prokaryotic sodium channels were cloned and characterized (6, 7). All studied prokaryotic Na V s form homotetramers with a structure thought to be similar to that of some potassium channels with known structures (8 -10). Furthermore, because the proteins could be expressed in large amounts in Escherichia coli and purified by metal chelate affinity chromatography (5, 7, 11), they are promising candidates for high resolution structure determination and structure-function analyses.The physiological role of prokaryotic Na V s may be related to pH homeostasis, motility, and chemotaxis (6, 12). Searching bacterial genomic data bases, we found 26 sequences of putative NaChBac homologues from bacteria living in various environments. We were able to clone the putative Na V genes from three of these bacteria, Bacillus licheniformis, Shewanella pu...