<p>Voltage-dependent sodium channels are transmembrane (TM) proteinsresponsible for the depolarising phase of the action potential in mostelectrically excitable cells [<cite idref="PUB00004785"/>]. They may exist in 3 states [<cite idref="PUB00000213"/>]: theresting state, where the channel is closed; the activated state, where thechannel is open; and the inactivated state, where the channel is closedand refractory to opening. Several different structurally and functionallydistinct isoforms are found in mammals, coded for by a multigene family, these being responsible for the different types of sodium ion currentsfound in excitable tissues.</p><p>The structure of sodium channels is based on 4 internal repeats of a 6-helixbundle [<cite idref="PUB00003989"/>] (in which 5 of the membrane-spanning segments are hydrophobic andthe other is positively charged), forming a 24-helical bundle. The chargedsegments are believed to be localised within clusters formed by their 5 hydrophobic neighbours: it is postulated that the charged domain may be thevoltage sensor region, possibly moving outward on depolarisation, causing aconformational change. This model [<cite idref="PUB00003989"/>] contrastswith another [<cite idref="PUB00000213"/>], in which the TM segments are juxtaposedoctagonally. The basic structural motif (the 6-helix bundle) is also found in potassium and calcium channel alpha subunits.</p><p>Cation channels are transport proteins responsible for the movement of cations through the membrane. These proteins contain 6 transmembrane helices in which the last two helices flank a loop which determines ion selectivity. In some sub-families (e.g. Na channels) the domain is repeated four times, whereas in others (e.g. K channels) the protein forms as a tetramer in the membrane. </p><p>This entry represents alpha subunits of the voltage-gated Na+ channelsuperfamily. For entries which define specifc alpha subunits see see <db_xref db="INTERPRO" dbkey="IPR008051"/>, <db_xref db="INTERPRO" dbkey="IPR008052"/>, <db_xref db="INTERPRO" dbkey="IPR008053"/>, <db_xref db="INTERPRO" dbkey="IPR008054"/>.</p> Voltage gated sodium channel, alpha subunit