A voltage-gated calcium-selective channel encoded by a sodium channel-like gene

BSC1, which was originally identified by its sequence similarity to voltage-gated Na+ channels, encodes a functional voltage-gated cation channel whose properties differ significantly from Na+ channels. BSC1 has slower kinetics of activation and inactivation than Na+ channels, it is more selective for Ba2+ than for Na+, it is blocked by Cd2+, and Na+ currents through BSC1 are blocked by low concentrations of Ca2+. All of these properties are more similar to voltage-gated Ca2+ channels than to voltage-gated Na+ channels. The selectivity for Ba2+ is partially due to the presence of a glutamate in the pore-forming region of domain III, since replacing that residue with lysine (normally present in voltage-gated Na+ channels) makes the channel more selective for Na+. BSC1 appears to be the prototype of a novel family of invertebrate voltage-dependent cation channels with a close structural and evolutionary relationship to voltage-gated Na+ and Ca2+ channels.