New molecular determinant for inactivation of the human L-type α(1C) Ca²⁺ channel

Molecular cloning of the human fibroblast Ca²⁺ channel pore-forming α(1C) subunit revealed (Soldatov, 1992. Proc. Natl. Acad. Sci. USA 89:4628-4632) a naturally occurring mutation g²²⁵⁴→ a that causes the replacement of the conservative alanine for threonine at the position 752 at the cytoplasmic end of transmembrane segment IIS6. Using stably transfected HEK293 cell lines, we have compared electrophysiological properties of the conventional α(1C,77) human recombinant L-type Ca²⁺ channel with those of its mutated isoform α(1C,94) containing the A752T replacement. Comparative quantification of steady-state availability of the current carried by α(1C,94) and α(1C,77) showed that A752T mutation prevented a large (≃25%) fraction of the current carried by Ca²⁺ or Ba²⁺ from fully inactivating. This mutation, however, did not appear to alter significantly the Ca²⁺-dependence and kinetics of decay of the inactivating fraction of the current or its voltage-dependence. The data suggests that Ala752 at the cytoplasmic end of IIS6 might serve as a molecular determinant of the Ca²⁺ channel inactivation, possibly regulating the voltage-dependence of its availability.