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

Molecular cloning of the human fibroblast Ca2+ channel pore-forming alpha(1 C) subunit revealed (Soldatov, 1992. Proc. Natl. Acad. Sci. USA 89:4628-463 2) a naturally occurring mutation g(2254) --> a that causes the replacement of the conservative alanine for threonine at the position 752 at the cytop lasmic end of transmembrane segment IIS6. Using stably transfected HEK293 c ell lines, we have compared electrophysiological properties of the conventi onal alpha(1C,77) human recombinant L-type Ca2+ channel with those of its m utated isoform alpha(1C,94) containing the A752T replacement. Comparative q uantification of steady-state availability of the current carried by alpha( 1C,94) and alpha(1C,77) showed that A752T mutation prevented a large (appro ximate to 25%) fraction of the current carried by Ca2+ or Ba2+ from fully i nactivating. This mutation, however, did not appear to alter significantly the Ca2+-dependence and kinetics of decay of the inactivating fraction of t he current or its voltage-dependence. The data suggests that Ala752 at the cytoplasmic end of IIS6 might serve as a molecular determinant of the Ca2channel inactivation, possibly regulating the voltage-dependence of its ava ilability.