NORMALIZATION OF CURRENT KINETICS BY INTERACTION BETWEEN THE ALPHA-1-SUBUNIT AND BETA-SUBUNIT OF THE SKELETAL-MUSCLE DIHYDROPYRIDINE-SENSITIVE CA2+ CHANNEL

PURIFICATION of skeletal muscle dihydropyridine binding sites has enabled protein complexes to be isolated from which Ca2+ currents have been reconstituted. Complementary DNAs encoding the five subunits of the dihydropyridine receptor, alpha-1, beta, gamma, alpha-2 and delta (ref. 1), have been cloned 2-6 and it is now recognized that alpha-2 and delta are derived from a common precursor 7,8. The alpha-1 subunit can itself produce Ca2+ currents, as was demonstrated using mouse L cells lacking alpha-2-delta (refs 9, 10), beta (ref. 10) and gamma (our unpublished results). In L cells, stable expression of skeletal muscle-alpha-1 alone was sufficient to generate voltage-sensitive, high-threshold L-type Ca2+ channel currents which were dihydropyridine-sensitive and blocked by Cd2+, but the activation kinetics were about 100 times slower than expected for skeletal muscle Ca2+ channel currents. This could have been due to the cell type in which alpha-1 was being expressed or to the lack of a regulatory component particularly one of the subunits that copurifies with alpha-1. We show here that coexpression of skeletal muscle-beta with skeletal muscle-alpha-1 generates cell lines expressing Ca2+ channel currents with normal activation kinetics as evidence for the participation of the dihydropyridine-receptor beta-subunits in the generation of skeletal muscle Ca2+ channel currents.