Modification of Ca2+ channel activity by deletions at the carboxyl terminus of the cardiac alpha 1 subunit (original) (raw)

1994, Journal of Biological Chemistry

Voltage-sensitive Ca2+ channels are multisubunit complexes that include, among others, a large al subunit, which by itself is sufficient to form a channel. Several al genes encoding L-, N-, and P-type Ca2+ channels have been cloned. These a1 genes share a high degree of sequence homology in the putative transmembrane regions, but vary substantially in the putative intracellular loops and the flanking amino and carboxyl termini. In the present study, we investigated the functional roles of the 665-amino acid long carboxyl terminus of a cardiac al by constructing deletion mutants. Expression in Xenopus oocytes of AC1866, AC1733, and AC1700, which lack from 307 to 472 amino acids at the carboxyl terminus, led to inward Ba2+ currents that were 4-to &fold greater than observed with the 2171-amino acid long wild type al. Ionic currents increased without a change in the amount of charge moved during voltagedependent gating, suggesting that the increase in ionic currents was not due to an increase in the number of channels that were expressed. Single channel analysis revealed an unaltered unitary conductance. Thus, removal of up to 70% of the carboxyl terminus increased current density by facilitating the coupling between the voltage-dependent gating and channel opening, leading to an increased opening probability of the channel. Voltage-sensitive Ca2+ channels, including the dihydropyridine-sensitive L-type, play important roles in excitation-contraction coupling, hormone secretion, neurotransmitter release, and a variety of other physiological processes. The L-type channels are modulated pharmacologically by Ca2+ channel antagonists and agonists such as the dihydropyridines (DHP),l and physiologically by, for example, /3-adrenergic stimulation in heart. Recent biochemical studies revealed that skeletal muscle L-type channels consist of 5 distinct components, al, /3, y, and a2-6 (1-41, which are encoded by 4 genes (5-9). Functional expression of the skeletal muscle al subunit in cells devoid of other subunits (/3, a2-8, and y) results in the appearance of Ca2+ currents and DHP binding, indicating that a1 alone is sufficient to fulfill the basic requirements of channel formation (10). Co-HL-37044 (to E. s. and L. B.), AR38970 (to E. s.), and HL-46702 (to E.