Ca2+ channel regulation by a conserved β subunit domain

The complete amino-acid sequence of the receptor for dihydropyridine calcium channel blockers from rabbit skeletal muscle is predicted by cloning and sequence analysis of DNA complementary to its messenger RNA. Structural and sequence similarities to the voltage-dependent sodium channel suggest that in the transverse tubule membrane of skeletal muscle the dihydropyridine receptor may act both as voltage sensor in excitation-contraction coupling and as a calcium channel.

The beta-subunit is an integral component of purified voltage-sensitive Ca2+ channels. Modulation of Ca2+ channel activity by the beta-subunit, which includes significant increases in transmembrane current and/or changes in kinetics, is observed on coexpression of six alpha 1-subunit genes with four beta-subunit genes in all alpha 1-beta combinations tested. Recent reports suggest that this regulation is not due to targeting of the alpha 1-subunit to the plasma membrane but is probably a result of a conformational change induced by the beta-subunit. Here we report that the beta-subunit binds to the cytoplasmic linker between repeats I and II of the dihydropyridine-sensitive alpha 1-subunits from skeletal (alpha 1S) and cardiac muscles (alpha 1C-a), and also with the more distantly related neuronal alpha 1A and omega-conotoxin GVIA-sensitive alpha 1B-subunits. Sequence analysis of the beta-subunit binding site identifies a conserved motif (QQ-E--L-GY--WI--E) positioned 24 amino acids from the IS6 transmembrane domain in each alpha 1-subunit. Mutations within this motif reduce the stimulation of peak currents by the beta-subunit and alter inactivation kinetics and voltage-dependence of activation. Conservation of the beta-subunit binding motif in these functionally distinct calcium channels suggests a critical role for the I-II cytoplasmic linker of the alpha 1-subunit in channel modulation by the beta-subunit.

The primary structure of a voltage-dependent calcium channel from rabbit brain has been deduced by cloning and sequencing the complementary DNA. Calcium channel activity expressed from the cDNA is dramatically increased by coexpression of the alpha 2 and beta subunits, known to be associated with the dihydropyridine receptor. This channel is a high voltage-activated calcium channel that is insensitive both to nifedipine and to omega-conotoxin. We suggest that it is expressed predominantly in cerebellar Purkinje cells and granule cells.