Studies on the structural requirements for the activity of the skeletal muscle dihydropyridine receptor/slow Ca2+ channel. Allosteric regulation of dihydropyridine binding in the absence of alpha 2 and beta components of the purified protein complex

A rabbit skeletal muscle dihydropyridine (DHP) receptor can be purified as an alpha 1-alpha 2-delta-beta-gamma complex, of which alpha 2 and delta are disulfide bonded. This complex has Ca2+ channel activity when incorporated into lipid bilayers. We reported recently that expression of alpha 1 in murine L cells (LCa cells) leads to appearance of both DHP binding and Ca2+ currents, and that we failed to detect alpha 2 by immunoblotting. LCa cell Ca2+ channel currents resembled those in rabbit skeletal muscle in their sensitivity to both voltage and the DHP agonist Bay K 8644, but differed in that they responded to depolarization much more slowly. We now report details of the molecular cloning of the cDNA encoding the 1857-amino acid long alpha 1 transfected into the L cells and results from studies on expression of beta, as well as, on allosteric regulation of DHP binding to these cells. The alpha 1 cDNA was cloned by a combination of cDNA library screening (5355 base pairs) and chemical synthesis (508 base pairs). Using rabbit labeled beta cDNA, which cross-reacts with murine beta mRNA, we failed to observe cross-hybridizing beta mRNA in LCa cells. Using a labeled single stranded 200-base long rabbit alpha 2 cDNA that cross-reacts with mouse alpha 2 mRNA, we likewise failed to observe cross-hybridizing alpha 2 mRNA in LCa cells and hence confirmed the absence of an endogenous murine alpha 2 in these cells. Using LCa cell membranes as DHP receptor source we found the binding of the DHP antagonist (+)-3H]PN200-110 to be regulated by both verapamil and diltiazem as it is in rabbit skeletal muscle membranes. However, we noted a difference; at concentrations above 10(-6) M, verapamil inhibited residual DHP binding in LCa but not in skeletal muscle membranes. We conclude that neither alpha 2 nor beta are essential for expression of alpha 1 on the cell surface, or for its functioning as a voltage-gated Ca2+ channel, or for its allosteric regulation of DHP binding by Ca2+ channel antagonists. The studies neither exclude roles for gamma and delta, nor for alpha 2 or beta in determining more subtle properties of this channel.