Subunit structure of dihydropyridine-sensitive calcium channels from skeletal muscle.

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Abstract
Purified dihydropyridine-sensitive calcium channels from rabbit transverse-tubule membranes consist of three noncovalently associated classes of subunits: .alpha. (167 kDa), .beta. (54 kDa) and .gamma. (30 kDa). Cleavage of disulfide bonds reveals two distinct .alpha. polypeptides and an additional component, .delta.. The .alpha.1 subunit, a 175-kDa polypeptide that is not N-glycosylated, contains the dihydropyridine binding site, cAMP-dependent protein kinase phosphorylation site(s), and substantial hydrophobic domain(s). .alpha.2, a 143-kDa glycoprotein, has none of the properties characteric of .alpha.1 but binds lectins and contains about 25% N-linked carbohydrate. .alpha.2 is disulfide-linked to .delta., a 24- to 27-kDa glycopeptide. .beta. (54 kDa) contains a cAMP-dependent phosphorylation site but is not N-glycosylated and does not have a hydrophobic domain. .gamma. (30 kDa) has a carbohydrate content of about 30% and extensive hydrophobic domain(s). Precipitation with affinity-purified anti-.alpha.1 antibodies or .alpha.2-specific lentil lectin-agarose demonstrated that .alpha.1.alpha.2.beta..gamma..delta. behaves as a complex in the presence of digitonin or 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate, whereas the .alpha.2.delta. complex dissociates from .alpha.1.beta..gamma. in the presence of Triton X-100. A model for subunit interaction and membrane insertion is proposed on the basis of these obesrvations.