Subunit structure of dihydropyridine-sensitive calcium channels from skeletal muscle

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 characteristic 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 observations.