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The purified calcium antagonist receptor of the voltage-sensitive calcium channel from skeletal muscle transverse tubule membrane consists of three subunits: alpha with Mr 135 000, beta with Mr 50 000, and gamma with Mr 33 000. Purified receptor preparations were incorporated into phosphatidylcholine (PC) vesicles by addition of PC in 3-(3-cholamidopropyl)dimethylammonio-1-propanesulfonate and removal of detergent by molecular sieve chromatography. Forty-five percent of the alpha, beta, and gamma polypeptides and the 3Hdihydropyridine/receptor complex were recovered in association with PC vesicles. The rate of dissociation of the purified and reconstituted dihydropyridine/receptor complex was identical with that in T-tubule membranes, and allosteric modulation by verapamil and diltiazem was retained. The reconstituted calcium antagonist receptor, when occupied by the calcium channel activator BAY K 8644, mediated specific 45Ca2+ and 133Ba2+ transport into the reconstituted vesicles. 45Ca2+ influx was blocked by the organic calcium antagonists PN200-110 (K0.5 = 0.2 microM), D600 (K0.5 = 1.0 microM), and verapamil (K0.5 = 1.5 microM) and by inorganic calcium channel antagonists (La3+ greater than Cd2+ greater than Ni2+ greater than Mg2+) as in intact T-tubules. A close quantitative correlation was observed between the presence of the alpha, beta, and gamma subunits of the calcium antagonist receptor and the ability to mediate 45Ca2+ or 133Ba2+ flux into reconstituted vesicles. Comparison of the number of reconstituted calcium antagonist receptors and functional channels supports the conclusion that only a few percent of the purified calcium antagonist receptor polypeptides are capable of mediating calcium transport as previously demonstrated for calcium antagonist receptors in intact T-tubules.
Curtis et al. (1986) studied this question.
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