Structural characterization of the nitrendipine receptor of the voltage-dependent Ca2+ channel: evidence for a 52,000 dalton subunit

The nitrendipine receptor of the voltage-dependent Ca2+ channel purified from rabbit skeletal muscle has been shown to contain four polypeptide components of 175,000, 170,000, 52,000, and 32,000 daltons. Despite the existence of a substantial amount of data on the composition of the nitrendipine receptor, little is known about the relationship between the 175,000 and 170,000 dalton subunits of the receptor and the lower molecular weight components of the receptor. A monoclonal antibody specific to the 52,000 dalton component of the receptor has now been produced. The monoclonal antibody is capable of specifically immunoprecipitating the [3H]dihydropyridine-labeled nitrendipine receptor from detergent-solubilized membranes. Immunoprecipitation experiments with 32P-labeled nitrendipine receptor have demonstrated a tight association between the 170,000 dalton nitrendipine binding subunit and the 52,000 dalton polypeptide of the receptor. Immunoblotting experiments have shown that the 52,000 dalton polypeptide copurifies with the 175,000 and 170,000 dalton subunits of the nitrendipine receptor at all stages of the purification. In addition, the higher molecular weight subunits of the receptor were not labeled by the antibody. Densitometric scanning of Coomassie blue stained SDS-polyacrylamide gels of the purified nitrendipine receptor has shown that the 175,000, 170,000, 52,000, and 32,000 dalton subunits of the nitrendipine receptor exists in a 1:1:1:1 stoichiometric ratio. In conclusion, we have demonstrated that the 52,000 dalton polypeptide is an integral and distinct subunit of the purified nitrendipine receptor of the voltage-dependent Ca2+ channel.