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Identification of 1,4-dihydropyridine binding regions within the alpha 1 subunit of skeletal muscle Ca2+ channels by photoaffinity labeling with diazipine.

To identify regions that are involved in the formation of the dihydropyridine receptor site of skeletal muscle L-type Ca2+ channels, the alpha 1 subunit of the channel complex was specifically labeled with the 1,4-dihydropyridine-receptor-selective photoaffinity probe [3H]diazipine. Photoaffinity-labeled regions were identified by probing labeled proteolytic fragments with several anti-peptide antibodies recognizing different segments of the alpha 1 sequence. Forty to 50% of the alpha 1-associated [3H]diazipine label was contained in the tryptic fragment between Arg-988 and Ala-1023 derived from the loop between segments S5 and S6 in domain III. This region corresponds to a portion of the channel that is believed to contribute to formation of the transmembrane pore. Twenty to 30% of the labeling occurred in a V8 protease fragment between Glu-1349 and Trp-1391. This fragment contains transmembrane segment S6 of domain IV and has previously been shown to form part of the drug receptor for phenylalkylamine Ca2+ antagonists. Our data suggest that the dihydropyridine receptor is formed by close apposition of two discontinuous regions of the alpha 1 subunit sequence in domains III and IV. In light of previous work localizing this receptor site to the extracellular surface of the lipid bilayer, it is proposed that amino acid residues at the extracellular surface in the loop connecting segments IIIS5 and IIIS6 and at the extracellular end of segment IVS6 contribute to formation of the dihydropyridine receptor site.