IDENTIFICATION OF A PHENYLALKYLAMINE BINDING REGION WITHIN THE ALPHA-1 SUBUNIT OF SKELETAL-MUSCLE CA-2+ CHANNELS

The α1 subunit of the skeletal muscle Ca2+ channel has been specifically photoaffinity labeled with the phenylalkylamine-receptor-selective verapamil derivative (-)-5-{3-azidophenethyl[N-methyl- 3H]methylamino}-2-(3,4,5-trimethoxyphenyl)-2-isopropylvaleronitrile ([N-methyl3H]LU49888). Proteolytic fragments generated by various endoproteases were probed by immunoprecipitation with several sequence-specific antibodies to determine the site of labeling within the primary structure of al. These results restrict the site of photolabeling by [N-methyl-3H]LU49888 to the region between Glu-1349 and Trp-1391. This segment of al contains transmembrane helix S6 of domain IV and the beginning of the long intracellular C-terminal tail. Because the phenylalkylamine receptor site is only accessible from the intracellular side of the Ca2+ channel, we propose that the intracellular end of helix IVS6 and the adjacent intracellular amino acid residues play an essential role in formation of the phenylalkylamine receptor site. The action of the phenylalkylamines as open-channel blockers suggests that this region may also contribute to formation of the intracellular opening of the transmembrane pore of the Ca2+ channel.