Molecular determinants of high affinity dihydropyridine binding in L-type calcium channels

The pore-forming α₁ subunit of L-type voltage-gated Ca²⁺ channels is pharmacologically modulated by dihydropyridine (DHP) Ca²⁺ antagonists and agonists. Site-directed mutation of amino acids within transmembrane segments IIIS6 and IVS6 to those characteristic of DHP-insensitive channels revealed 2 mutations in IIIS6 (I1049F and I1052F) and 4 mutations in IVS6 (Y1365I, M1366F, I1372M, and I1373L) with increased K(D) values for (+)-[³H]PN200- 110 binding. A tyrosine residue (Y1048) in IIIS6 that is conserved between DHP-sensitive and -insensitive Ca²⁺ channels was also altered by mutagenesis. Y1048F had a K(D) for (+)-[³H]PN200-110 binding that was increased 12-fold, and Y1048A had a K(D) at least 1000-fold higher than that of wild-type. These results support the hypothesis that transmembrane segments IIIS6 and IVS6 both contribute critical amino acid residues to the DHP receptor site and that Tyr-1048 within transmembrane segment IIIS6 is required for high affinity DHP binding, even though it is conserved between DHP-sensitive and -insensitive Ca²⁺ channels.