Identification of the minimum essential region in the II-III loop of the dihydropyridine receptor α1 subunit required for activation of skeletal muscle-type excitation-contraction coupling

We have previously shown that among several peptides encompassing various regions of the II-III loop of the dihydropyridine receptor al subunit, only one peptide corresponding to the Thr(671)-Leu(690) region (designated as peptide A) activated ryanodine binding to and induced calcium release from the sarcoplasmic reticulum [El-Hayek et al, (1995) J, Biol, Chem. 270; 22116-22118], To further localize within peptide A the minimum unit essential for activating the sarcoplasmic reticulum calcium release channel, we synthesized variously truncated forms of peptide A and examined their ability to activate ryanodine binding. We found that the carboxy-terminal 10-residue region of peptide A encompassing Arg(681)-Leu(690) (peptide As-10; s, skeletal muscle-type sequence) activated ryanodine binding in a RyR1-specific manner and induced calcium release even more efficiently than the 20-residue peptide A. Further truncation of one or more residue(s) of peptide As-10 virtually abolished both functions of activating ryanodine binding and inducing Ca2+ release. The activating ability of As-10 seems to be determined by at least two factors: (1) the distribution of the positively charged residues, and (2) the skeletal muscle-type amino acid sequence as deduced from the comparison of various peptides with modified structures. These results provide evidence that the minimum essential unit for the in situ trigger of skeletal muscle excitation-contraction coupling is localized in the Arg(681)-Leu(690) region of the II-III loop of the alpha(1) subunit of the dihydropyridine receptor.