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In an attempt to identify and characterize functional domains of the rabbit skeletal muscle dihydropyridine receptor alpha 1 subunit II-III loop, we synthesized several peptides corresponding to different regions of the loop: peptides A, B, C, C1, C2, D (cf. Fig. 1). Peptide A (Thr671-Leu690) activated 3Hryanodine binding to, and induced Ca2+ release from, rabbit skeletal muscle triads, but none of the other peptides had such effects. Peptide A-induced Ca2+ release and activation of ryanodine binding were partially suppressed by an equimolar concentration of peptide C (Glu724-Pro760) but were not affected by the other peptides. These results suggest that the short stretch in the II-III loop, Thr671-Leu690, is responsible for triggering SR Ca2+ release, while the other region, Glu724-Pro760, functions as a blocker of the release trigger. A hypothesis is proposed to account for how these subdomains interact with the sarcoplasmic reticulum Ca2+ release channel protein during excitation-contraction coupling.
El-Hayek et al. (Fri,) studied this question.