The random-coil 'C' fragment of the dihydropyridine receptor II-III loop can activate or inhibit native skeletal ryanodine receptors

The actions of peptide C, corresponding to ⁷²⁴Glu-Pro⁷⁶⁰ of the II-III loop of the skeletal dihydropyridine receptor, on ryanodine receptor (RyR) channels incorporated into lipid bilayers with the native sarcoplasmic reticulum membrane show that the peptide is a high-affinity activator of native skeletal RyRs at cytoplasmic concentrations of 100 nM-10 μM. In addition, we found that peptide C inhibits RyRs in a voltage-independent manner when added for longer times or at higher concentrations (up to 150 μM). Peptide C had a random-coil structure indicating that it briefly assumes a variety of structures, some of which might activate and others which might inhibit RyRs. The results suggest that RyR activation and inhibition by peptide C arise from independent stochastic processes. A rate constant of 7.5 × 10⁵ s^-1 · M^-1 was obtained for activation and a lower estimate for the rate constant for inhibition of 5.9 × 103 s^-1 · M^-1. The combined actions of peptide C and peptide A (II-III loop sequence ⁶⁷¹Thr-Leu⁶⁹⁰) showed that peptide C prevented activation but not blockage of RyRs by peptide A. We suggest that the effects of peptide C indicate functional interactions between a part of the dihydropyridine receptor and the RyR. These interactions could reflect either dynamic changes that occur during excitation-contraction coupling or interactions between the proteins at rest.