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

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Abstract
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.