Role of calmodulin methionine residues in mediating productive association with cardiac ryanodine receptors

Calmodulin (CaM) binds to the cardiac ryanodine receptor Ca²⁺ release channel (RyR2) with high affinity and may act as a regulatory channel subunit. Here we determine the role of CaM Met residues in the productive association of CaM with RyR2, as assessed via determinations of [ ³H]ryanodine and [³⁵S]CaM binding to cardiac muscle sarcoplasmic reticulum (SR) vesicles. Oxidation of all nine CaM Met residues abolished the productive association of CaM with RyR2. Substitution of the COOH-terminal Mets of CaM with Leu decreased the extent of CaM inhibition of cardiac SR (CSR) vesicle [³H]ryanodine binding. In contrast, replacing the NH₂-terminal Met of CaM with Leu increased the concentration of CaM required to inhibit CSR [³H]ryanodine binding but did not alter the extent of inhibition. Site-specific substitution of individual CaM Met residues with Gln demonstrated that Met124 was required for both high-affinity CaM binding to RyR2 and for maximal CaM inhibition. These results thus identify a Met residue critical for the productive association of CaM with RyR2 channels.