S-adenosyl-L-methionine regulation of the cardiac ryanodine receptor involves multiple mechanisms.

ABSTRACT

Cardiac contraction is triggered by the release of Ca(2+) via the ryanodine receptor (RyR2), a sarcoplasmic reticulum (SR) resident ion channel. RyR2 channel activity is modulated through ligand binding and posttranslational regulatory mechanisms. S-Adenosyl-l-methionine (SAM), the primary methyl group donor for enzyme-mediated methylation of proteins and other biological targets, activates RyR2 via an unknown mechanism. Here we show that the SAM-induced increase in cardiac SR (CSR) vesicle [(3)H]ryanodine binding is unaffected by methyltransferase inhibitors and immunoprecipitation of RyR2 from S-adenosyl-l-[methyl-(3)H]methionine ([(3)H]SAM) pretreated CSR indicates that RyR2 is not a target of SAM-mediated protein methylation. Because SAM contains an adenosine moiety and RyR2 is activated by ATP, we investigated whether SAM exerts its effects through the adenine nucleotide binding sites on the RyR2 channel. In support of this hypothesis, the SAM and ATP concentration dependence of CSR vesicle [(3)H]ryanodine binding virtually overlaps. Furthermore, ryanodine binding assays show that SAM competes with adenine nucleotide activation of RyR2, and the effects of SAM on mean channel open and closed times follow similar trends as those observed for ATP. Interestingly, SAM but not ATP activation of RyR2 was associated with a marked increase in the percent of channel openings to a subconductance level approximately 60% of the maximal single channel conductance. This work highlights the complexity underlying SAM regulation of RyR2 and suggests ligand binding is among the multiple mechanisms responsible for SAM regulation of RyR2.