Unique phosphorylation site on the cardiac ryanodine receptor regulates calcium channel activity

TLDR

Ryanodine receptors are the sarcoplasmic reticulum Ca²⁺ release channels in cardiac and skeletal muscle, yet no definitive regulatory phosphorylation sites have been identified. The study seeks to identify a regulatory phosphorylation site on the cardiac ryanodine receptor. By showing that the cardiac isoform is a preferred substrate for CaM kinase, the authors localize serine 2809 as the phosphorylation site. Phosphorylation of serine 2809 by CaM kinase activates the Ca²⁺ channel, and antibodies against this site recognize only the cardiac isoform, indicating a unique regulatory mechanism in heart and brain.

Abstract

Ryanodine receptors have recently been shown to be the Ca2+ release channels of sarcoplasmic reticulum in both cardiac muscle and skeletal muscle. Several regulatory sites are postulated to exist on these receptors, but to date, none have been definitively identified. In the work described here, we localize one of these sites by showing that the cardiac isoform of the ryanodine receptor is a preferred substrate for multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase). Phosphorylation by CaM kinase occurs at a single site encompassing serine 2809. Antibodies generated to this site react only with the cardiac isoform of the ryanodine receptor, and immunoprecipitate only cardiac [3H]ryanodine-binding sites. When cardiac junctional sarcoplasmic reticulum vesicles or partially purified ryanodine receptors are fused with planar bilayers, phosphorylation at this site activates the Ca2+ channel. In tissues expressing the cardiac isoform of the ryanodine receptor, such as heart and brain, phosphorylation of the Ca2+ release channel by CaM kinase may provide a unique mechanism for regulating intracellular Ca2+ release.