Regulation of RYR1 activity by Ca2+ and calmodulin

The skeletal muscle calcium release channel (RYR1) is a Ca2+-binding protein that is regulated by another Ca2+-binding protein, calmodulin. The functional consequences of calmodulin's interaction with RYR1 are dependent on Ca2+ concentration. At nanomolar Ca2+ concentrations, calmodulin is an activator, but at micromolar Ca2+ concentrations, calmodulin is an inhibitor of RYR1. This raises the question of whether the Ca2+-dependent effects of calmodulin on RYR1 function are due to Ca2+ binding to calmodulin, RYR1, or both. To distinguish the effects of Ca2+ binding to calmodulin from those of Ca2+ binding to RYR1, a mutant calmodulin that cannot bind Ca2+ was used to evaluate the effects of Ca2+-free calmodulin on Ca2+-bound RYR1. We demonstrate that Ca2+-free calmodulin enhances the affinity of RYR1 for Ca2+ while Ca2+ binding to calmodulin converts calmodulin from an activator to an inhibitor. Furthermore, Ca2+ binding to RYR1 enhances its affinity for both Ca2+-free and Ca2+-bound calmodulin.