Authors: Kelvin Lau, Mandy M. Y. Chan, and Filip Van Petegem
Ryanodine receptors (RyRs) are large ion channels that are responsible for the release of Ca2+ from the sarcoplasmic/endoplasmic reticulum. Calmodulin (CaM) is a Ca2+ binding protein that can affect the channel open probability at both high and low Ca2+ concentrations, shifting the Ca2+ dependencies of channel opening in an isoform-specific manner. Here we analyze the binding of CaM and its individual domains to three different RyR regions using isothermal titration calorimetry. We compared binding to skeletal muscle (RyR1) and cardiac (RyR2) isoforms, under both Ca2+-loaded and Ca2+-free conditions. CaM can bind all three regions in both isoforms, but the binding modes differ appreciably in two segments. The results highlight a Ca2+/CaM and apoCaM binding site in the C-terminal fifth of the channel. This binding site is the target for malignant hyperthermia and central core disease mutations in RyR1, which affect the energetics and mode of CaM binding.