Interplay between Mg²⁺and Ca²⁺at multiple sites of the ryanodine receptor

Abstract

RyR1 is an intracellular Ca2+channel important in excitable cells. Ca2+activates it at low concentrations and inhibits it at high concentrations. Mg2+is the main physiological RyR1 inhibitor, an effect that is overridden upon activation. Despite the significance of Mg2+-mediated inhibition, the molecular-level mechanisms remain unclear. We determined two cryo-EM structures of RyR1 with Mg2+up to 2.8 Å resolution, identifying multiple Mg2+binding sites. Mg2+inhibits at the known Ca2+activating site and we propose that the EF hand domain is an inhibitory divalent cation sensor. Both divalent cations bind to ATP within a crevice, contributing to the precise transmission of allosteric changes within the enormous channel protein. Notably, Mg2+inhibits RyR1 by interacting with the gating helices as validated by molecular dynamics. This structural insight enhances our understanding of how Mg2+inhibition is overcome during excitation.