Interplay between Mg2+ and Ca2+ at multiple sites of the ryanodine receptor
Ashok R. Nayak, Warin Rangubpit, Alex H. Will, Yifan Hu, Pablo Castro‐Hartmann, Joshua Lobo, Kelly A. Dryden, Graham D. Lamb, Pornthep Sompornpisut, Montserrat Samsó
Abstract
Abstract RyR1 is an intracellular Ca 2+ channel important in excitable cells such as neurons and muscle fibers. Ca 2+ activates it at low concentrations and inhibits it at high concentrations. Mg 2+ is the main physiological RyR1 inhibitor, an effect that is overridden upon activation. Despite the significance of Mg 2+ -mediated inhibition, the molecular-level mechanisms remain unclear. In this work we determined two cryo-EM structures of RyR1 with Mg 2+ up to 2.8 Å resolution, identifying multiple Mg 2+ binding sites. Mg 2+ inhibits at the known Ca 2+ 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, Mg 2+ inhibits RyR1 by interacting with the gating helices as validated by molecular dynamics. This structural insight enhances our understanding of how Mg 2+ inhibition is overcome during excitation.