A PUTATIVE INHIBITORY DIVALENT CATION BINDING SITE ON TYPE 1 RYANODINE RECEPTOR
Ryanodine receptors (RyR) are intracellular cation channels that provide a pathway to release Ca2+ ions from intracellular stores into the cytosol, which is essential for numerous cellular functions including excitation-contraction (EC) coupling in striated muscles. They are comprised of four identical ~565 KD protomers, regulatory proteins such as FKBP12/12.6 and calmodulin, enzymes and their respective target proteins in a huge macromolecular complex.1,2 RyRs are activated by calcium ions and ATP3 and are inhibited by high (1mM) calcium concentrations and by magnesium ions (Mg2+).4 1mM Mg2+ is sufficient to reduce RyR channel open probability in lipid bilayer. Lack of sufficient structural information has impeded our understanding of RyRs function and regulation. The closed state structure of type 1 RyR (RyR1) has been solved by single particle cryo-electron microscopy (Cryo-EM).5 And the gating mechanism was revealed by a series of activated RyR1 structures.3 Here we have reconstructed a 3D cryo-EM map of type 1 Ryanodine receptor (RyR1) in presence of magnesium ion (1mM) and we have identified a potential magnesium ion-binding site. We anticipated that Mg2+ ions are locked between two highly conserved negatively charged residues, coming from two different protomers. This new Cryo-EM map suggests a structural basis for divalent cations inhibition of RyR1 channel activity.