The structural basis of ryanodine receptor ion channel function.

Large-conductance Ca release channels known as ryanodine receptors (RyRs) mediate the release of Ca from an intracellular membrane compartment, the endo/sarcoplasmic reticulum. There are three mammalian RyR isoforms: RyR1 is present in skeletal muscle; RyR2 is in heart muscle; and RyR3 is expressed at low levels in many tissues including brain, smooth muscle, and slow-twitch skeletal muscle. RyRs form large protein complexes comprising four 560-kD RyR subunits, four ∼12-kD FK506-binding proteins, and various accessory proteins including calmodulin, protein kinases, and protein phosphatases. RyRs share ∼70% sequence identity, with the greatest sequence similarity in the C-terminal region that forms the transmembrane, ion-conducting domain comprising ∼500 amino acids. The remaining ∼4,500 amino acids form the large regulatory cytoplasmic "foot" structure. Experimental evidence for Ca, ATP, phosphorylation, and redox-sensitive sites in the cytoplasmic structure have been described. Exogenous effectors include the two Ca releasing agents caffeine and ryanodine. Recent work describing the near atomic structures of mammalian skeletal and cardiac muscle RyRs provides a structural basis for the regulation of the RyRs by their multiple effectors.