The structural biology of ryanodine receptors

Ryanodine receptors are ion channels that allow for the release of Ca(2+) from the endoplasmic or sarcoplasmic reticulum. They are expressed in many different cell types but are best known for their predominance in skeletal and cardiac myocytes, where they are directly involved in excitation-contraction coupling. With molecular weights exceeding 2 MDa, Ryanodine Receptors are the largest ion channels known to date and present major challenges for structural biology. Since their discovery in the 1980s, significant progress has been made in understanding their behaviour through multiple structural methods. Cryo-electron microscopy reconstructions of intact channels depict a mushroom-shaped structure with a large cytoplasmic region that presents many binding sites for regulatory molecules. This region undergoes significant motions during opening and closing of the channel, demonstrating that the Ryanodine Receptor is a bona fide allosteric protein. High-resolution structures through X-ray crystallography and NMR currently cover ∼11% of the entire protein. The combination of high- and low-resolution methods allows us to build pseudo-atomic models. Here we present an overview of the electron microscopy, NMR, and crystallographic analyses of this membrane protein giant.