Cryo-EM reveals local and global structural rearrangements in RYR mutants

Abstract

Single-point mutations in ryanodine receptors (RYRs), large intracellular Ca2+ channels that play a critical role in EC coupling, are linked to debilitating and lethal disorders such as central core disease, malignant hyperthermia (for the skeletal isoform, RYR1), catecholaminergic polymorphic ventricular tachycardia, and ARVD2 (for the cardiac isoform, RYR2). Mutant RYRs result in elevated [Ca2+]cyto due to steady leak from the sarcoplasmic reticulum. To explore the nature of long-range allosteric mechanisms of malfunction, we determined the structure of two N-terminal domain mutants of RYR1, situated far away from the pore. Cryo-electron microscopy of the N-terminal subdomain A (NTDA) and subdomain C (NTDC) full-length mutants, RYR1 R163C (determined to 3.5 Å resolution), and RYR1 Y522S (determined to 4.0 Å resolution), respectively, reveal large-scale conformational changes in the cytoplasmic assembly under closed-state conditions (i.e., absence of activating Ca2+). The multidomain changes suggest that the mutations induce a preactivated state of the channel in R164C by altering the NTDA+/CD interface, and in Y522S by rearrangement of the α-helical bundle in NTDC. However, the extent of preactivation is considerably higher in Y522S as compared with R163C, which agrees with the increased severity of the Y522S mutation as established by various functional studies. The Y522S mutation represents loss of a spacer residue that is crucial for maintaining optimal orientation of α helices in NTDC, alteration of which has long-range effects felt as far away as ∼100 Å. Additionally, the structure of the Y522S mutant channel under open-state conditions also differs from RYR1 WT open channels. Our developing work with RYR mutants exhibits the diverse mechanisms by which these single-point mutations exert an effect on the channel's function and highlight the complexity of the multidomain channel, as well as the need for targeted therapies.