Crystal structure of type 1 ryanodine receptor amino-terminal β-trefoil domain reveals a disease-associated mutation "hot spot" loop

Muscle contraction and relaxation is regulated by transient elevations of myoplasmic Ca2+center dot Ca2+ is released from stores in the lumen of the sarco(endo) plasmic reticulum (SER) to initiate formation of the Ca2+ transient by activation of a class of Ca2+ release channels referred to as ryanodine receptors (RyRs) and is pumped back into the SER lumen by Ca2+-ATPases (SERCAs) to terminate the Ca2+ transient. Mutations in the type 1 ryanodine receptor gene, RYR1, are associated with 2 skeletal muscle disorders, malignant hyperthermia (MH), and central core disease (CCD). The evaluation of proposed mechanisms by which RyR1 mutations cause MH and CCD is hindered by the lack of high-resolution structural information. Here, we report the crystal structure of the N-terminal 210 residues of RyR1 (RyR(NTD)) at 2.5 angstrom. The RyR(NTD) structure is similar to that of the suppressor domain of type 1 inositol 1,4,5-trisphosphate receptor (IP(3)Rsup), but lacks most of the long helix-turn-helix segment of the "arm" domain in IP(3)Rsup. The N-terminal beta-trefoil fold, found in both RyR and IP3R, is likely to play a critical role in regulatory mechanisms in this channel family. A disease-associated mutation "hot spot" loop was identified between strands 8 and 9 in a highly basic region of RyR1. Biophysical studies showed that 3 MH-associated mutations (C36R, R164C, and R178C) do not adversely affect the global stability or fold of RyR(NTD), supporting previously described mechanisms whereby mutations perturb protein-protein interactions.