Molecular basis for allosteric regulation of the type 2 ryanodine receptor channel gating by key modulators

The type-2 ryanodine receptor (RyR2) is responsible for releasing Ca2+ from the sarcoplasmic reticulum of cardiomyocytes, subsequently leading to muscle contraction. Here, we report four cryo-EM structures of porcine RyR2 bound to distinct modulators that collectively provide mechanistic insight into RyR2 regulation. Ca2+ alone induces a contraction of the Central domain that facilitates the dilation of S6 bundle, but is insufficient to open the pore. The small molecule agonist PCB95 helps Ca2+ to overcome the barrier for opening. FKBP12.6 induces a relaxation of the Central domain that decouples it from the S6 bundle, stabilizing RyR2 in a closed state. Caffeine locks the Central domain in a constitutively contracted state, while further addition of ATP opens the channel by strengthening the coupling between the U-motif and S6. Our study marks an important step towards mechanistic understanding of the complicated regulation of this key channel whose aberrant activity engenders life-threatening cardiac disorders.