The type-1 ryanodine receptor (RyR1) is an intracellular calcium release channel for skeletal muscle excitation-contraction coupling. Published structures of RyR1 showed RyR1 is open only in the assistance of exogenous regulators, such as caffeine, ryanodine, PCB-95 and diamide. Here, we report that with a mild purification procedure, a single transmembrane protein junctin is co-purified with RyR1. RyR-junctin complex can be activated to open state by Ca2+ only. Junctin inserts its transmembrane helix next to S1, S4 and S4-5 linker of RyR1, exerting an out-ward pushing force to the TM domain, facilitating the dilation of S6. Junctin regulates the channel conformation by directly affecting the TM domain via Jun/S1-4–S4-5 linker. Additional ATP increases the channel open probability by further stabilizing the channel in the open state. Our results demonstrate junctin forms an intrinsic complex with RyR1 and play a key role in the channel gating of RyR1 in physiological context.
