Three-Dimensional Location of the Imperatoxin a Binding Site on the Ryanodine Receptor

Cryo-electron microscopy and three-dimensional, single-particle image analysis have been used to reveal the specific binding site of imperatoxin A (IpTx_a) on the architecture of the calcium release channel/ryanodine receptor from skeletal muscle (RyR1). IpTx_a is a peptide toxin that binds with high affinity to RyR1 and affects its functioning. The toxin was derivatized with biotin to enhance its detection with streptavidin. IpTx_a binds to the cytoplasmic moiety of RyR1 between the clamp and handle domains, 11 nm away from the transmembrane pore. The proposed mimicry by IpTx_a of the dihydropyridine receptor (DHPR) II-III loop, thought to be a main physiological excitation-contraction trigger, suggests that the IpTx_a binding location is a potential excitation-contraction signal transduction site.