Cryo-EM investigation of the subconductance state of ryanodine receptor calcium release channels induced by scorpion toxins.

Ryanodine Receptors (RyRs) are ion channels that release Ca2+ from the endoplasmic or sarcoplasmic reticulum. They are known to undergo subconductance states, but such states have thus far remained elusive. Calcins are scorpion-derived peptides that can permeate the plasma membrane, thus gaining access to intracellular ion channels. They bind to RyRs with high potency and induce long-lived subconductance states. Using cryo-EM and functional assays, we describe the binding site of imperacalcin on RyR1, and show that binding of calcins results in extended ion conduction pathways with radii and electrostatic profiles that explain the subconductance. Calcin binding also results in large asymmetry throughout the RyR, including intersubunit breaks. Kinase assays show that calcins are excellent substrates for cAMP-dependent kinase, and our data show that this obliterates binding to the RyR, explaining how post-translational modifications can affect the fate of a cell-permeating peptide.