Effect of FKBP12 in the Conformational Dynamics of RyR1

Abstract

The ryanodine receptor (RyR) is an intracellular ion channel with an important role in depolarization-induced Ca2+ release in excitable cells. For the skeletal muscle isoform RyR1, the open and closed conformations were determined at 10 A resolution by cryoEM, and their structural analysis suggest that RyR1, with four inner helices forming a helical bundle, has a similar gating mechanism to that of K+ channels. Upon gating, RyR1's large cytoplasmic domain also undergoes conformational changes, underlying a long-range communication pathway between the ion gate and effectors bound more than 130 A away. One of such effectors is the FK506-binding protein of 12 kDa (FKBP12), which we localized at the interface of the “handle” and the “clamp” domains of RyR1. At the functional level, and under activating [Ca2+] concentrations, single channel experiments with purified RyR1 show that FKBP12 increased RyR1's Po and redistributed its subconductance states towards higher conductance levels. At the structural level, we found that under sub-activating [Ca2+] concentrations and in the absence of FKBP12, RyR1's cytoplasmic domain appears to be a structural intermediate of the well-defined open- and closed-state RyR1-FKBP12 conformations, whereas the inner helices remain in the closed conformation. Overall, our studies suggest that while [Ca2+] exerts a dominant effect in RyR1's gating, FKBP12 changes the energy landscape of RyR1's structural transitions.