Ion-pulling simulations provide insights into the mechanisms of channel opening of the skeletal muscle ryanodine receptor

Abstract

The type 1 ryanodine receptor (RyR1) mediates Ca²⁺ release from the sarcoplasmic reticulum to initiate skeletal muscle contraction and is associated with muscle diseases, malignant hyperthermia, and central core disease. To better understand RyR1 channel function, we investigated the molecular mechanisms of channel gating and ion permeation. An adequate model of channel gating requires accurate, high-resolution models of both open and closed states of the channel. To this end, we generated an open-channel RyR1 model using molecular simulations to pull Ca²⁺ through the pore constriction site of a closed-channel RyR1 structure determined at 3.8 Å resolution. Importantly, we find that our open-channel model is consistent with the RyR1 and cardiac RyR (RyR2) openchannel structures reported while this paper was in preparation. Both our model and the published structures show similar rotation of the upper portion of the pore-lining S6 helix away from the four-fold channel axis and twisting of I4937 at the channel constriction site out of the channel pore. These motions result in a minimum open-channel pore radius of ~3 Å formed by Q4933, rather than I4937 in the closed-channel structure. We also present functional support for our model by mutations around the closed- and open-channel constriction sites (Q4933 and I4937). Our results indicate that use of ion-pulling simulations produces a RyR1 open-channel model, which can provide insights into the mechanisms of channel opening complementing those from the structural data.