Structural insights into Ca2+-activated long-range allosteric channel gating of RyR1

Abstract

Ryanodine receptors (RyRs) are a class of giant ion channels with molecular mass over 2.2 mega-Daltons. These channels mediate calcium signaling in a variety of cells. Since more than 80% of the RyR protein is folded into the cytoplasmic assembly and the remaining residues form the transmembrane domain, it has been hypothesized that the activation and regulation of RyR channels occur through an as yet uncharacterized long-range allosteric mechanism. Here we report the characterization of a Ca2+-activated open-state RyR1 structure by cryo-electron microscopy. The structure has an overall resolution of 4.9 Å and a resolution of 4.2 Å for the core region. In comparison with the previously determined apo/closed-state structure, we observed long-range allosteric gating of the channel upon Ca2+ activation. In-depth structural analyses elucidated a novel channel-gating mechanism and a novel ion selectivity mechanism of RyR1. Our work not only provides structural insights into the molecular mechanisms of channel gating and regulation of RyRs, but also sheds light on structural basis for channel-gating and ion selectivity mechanisms for the six-transmembrane-helix cation channel family.

Ryanodine Receptor Structure and Function in Health and Disease

“…In 2015, the first atomic structures of RyR1 in its closed state were published by three groups, all taking advantage of this new data collection technology (Efremov et al 2015; Yan et al 2015; Zalk et al 2015). Meanwhile several additional structures in different states have been described for RyR1 (des Georges et al 2016; Wei et al 2016; Bai et al 2016) and for RyR2 (Peng et al 2016). The vast increase of knowledge presented by the cryo-EM studies in the past 2 years will be illuminated in the following sections of this review, and its implications will be discussed in the context of human disease.…”

“…11.2), thereby providing the allosteric coupling between the pore and the shell of the receptor. Presence of an EF-hand motif in the activation domain interacting with the pVSD hinted at a putative mechanism of activation by Ca 2+ (Wei et al 2016). This putative mechanism later proved not to be correct (Guo et al 2016; des Georges 2016), but this interaction between the EF-hand and the pVSD may have an important functional role, which remains to be determined.…”

“…The S2 helix and RyR-specific S2-S3 domain of the pVSD, together with the EF1/EF2 domain have also been implicated in Ca 2+ -dependent inactivation of RyR (Gomez and Yamaguchi 2014; Gomez et al 2016). It has been observed that the EF hand domain comes closer to the S2-S3 domain upon channel opening (des Georges et al 2016; Wei et al 2016). A tentative hypothesis could be that low affinity Ca 2+ -binding on one of those domains triggers a repulsion, pushing those two domains apart and thereby preventing channel opening.…”

How to open a Ryanodine Receptor

“…Again, cryo-EM has proven to be invaluable in solving this puzzle. In 2009, Samso et al [5] compared open and closed RyR1 at ~10 Å resolution, and this work has now been further refined at higher resolution by Wei et al [6] and Bai et al [7] in Cell Research. Both groups come to similar conclusions on the movements within the transmembrane region, and offer distinct viewpoints about the role of the cytosolic cap.…”

“…This offers two pathways to open the RyR: movement of the extended S6 helix, coupled to the U-motif, or movement of the VSL, which is also intimately coupled to the pore domain. Wei et al describe additional details, including a widening of the selectivity filter, and a movement of the S4 segment towards the lumen upon opening [6].…”