RyR1–RyR1 structures in native membranes. (A) RyR1 in TC-enriched SR fractions from rabbit muscle. Images, obtained from cryo-electron tomography and subtomogram averaging, reached a resolution of 17.5 Å. ryRyR1 names preparations in high Ca²⁺ and ryanodine, presumably an open channel configuration. Views are through the middle slice of the reconstruction (top panels) and at the level of intra-SR extensions (bottom panel). In the left panel, the outer domains in the mushroom cap are displaced downward from their positions in the closed structure (in red oval; movement also indicated by red arrows). In the lower panel, arcs in red trace the fitted curvatures of the SR membrane. (B) Rendering of apoRyR1 and ryRyR1 (mirror-flipped) viewed from the cytoplasm. Again, red arrows mark regions that move the most, in this case on the putatively open structure. (C) Section through the isosurface, with plane indicated by the dashed line in B. Red arrows indicate movement of the SR extensions (dark orange circle: apoRyR1; blue, ryRyR1). Extensions are visible also in-between the SR membrane leaflets (green arrow in A). Reproduced with minor modifications from Fig. 4 of Chen and Kudryashev (2020) by courtesy of Misha Kudryashev.