A slice through a filtered tomogram showing SR vesicles in contact with a putative T-tubule. Individual RyR1 molecules are indicated by the red circles. The dense protein coat on the surface of the SR vesicles correspond to the SERCA pump. Areas circled in yellow indicate accumulation of density inside the SR. Scale bar: 20 nm. A slice through the structure of RyR1 in contact with putative TT-membrane. A middle slice through the in situ structure of RyR1 at 12.6-Å resolution. The observed additional transmembrane density is indicated by the red arrows. Scale bars in B, C: 10 nm. 