Panel A shows electron microscopy images and a structural schematic of densely packed membrane protein assemblies arranged in an ordered lattice at a membrane junction. Panel B shows an electron microscopy image revealing repeated groups of four particles distributed in a regular pattern across the membrane surface. Panel C shows a simplified molecular arrangement illustrating alternating protein interactions and a right-handed geometric organization within the junctional complex.
Components of a junction between a transverse (T) tubule and an SR terminal cisterna in skeletal muscle. (A) Freeze-dried junctional SR membrane from guinea pig showing RyR1 tetramers in checkerboard formation. (B) Tetrads of CaVs in a freeze-fractured T tubule membrane from toadfish muscle, presented with the same orientation and magnification. (C) Canonical couplon, in which every other RyR is in contact with CaVs (orange elements), thus constituting the “skipping pattern.” The diagram illustrates chirality or handedness, as viewed from outside the cell, in the way RyR tetramers make mutual contact. This orientation, called right-handed, has the inter-tetramer approaches occurring at the tetramers’ edge, to the right-hand side of every corner. Relabeled Fig. 1 of Ríos et al. (2019), which includes images from Block et al. (1988).