Block B A, Imagawa T, Campbell K P, Franzini-Armstrong C
Department of Biology, University of Pennsylvania, Philadelphia 19104.
J Cell Biol. 1988 Dec;107(6 Pt 2):2587-600. doi: 10.1083/jcb.107.6.2587.
The architecture of the junctional sarcoplasmic reticulum (SR) and transverse tubule (T tubule) membranes and the morphology of the two major proteins isolated from these membranes, the ryanodine receptor (or foot protein) and the dihydropyridine receptor, have been examined in detail. Evidence for a direct interaction between the foot protein and a protein component of the junctional T tubule membrane is presented. Comparisons between freeze-fracture images of the junctional SR and rotary-shadowed images of isolated triads and of the isolated foot protein, show that the foot protein has two domains. One is the large hydrophilic foot which spans the junctional gap and is composed of four subunits. The other is a hydrophobic domain which presumably forms the SR Ca2+-release channel and which also has a fourfold symmetry. Freeze-fracture images of the junctional T tubule membranes demonstrate the presence of diamond-shaped clusters of particles that correspond exactly in position to the subunits of the feet protein. These results suggest the presence of a large junctional complex spanning the two junctional membranes and intervening gap. This junctional complex is an ideal candidate for a mechanical coupling hypothesis of excitation-contraction coupling at the triadic junction.
已对连接肌浆网（SR）和横小管（T小管）膜的结构以及从这些膜中分离出的两种主要蛋白质——兰尼碱受体（或足蛋白）和二氢吡啶受体的形态进行了详细研究。文中提供了足蛋白与连接T小管膜的一种蛋白质成分之间直接相互作用的证据。连接SR的冷冻蚀刻图像与分离的三联体以及分离的足蛋白的旋转阴影图像之间的比较表明，足蛋白有两个结构域。一个是大的亲水性足，它跨越连接间隙，由四个亚基组成。另一个是疏水性结构域，推测它形成了SR Ca2+释放通道，并且也具有四重对称性。连接T小管膜的冷冻蚀刻图像显示存在菱形颗粒簇，其位置与足蛋白的亚基完全对应。这些结果表明存在一个跨越两个连接膜和中间间隙的大型连接复合体。这个连接复合体是三联体连接处兴奋-收缩偶联的机械偶联假说的理想候选者。
