Calcium release units (CRUs) are stable sites of junction between junctional domains of sarcoplasmic reticulum (SR) and the exterior membranes (plasmalemma and T tubules) of muscle cells. Rapid calcium release from the SR, via the ryanodine receptor (RyR), is under the control of the dihydropyridine receptor (DHPR) of exterior membranes, a slowly activating calcium channel. At the heart of this interaction is a large supramolecular complex that reaches from the lumen of the SR to the extracellular surface of the plasmalemma (or lumen of T tubules) and includes calsequestrin; the luminal and intramembrane domains of triadin and junctin; the intramembrane and cytoplasmic domains of RyR; the DHPR. A number of accessory proteins in and around the interacting RyR/DHPR pair contribute to the mechanical stability of the junction and modulate the DHPR./RyR interaction and/or the properties of the two channels. Structural probes of calcium release units (CRU) have been instrumental in defining the architecture of the junction and establishing a correlation between a DHPR/RyR link and the functional cross talk the two molecules. In the continuation of this project, we will further probe the molecular domains that allow the RyR/DHPR link to be formed, and search for direct evidence for the conformational coupling between RyR and DHPR that must underlie their interaction. Still using structural approaches, we further propose to expand into a quest for the structural role of proteins that may play a specific role in the SR-plasmalemma docking (junctophilin) and on the recruiting and/or maintenance of a high concentration of RyR and DHPR withi9n CRUs (possibly Homer). Finally, we will define the role of triadin and junctin in establishing and maintaining location of calsequestrin within the CRU. Appropriate assembly and function of CRUs are essential for normal muscle physiology.
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