Architecture and regulation of the Ca2+ delivery system in muscle cells.

The junctional domain of sarcoplasmic reticulum (jSR) is specialized for receiving signals from the plasmalemma-transverse tubules and for releasing Ca2+ during muscle activation. The junctional face of the jSR, facing the transverse tubules, is occupied by a molecular complex composed of the transmembrane Ca2+ release channels (ryanodine receptors); the luminal protein calsequestrin (CSQ); the 2 membrane proteins, junctin (Jct), and triadin (Tr), which mediate CSQ-ryanodine receptor interactions; and several other components. Under the conditions prevailing within the sarcoplasmic reticulum lumen (physiological ionic strength, mostly due to K+ and Ca2+ ions), CSQ forms long linear polymers and the fixed protein gel is clearly visible in the electron microscope. The luminal domains of Jct and Tr are detectable but, overall, the 2 molecules are not clearly delineated. Cardiac muscles either overexpressing or bearing null mutations for 3 proteins of the junctional complex (CSQ, Jct, and Tr) reveal the contribution of these 3 components to the general architecture of the jSR.