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.