Architecture and regulation of the Ca2+delivery system in muscle cellsThis paper is one of a selection of papers published in this Special Issue, entitled 14th International Biochemistry of Exercise Conference – Muscles as Molecular and Metabolic Machines, and has undergone the Journal’s usual peer review process.

Abstract

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.