Junctin

Junctin is a small, single-pass transmembrane protein located in the membrane of the sarcoplasmic reticulum (SR) in skeletal and cardiac muscle. It is a component of the calcium release unit that governs excitation-contraction coupling, contributing to the regulation of calcium release from the SR during muscle contraction.

Junctin has a luminal region that resides in the SR lumen and a cytoplasmic region that extends into the cytosol. In the SR lumen it binds to calsequestrin, the major calcium-binding protein inside the SR. On the cytoplasmic side, junctin interacts with triadin, another SR membrane protein, and these interactions help assemble a complex that links the SR calcium store to the ryanodine receptor (RyR) calcium release channel. Through these associations, junctin participates in modulating RyR activity in response to luminal calcium levels, thereby influencing the magnitude and timing of calcium release during muscle contraction.

Expression of junctin is detected in striated muscle, including cardiac and skeletal muscle, and it is generally discussed in the context of the cardiac RyR2 and skeletal RyR1 systems. In humans, junctin is encoded by the JCN gene.

Functionally, junctin contributes to the stability and regulation of the calcium release complex. Alterations in junctin expression or function have been studied in experimental models and are associated with disruptions to calcium homeostasis and excitation-contraction coupling, which can affect muscle performance and rhythmogenesis.

See also: ryanodine receptor, triadin, calsequestrin, junctophilin.