8.A.28.1.5
Junctate, Junctin, Aspartate beta-hydroxylase, ASPH, BAH of 758 aas and 1 N-terminal TMS. It has enzyme activity, but is also a Ca²⁺-sensing ER protein, a structural component of ER-PM junctions where Orai1 and STIM1 cluster and interact in T cells (Srikanth et al. 2012). It plays a role in cardiac Ca²⁺ handeling (homeostatis), contractility and heart failure (Gergs et al. 2007).  Two other proteins of the complex are: Triadin and Calsequestrin. Junctin is a transmembrane protein of striated muscles, located at the junctional sarcoplasmic reticulum (SR). It is characterized by a luminal C-terminal tail, through which it functionally interacts with calsequestrin and the ryanodine receptor (RyR). In non-muscle cells, junctin and calsequestrin assemble in long linear regions within the endoplasmic reticulum, mirroring the formation of calsequestrin polymers (Rossi et al. 2022). In differentiating myotubes, the two proteins colocalize at triads, where they assemble with other proteins of the junctional SR. Two KEKE motifs can bind calsequestrin, and stretches of charged amino acids downstream of these motifs can also bind calsequestrin and the RyR. Deletion of even one of these regions impaired the ability of junctin to localize at the junctional SR, suggesting that interaction with other proteins at this site represents a key element in junctin targeting (Rossi et al. 2022).