The sarcoplasmic reticulum (SR) is an intracellular membrane compartment that controls intracellular calcium concentration and therefore plays a role in excitation-contraction coupling. JP-45 (also JP45; encoded by JSRP1) is an integral protein constituent of the skeletal muscle sarcoplasmic reticulum junctional face membrane interacting with Ca(v) 1.1 (the α.1 subunit of the voltage-sensing dihydropyridine receptor, DHPR) and the luminal calcium-binding protein calsequestrin. Mouse and human JP45 share 50% amino acid identity overall and 91% identity within the transmembrane domain. Two JSRP1 variants have been found in the human population: c.323C>T (p.P108L) in exon 5 and c.449G>C (p.G150A) in exon 6.