Inhibits the activity of ATP2A1/SERCA1 ATPase in sarcoplasmic reticulum by decreasing the apparent affinity of the ATPase for Ca(2+), thereby acting as a key regulator of skeletal muscle activity. Its high expression in adult skeletal muscle, suggests that it constitutes the predominant regulator of ATP2A1/SERCA1 in adult skeletal muscle. Interacts with ATP2A1/SERCA1. Specifically expressed in all skeletal muscles. Not expressed in cardiac or smooth muscles. During embryogenesis, expressed in the myotomal compartment of the somites and the anlagen of skeletal muscle. During fetal and adult stages, strongly expressed in all skeletal muscles. Not detectable in cardiac or smooth muscles. Expression is regulated by MYEF2 and MYOD1. Mice were born at expected Mendelian ratios and did not show no obvious morphological abnormalities or differences in body or muscle weights. They however show enhanced Ca(2+) handling in skeletal muscle and improved exercise performance. Extended N-terminus.