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. Also inhibits the activity of ATP2A2/SERCA2 and ATP2A3/SERCA3. . This is the function of MRLN (myoregulin, ENSG00000227877).