Enhances the activity of ATP2A1/SERCA1 ATPase in sarcoplasmic reticulum by displacing ATP2A1/SERCA1 inhibitors, thereby acting as a key regulator of skeletal muscle activity. Also enhances the activity of the ATP2A2/SERCA2 ATPase. Does not directly stimulate SERCA pump activity. Binds preferentially to the phosphorylated E1 and E2 conformational forms of ATP2A2 which predominate at high Ca(2+) concentrations during the systolic phase of the cardiac cycle. Competes with ATP2A2 inhibitor phospholamban (PLN) for binding to ATP2A2 and displaces PLN. Can activate ATP2A2 directly in the absence of PLN. Also enhances sarcoplasmic reticulum Ca(2+) uptake and myocyte contractility by displacing the SERCA inhibitory peptides sarcolipin (SLN) and myoregulin (MRLN)