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 (PubMed:26816378, PubMed:30299255). Does not directly stimulate SERCA pump activity (PubMed:26816378). Enhances sarcoplasmic reticulum Ca(2+) uptake and myocyte contractility by displacing the SERCA inhibitory peptides sarcolipin (SLN), phospholamban (PLN) and myoregulin (MRLN) (PubMed:26816378, PubMed:30299255). Monomer (PubMed:30299255). Interacts with ATP2A1/SERCA1 (PubMed:26816378, PubMed:30299255). Highly expressed in heart (at protein level). Detected in heart and soleus, a postural muscle group of the hindlimb containing the highest enrichment of slow-twitch muscle fibers. Also expressed in diaphragm, which contains some slow-twitch fibers. Not detected in the quadriceps, a fast-twitch muscle group, or in cardiac atrial muscle. Not expressed in the prenatal heart but gradually increases in abundance postnatally. Mice produce offspring at expected Mendelian ratios. Slow-twitch skeletal muscle fibers display delayed Ca(2+) clearance and relaxation and reduced SERCA activity. No significant differences are observed in peak muscle force and no differences are observed in relaxation rates at low, non-tetanic stimulation frequencies. However at tetanus-inducing frequencies, relaxation rates are significantly slowed after tetanus.