Reversibly inhibits the activity of ATP2A1 and ATP2A2 in sarcoplasmic reticulum by decreasing the apparent affinity of the ATPase for Ca(2+). Modulates calcium re-uptake during muscle relaxation and plays an important role in calcium homeostasis in muscle (PubMed:21697544, PubMed:26816378). Required for muscle-based, non-shivering thermogenesis (PubMed:22961106). Interacts with calcium ATPase ATP2A1/SERCA1. Interact with ATP2A2; the inhibition decreases ATP2A1 Ca(2+) affinity. Interacts with VMP1; VMP1 competes with PLN and SLN to prevent them from forming an inhibitory complex with ATP2A2 (By similarity). Highly expressed in heart atrium, red gastrocnemius muscle and soleus. Detected at lower levels in the extensor digitorum longus muscle (at protein level). No visible phenotype at 22 degrees Celsius. Increased heat-loss and failure to maintain body temperature at 4 degrees Celsius when the shivering response is prevented. Mice show increased weight gain and obesity when kept on a high-fat diet. Mice display faster muscle relaxation rates in soleus due to an increase in the affinity of ATP2A1 for Ca(2+). Belongs to the sarcolipin family.