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. Required for muscle-based, non-shivering thermogenesis (By similarity). 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. Belongs to the sarcolipin family.