Reversibly inhibits the activity of ATP2A1/SERCA1 and ATP2A2/SERCA2 in sarcoplasmic reticulum by decreasing the apparent affinity of the ATPase for Ca(2+) (PubMed:11781085, PubMed:9575189). Also inhibits the activity of ATP2A3/SERCA3 (By similarity). 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). {ECO:0000250|UniProtKB:Q9CQD6, ECO:0000269|PubMed:11781085, ECO:0000269|PubMed:9575189}. This is the function of SLN (sarcolipin, Ensembl gene identifier ENSG00000170290).