The function of STRIT1 (small transmembrane regulator of ion transport 1, ENSG00000240045) is as follows. 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 (By similarity). Also enhances the activity of the ATP2A2/SERCA2 ATPase (PubMed:33581112). Does not directly stimulate SERCA pump activity (By similarity). 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 (By similarity). Competes with ATP2A2 inhibitor phospholamban (PLN) for binding to ATP2A2 and displaces PLN (PubMed:33581112). Can activate ATP2A2 directly in the absence of PLN (PubMed:33581112). Also enhances sarcoplasmic reticulum Ca(2+) uptake and myocyte contractility by displacing the SERCA inhibitory peptides sarcolipin (SLN) and myoregulin (MRLN) (By similarity). {ECO:0000250|UniProtKB:P0DN83, ECO:0000269|PubMed:33581112}.