Associates with and regulates the activity of the sodium/potassium-transporting ATPase (NKA) which transports Na(+) out of the cell and K(+) into the cell. Inhibits NKA activity in its unphosphorylated state and stimulates activity when phosphorylated. Reduces glutathionylation of the NKA beta-1 subunit ATP1B1, thus reversing glutathionylation-mediated inhibition of ATP1B1. Contributes to female sexual development by maintaining the excitability of neurons which secrete gonadotropin-releasing hormone. Homotetramer (PubMed:16597826). Monomer (PubMed:17511473). Regulatory subunit of the sodium/potassium-transporting ATPase (NKA) which is composed of a catalytic alpha subunit, an auxiliary non-catalytic beta subunit and an additional regulatory subunit (By similarity). The monomeric form associates with NKA while the oligomeric form does not (By similarity). Interacts with the catalytic alpha-1 subunit ATP1A1 (By similarity). Also interacts with the catalytic alpha-2 and alpha-3 subunits ATP1A2 and ATP1A3 (By similarity). Very little interaction with ATP1A1, ATP1A2 or ATP1A3 when phosphorylated at Ser-83 (By similarity). Interacts with the non-catalytic beta-1 subunit ATP1B1 (By similarity). Oxidative stress decreases interaction with ATP1A1 but increases interaction with ATP1B1 (By similarity). Detected in the apical cell membrane in brain. In myocytes, localizes to sarcolemma, t-tubules and intercalated disks. Highest expression in skeletal muscle and heart. Moderate levels in brain, placenta, lung, liver, pancreas, uterus, bladder, prostate, small intestine and colon with mucosal lining. Very low levels in kidney, colon and small intestine without mucosa, prostate without endothelial lining, spleen, and testis. The cytoplasmic domain is sufficient to regulate sodium/potassium-transporting ATPase activity. Major plasma membrane substrate for cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) in several different tissues (By similarity). Phosphorylated in response to insulin and adrenergic stimulation (By similarity). Phosphorylation at Ser-88 stimulates sodium/potassium-transporting ATPase activity while the unphosphorylated form inhibits sodium/potassium-transporting ATPase activity (By similarity). Phosphorylation increases tetramerization, decreases binding to ATP1A1 and reduces inhibition of ATP1A1 activity (By similarity). Phosphorylation at Ser-83 leads to greatly reduced interaction with ATP1A1, ATP1A2 and ATP1A3 (By similarity). May be phosphorylated by DMPK (PubMed:10811636). Palmitoylation increases half-life and stability and is enhanced upon phosphorylation at Ser-88 by PKA. Belongs to the FXYD family.