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 (By similarity). Monomer (By similarity). Regulatory subunit of the sodium/potassium-transporting ATPase (NKA) which is composed of a catalytic alpha subunit, a non-catalytic beta subunit and an additional regulatory subunit (PubMed:15563542). The monomeric form associates with NKA while the oligomeric form does not (By similarity). Interacts with the catalytic alpha-1 subunit ATP1A1 (PubMed:15563542). 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. 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. Phosphorylated in response to insulin and adrenergic stimulation. Phosphorylation at Ser-83 stimulates sodium/potassium-transporting ATPase activity while the unphosphorylated form inhibits sodium/potassium-transporting ATPase activity. Phosphorylation increases tetramerization, decreases binding to ATP1A1 and reduces inhibition of ATP1A1 activity. Phosphorylation at Ser-75 leads to greatly reduced interaction with ATP1A1, ATP1A2 and ATP1A3. May be phosphorylated by DMPK. Palmitoylation increases half-life and stability and is enhanced upon phosphorylation at Ser-83 by PKA. Belongs to the FXYD family.