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 (By similarity). Inhibits NKA activity in its unphosphorylated state and stimulates activity when phosphorylated (PubMed:17283221). Reduces glutathionylation of the NKA beta-1 subunit ATP1B1, thus reversing glutathionylation-mediated inhibition of ATP1B1 (By similarity). Contributes to female sexual development by maintaining the excitability of neurons which secrete gonadotropin-releasing hormone (By similarity). 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 (By similarity). The monomeric form associates with NKA while the oligomeric form does not (By similarity). Interacts with the catalytic alpha-1 subunit ATP1A1 (PubMed:17283221, PubMed:19339511, PubMed:23532852). Also interacts with the catalytic alpha-2 and alpha-3 subunits ATP1A2 and ATP1A3 (PubMed:23532852). Very little interaction with ATP1A1, ATP1A2 or ATP1A3 when phosphorylated at Ser-83 (PubMed:23532852). Interacts with the non-catalytic beta-1 subunit ATP1B1 (PubMed:23532852). Oxidative stress decreases interaction with ATP1A1 but increases interaction with ATP1B1 (By similarity). Detected in the apical cell membrane in brain (PubMed:12657675). In myocytes, localizes to sarcolemma, t-tubules and intercalated disks (PubMed:23532852). In adult brain, highest levels are found in the cerebellum and in the lateral, third and fourth ventricles of the choroid plexus (at protein level) (PubMed:12657675). Also detected in cells of a portion of the ependymal lining of the lateral ventricle on its rostral surface posterior to the caudate putamen (at protein level) (PubMed:12657675). Expressed in a subset of neurons which secrete gonadotropin-releasing hormone (PubMed:19187398). In the medial basal hypothalamus, levels are low at birth and increase during neonatal and infantile development to reach a maximum during the mid-to-late juvenile period at postnatal days 24-30. 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 (PubMed:17283221). 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 (PubMed:23532852). May be phosphorylated by DMPK (By similarity). Palmitoylation increases half-life and stability and is enhanced upon phosphorylation at Ser-88 by PKA. Belongs to the FXYD family.