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 (PubMed:15743908). Reduces glutathionylation of the NKA beta-1 subunit ATP1B1, thus reversing glutathionylation-mediated inhibition of ATP1B1 (By similarity). Induces a hyperpolarization-activated chloride current when expressed in Xenopus oocytes (By similarity). Regulatory subunit of the sodium/potassium-transporting ATPase which is composed of a catalytic alpha subunit, a non-catalytic beta subunit and an additional regulatory subunit (PubMed:15743908). Interacts with catalytic alpha subunit ATP1A1 (PubMed:15743908). Also interacts with non-catalytic beta subunit ATP1B1 (PubMed:15743908). Interacts with the ATP1A1-ATP1B1, ATP1A2-ATP1B1 and ATP1A3-ATP1B1 NKA isozymes (By similarity). Expressed at high levels in heart, skeletal muscle and liver with low levels of expression in breast, brain, lung, stomach and colon (PubMed:7836447). In the gastric gland, mainly expressed in the mucus cells forming the upper part of the gland and is absent from the parietal cells (PubMed:15743908). Glutathionylated. Marker of a cell type preferentially transformed by neu or ras oncoprotein. Belongs to the FXYD family.