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:17077088). Reduces glutathionylation of the NKA beta-1 subunit ATP1B1, thus reversing glutathionylation-mediated inhibition of ATP1B1 (PubMed:21454534). Induces a hyperpolarization-activated chloride current when expressed in Xenopus oocytes (PubMed:7836447). Decreases the apparent K+ and Na+ affinity of the sodium/potassium-transporting ATPase over a large range of membrane potentials. Decreases the apparent K+ affinity of the sodium/potassium-transporting ATPase only at slightly negative and positive membrane potentials and increases the apparent Na+ affinity over a large range of membrane potentials. 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:17077088). Interacts with catalytic alpha subunit ATP1A1 (PubMed:21454534). Also interacts with non-catalytic beta subunit ATP1B1 (By similarity). Interacts with the ATP1A1-ATP1B1, ATP1A2-ATP1B1 and ATP1A3-ATP1B1 NKA isozymes (PubMed:17077088). Isoform 1: Expressed mainly in differentiated cells (at protein level). Isoform 2: Expressed mainly in undifferentiated cells (at protein level). Glutathionylated. Marker of a cell type preferentially transformed by neu or ras oncoprotein. Belongs to the FXYD family.