The sodium/potassium-transporting ATPase (ATP1A:ATP1B:FXYD) is composed of three subunits - alpha (catalytic part), beta and gamma. The trimer catalyzes the hydrolysis of ATP coupled with the exchange of sodium and potassium ions across the plasma membrane, creating the electrochemical gradient which provides energy for the active transport of various nutrients.
Four human genes encode the catalytic alpha subunits, ATP1A1-4 (Kawakami et al, 1986; Shull et al, 1989; Ovchinnikov et al, 1988; Keryanov and Gardner, 2002). Defects in ATP1A2 cause alternating hemiplegia of childhood (AHC) (Swoboda et al, 2004). Another defect in ATP1A2 causes familial hemiplegic migraine type 2 (FHM2) (Vanmolkot et al, 2003). Defects in ATP1A3 are the cause of dystonia type 12 (DYT12) (de Carvalho Aguiar et al, 2004).
Three human genes encode the non-catalytic beta subunits, ATP1B1-3. The beta subunits are thought to mediate the number of sodium pumps transported to the plasma membrane (Lane et al, 1989; Ruiz et al, 1996; Malik et al, 1996). FXYD proteins belong to a family of small membrane proteins that are auxiliary gamma subunits of Na-K-ATPase. At least six members of this family, FYD1-4, 6 and 7, have been shown to regulate Na-K-ATPase activity (Geering 2006, Choudhury et al. 2007). Defects in FXYD2 are the cause of hypomagnesemia type 2 (HOMG2) (Meij et al, 2000). ATP1A1-4 and ATP1B1-4 play a minor role during phase 2, since they begin to restore ion concentrations. The high concentration of intracellular calcium starts contraction of those cells, which is sustained in the plateau phase.