The catalytic subunit of a H(+)/K(+) ATPase and/or Na(+)/K(+) ATPase pump which transports K(+) ions in exchange for Na(+) and/or H(+) ions across the apical membrane of epithelial cells. Uses ATP as an energy source to pump K(+) ions into the cell while transporting Na(+) and/or H(+) ions to the extracellular compartment (By similarity). Involved in the maintenance of electrolyte homeostasis through K(+) ion absorption in kidney and colon (By similarity). In the airway epithelium, may play a primary role in mucus acidification regulating its viscosity and clearance (By similarity). ATP + H(+)(in) + H2O + K(+)(out) = ADP + 2 H(+)(out) + K(+)(in) + phosphate ATP + H2O + K(+)(out) + Na(+)(in) = ADP + H(+) + K(+)(in) + Na(+)(out) + phosphate The X(+)/K(+) ATPase pump is composed of a catalytic alpha subunit and an auxiliary non-catalytic beta subunit. The alpha subunit pairs with the beta subunit of gastric H(+)/K(+) ATPase ATP4B or the beta subunit of Na(+)/K(+) ATPases ATP1B1 and ATP1B3; this interaction is required for the formation of a functionally active pump and its targeting at the plasma membrane. Found in the skin, kidney, distal colon and brain. In the kidney it is found in the connecting tubule, cortical collecting duct and outer medullary collecting duct while in the brain it is specific to choroid plexus and cortex. Belongs to the cation transport ATPase (P-type) (TC 3.A.3) family. Type IIC subfamily.