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 (PubMed:9449685). 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 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 skin, kidney and distal colon. Up-regulated in kidney and down-regulated in colon in response to K(+) ion free diet. Mice are born at the expected Mendelian rate. Belongs to the cation transport ATPase (P-type) (TC 3.A.3) family. Type IIC subfamily.