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 (PubMed:10644526, PubMed:7560093). 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 Up-regulated by K(+) ions in a dose-dependent way. 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. Expressed at high levels in distal colon, coagulating and preputial glands; at much lower levels in proximal colon, kidney, uterus, brain, placenta and lung; and at trace levels in heart and forestomach (PubMed:9872395). Expressed in distal colon epithelium (at protein level) (PubMed:7560093). Expressed in anterior prostate (at protein level) (PubMed:14749213). Belongs to the cation transport ATPase (P-type) (TC 3.A.3) family. Type IIC subfamily.