K(+)/H(+) antiporter that extrudes potassium in exchange for external protons. Can also catalyze NH(4)(+)/H(+) antiport. Could have weak activity with Na(+). Optimum pH is 7.5. Exhibits K(+)/H(+) antiporter activity between pH 7.5 and 9.5. Belongs to the monovalent cation:proton antiporter 1 (CPA1) transporter (TC 2.A.36) family.