Part of the high-affinity ATP-driven potassium transport (or Kdp) system, which catalyzes the hydrolysis of ATP coupled with the electrogenic transport of potassium into the cytoplasm. This subunit is responsible for energy coupling to the transport system and for the release of the potassium ions to the cytoplasm. ATP + H2O + K(+)(out) = ADP + H(+) + K(+)(in) + phosphate The system is composed of three essential subunits: KdpA, KdpB and KdpC. Belongs to the cation transport ATPase (P-type) (TC 3.A.3) family. Type IA subfamily.