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 binds the extracellular potassium ions and delivers the ions to the membrane domain of KdpB through an intramembrane tunnel. The system is composed of three essential subunits: KdpA, KdpB and KdpC. By low potassium concentration. Belongs to the KdpA family.