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. Present in detergent-resistant membrane (DRM) fractions that may be equivalent to eukaryotic membrane rafts; these rafts include proteins involved in signaling, molecule trafficking and protein secretion. Belongs to the KdpA family.