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 (By similarity). The Kdp system is essential for growth under K(+) limitation, and for survival under desiccation and salt crystal inclusion (PubMed:18633573, PubMed:23757278). The system is composed of three essential subunits: KdpA, KdpB and KdpC. The complex also contains KdpF, a small non-essential subunit. Up-regulated in response to K(+) limitation (PubMed:18633573, PubMed:21947979). Induced under desiccating conditions (PubMed:23757278). kdpFABCQ and kdpFABC deletion strains are only able to grow in the presence of K(+) concentrations above 60 uM. Belongs to the KdpA family.