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 acts as a catalytic chaperone that increases the ATP-binding affinity of the ATP-hydrolyzing subunit KdpB by the formation of a transient KdpB/KdpC/ATP ternary complex