Kdp of E.coli was first identified from the analysis of matants that could not growth on low concentrations of potassium. It three-subunit enzyme whose subunit ratio appears to be ABC, although the exact stoichiometry is unknow. The KdpB protein spans the membrane seven times, is homologous to other P-type ATPases, and is the subunit that is phosphorelated by ATP (putative phosphorylation site Asp307), which couples energy to transport. The KdpA subunit, which tranverythe membrane 10 times, binds K+ and probably forms most, if not all, of the membrane channel for K+