Kdp-ATPase consists of three large protein subunits each having a distinct function. The 72-kD KdpB energy-coupling sub-unit is homologous to other P-type ATPases, and is the site of acylphosphorylation. Evidence points to Asp307 as the site of phosphorylation. The 59-kD KdpA subunit, predicted to span the membrane 12 times, appears to bind K+ for transport and to form the transmembrane channel for K+ movement. The 20.5-kD KdpC subunit appears to be necessary for assembly of the complex. KdpF, a 29-residue hydrophobic peptide, is also made, but its role is not known. Expression of Kdp is controlled at the transcriptional level by the 98.5-kD inner membrane-bound KdpD sensor kinase and the soluble, cytoplasmic 25-kD KdpE response regulator. KdpD is autophosphorylated and is able to transfer phosphate to KdpE. Reduced turgor pressure is believed to be the signal that stimulates the kinase activity of KdpD to create phospho-KdpE, which in turn stimulates transcription of the operon encoding the Kdp-ATPase.