KDPD AND KDPE, PROTEINS THAT CONTROL EXPRESSION OF THE KDPABC OPERON,ARE MEMBERS OF THE 2-COMPONENT SENSOR-EFFECTOR CLASS OF REGULATORS

The Kdp system of Escherichia coli, a transport ATPase with high affin ity for potassium, is expressed when turgor pressure is low. Expressio n requires KdpD, a 99-kDa membrane protein, and KdpE, a 25-kDa soluble cytoplasmic protein. The sequences of KdpD and KdpE show they are mem bers of the sensor-effector class of regulatory proteins: the C-termin al half of KdpD is homologous to sensors such as EnvZ and PhoR, and Kd pE is homologous to effectors such as OmpR and PhoB. The predicted str ucture of KdpD suggests that it is anchored to the membrane by four me mbrane-spanning segments near its middle, with both C- and N-terminal portions in the cytoplasm. Subcellular fractionation confirms the expe cted location of the protein in the inner membrane. The N-terminal reg ion has no homology to known proteins and is the site of mutations tha t make Kdp expression partially constitutive; this portion may serve t o sense turgor pressure. Since several other sensor-effectors have bee n shown to mediate control through phosphorylation, this mechanism is proposed to control expression of Kdp.