Domain swapping reveals that the N-terminal domain of the sensor kinase KdpD in Escherichia coli is important for signaling

Full response to salt stress was only achieved with a chimera that contains UspC, probably due to unaffected scaffolding of the KdpD/KdpE signaling cascade by soluble UspC. Unexpectedly, chimeras containing either UspF or UspG not only prevented kdpFABC expression under salt stress but also under K+ limiting conditions, although these hybrid proteins exhibited kinase and phosphotransferase activities in vitro. These are the first KdpD derivatives that do not respond to K+ limitation due to alterations in the N-terminal domain. Analysis of the KdpD-Usp tertiary structure revealed that this domain has a net positively charged surface, while UspF and UspG are characterized by net negative surface charges.The Usp domain within KdpD not only functions as a binding surface for the scaffold UspC, but it is also important for KdpD signaling. We propose that KdpD sensing/signaling involves alterations of electrostatic interactions between the large N- and C-terminal cytoplasmic domains.K+ plays an important role in turgor maintenance in bacteria [1]. KdpFABC is a high affinity K+ uptake system that serves as an emergency system to scavenge K+ when other transporters cannot sustain the cellular requirement for K+. The corresponding kdpFABC operon is under control of the two-component system KdpD/KdpE, which induces kdpFABC expression under K+ limiting conditions or under osmotic stress imposed by a salt [2,3]. Upon stimulus perception, KdpD undergoes autophosphorylation and subsequently, the phosphoryl group is transferred to the cytoplasmic response regulator KdpE [4]. Phosphorylated KdpE exhibits increased affinity for a 23-base pair sequence upstream of the canonical -35 and -10 regions of the kdpFABC promoter and triggers kdpFABC expression [5]. The enzymatic activities of purified KdpD and KdpE were determined in vitro [4]. All data known thus far indicate that KdpD does not sense a single specific parameter, but integrates the information of intracellular parameters imp