The universal stress protein UspC scaffolds the KdpD/KdpE signaling cascade of Escherichia coli under salt stress

A new mechanism of phosphoregulation in signal transduction pathways

Histidine protein kinases and serine, threonine, or tyrosine protein kinases play essential roles in signal transduction in prokaryotes and eukaryotes. A third type of protein kinase, an arginine protein kinase, has been identified. McsB of Bacillus subtilis phosphorylates the heat shock transcriptional regulator CtsR and can be regarded as the founding member of arginine protein kinases.

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

BACKGROUND

The KdpD/KdpE two-component system of Escherichia coli regulates expression of the kdpFABC operon encoding the high affinity K+ transport system KdpFABC. The input domain of KdpD comprises a domain that belongs to the family of universal stress proteins (Usp). It has been previously demonstrated that UspC binds to this domain, resulting in KdpD/KdpE scaffolding under salt stress. However the mechanistic significance of this domain for signaling remains unclear. Here, we employed a "domain swapping" approach to replace the KdpD-Usp domain with four homologous domains or with the six soluble Usp proteins of E. coli.

RESULTS

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.

CONCLUSION

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.

Stimulation of the potassium sensor KdpD kinase activity by interaction with the phosphotransferase protein IIA(Ntr) in Escherichia coli

Proteins EI(Ntr), NPr and IIA(Ntr) form a phosphoryl group transfer chain (Ntr-PTS) working in parallel to the phosphoenolpyruvate:carbohydrate phosphotransferase system (transport-PTS) in Escherichia coli. Recently, it was shown that dephosphorylated IIA(Ntr) binds and inhibits TrkA, a low-affinity potassium transporter. Here we report that the Ntr-PTS also regulates expression of the high-affinity K+ transporter KdpFABC, which rescues K+ uptake at limiting K+ concentrations. Transcription initiation at the kdpFABC promoter is positively controlled by the two-component system KdpD/KdpE in response to K+ availability. We found that kdp promoter activity is stimulated by the dephosphorylated form of IIA(Ntr). Two-hybrid data and biochemical analysis revealed that IIA(Ntr) interacts with sensor kinase KdpD and stimulates kinase activity, resulting in increased levels of phosphorylated response regulator KdpE. The data suggest that exclusively dephosphorylated IIA(Ntr) binds and activates KdpD. As there is cross-talk between the Ntr-PTS and the transport-PTS, carbon source utilization affects kdpFABC expression. Expression is enhanced, when cells utilize preferred carbohydrates like glucose, which results in preferential dephosphorylation of the transport-PTS and also of IIA(Ntr). Taken together, the data show that the Ntr-PTS has an important role in maintaining K+ homeostasis and links K+ uptake to carbohydrate metabolism.