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.


CONCLUSIONS
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.
Heermann
Ralf
R
Ludwig-Maximilians-Universität München, Biozentrum, Bereich Mikrobiologie, D-82152 Martinsried, Germany. heermann@lmu.de
Lippert
Marie-Luise
ML
Jung
Kirsten
K
D000595:Amino Acid Sequence; D001665:Binding Sites; D004926:Escherichia coli / Q000235:genetics* / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000235:genetics / Q000378:metabolism*; D006360:Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D010766:Phosphorylation; D011188:Potassium / Q000378:metabolism; D054730:Protein Interaction Domains and Motifs; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D016415:Sequence Alignment; D015398:Signal Transduction; D012965:Sodium Chloride / Q000378:metabolism; D013312:Stress, Physiological
D029968:Escherichia coli Proteins; D006360:Heat-Shock Proteins; D011994:Recombinant Proteins; C538839:universal stress protein A, E coli; D012965:Sodium Chloride; C489580:KdpD protein, E coli; D011494:Protein Kinases; D011188:Potassium