The hydrophilic N-terminal domain complements the membrane-anchored C-terminal domain of the sensor kinase KdpD of Escherichia coli.

The putative turgor sensor KdpD is characterized by a large, N-terminal domain of about 400 amino acids, which is not found in any other known sensor kinase. Comparison of 12 KdpD sequences from various microorganisms reveals that this part of the kinase is highly conserved and includes two motifs (Walker A and Walker B) that are very similar to the classical ATP-binding sites of ATP-requiring enzymes. By means of photoaffinity labeling with 8-azido-[alpha-(32)P]ATP, direct evidence was obtained for the existence of an ATP-binding site located in the N-terminal domain of KdpD. The N-terminal domain, KdpD/1-395, was overproduced and purified. Although predicted to be hydrophilic, it was found to be membrane-associated and could be solubilized either by treatment with buffer of low ionic strength or detergent. The membrane-associated form, but not the solubilized one, retained the ability to bind 8-azido-[alpha-(32)P]ATP. Previously, it was shown that the phosphatase activity of a truncated KdpD, KdpD/Delta12-395, is deregulated in vitro (Jung, K., and Altendorf, K. (1998) J. Biol. Chem. 273, 17406-17410). Here, we demonstrated that this effect was reversed in vesicles containing both the truncated KdpD and the N-terminal domain. Furthermore, coexpression of kdpD/Delta12-395 and kdpD/1-395 restored signal transduction in vivo. These results highlight the importance of the N-terminal domain for the function of KdpD and provide evidence for an interaction of this domain and the transmitter domain of the sensor kinase.