Cells were grown in minimal medium containing 5 mM K+ with or without 0.4 M sodium chloride. Cells were harvested in the mid-logarithmic growth phase, and β-galactosidase activity was determined, given in Miller Units [39]. The data are average values obtained from at least three independent experiments, and error bars represent standard deviations. Usp proteins form homodimers and oligomers, thus it is conceivable that UspC interacts with KdpD-UspC and buy BIBW2992 thereby facilitates scaffolding. Although the Salmonella KdpD-Usp domain has the highest degree of similarity to the E. coli KdpD-Usp-domain, scaffolding find more by UspC seemed to be abolished. The induction level supported by this chimera

was comparable to wild-type KdpD in a ΔuspC mutant [19]. Scaffolding might also be prevented in Agrocoli-KdpD. These data underline the importance of the KdpD-Usp domain for scaffolding the KdpD/KdpE signaling cascade under salt stress. The negative results obtained for all other KdpD chimeras might be explained by steric hindrance of the protein dynamics due to binding of other Usp proteins, major structural

changes, or altered enzymatic activities. The response of KdpD-Usp chimeras towards K+ limitation All KdpD derivatives with altered osmosensing properties characterized thus far [8, 10, 12] were able to respond to K+ limitation. To test the response toward K+ limitation, cells producing the KdpD-Usp chimeras were grown in minimal this website media containing different K+ concentrations. In wild-type cells, kdpFABC expression is repressed when cells are grown in medium that contains 10 mM K+, and induced under K+ limiting conditions (0.2 mM K+) (Fig. 5). As shown earlier [19, 27, 28], the Kdp system is induced under K+ limitation to a much higher level than in response to salt stress. None of the KdpD-Usp chimeras induced kdpFABC expression at a high K+ concentration. As expected from the salt stress study, cells producing KdpD-UspC, Streptocoli-KdpD, or

Agrocoli-KdpD induced kdpFABC expression similar to wild-type KdpD. Moreover, KdpD-UspA, KdpD-UspD and Pseudocoli-KdpD were able to respond to K+ limitation, although the β-galactosidase activities were significantly reduced in Pseudocoli-KdpD and KdpD-UspD. Cells Cepharanthine producing these chimeras were exposed to even more severe K+ limitation (0.1 mM), and kdpFABC expression levels increased to wild-type levels, indicating that these two chimeras retain the ability to sense K+ limitation (data not shown). Unexpectedly, KdpD-UspF and KdpD-UspG were unable to induce kdpFABC expression under all conditions tested ([K+] = 0.1 – 115 mM, data not shown). These are the first two KdpD derivatives with alterations in the N-terminal domain that completely prevent kdpFABC expression. These results reveal that the KdpD-Usp domain is not only a binding partner for UspC but is somehow involved in KdpD/KdpE signaling. Figure 5 The response of different KdpD-Usp chimeras to K + limitation.