, 1996). As described above, this domain contains a ‘Walker-type’ ATP-binding site (Jung & Altendorf, 1998b). Truncated forms of KdpD lacking this site (KdpD/Δ12–228, KdpD/Δ12–395) were characterized Z-VAD-FMK in vivo by a deregulated phosphatase activity. In contrast, the sole N-terminal cytoplasmic domain (KdpD/1–395)

caused constitutive expression of kdpFABC in vivo (Heermann et al., 2003a). Detailed biochemical studies revealed a stabilizing function of the N-terminal domain of KdpD in complex with phospho-KdpE and the corresponding DNA-binding site (Heermann et al., 2003a, 2009a). Many bacteria, for example the cyanobacterium Anabaena sp., have a KdpD homologue that comprises only the N-terminal domain without the C-terminal transmitter domain and the transmembrane helices (Ballal et al., 2005). selleck chemicals llc Replacement of the N-terminal domain of E. coli KdpD with the short KdpD version of Anabaena resulted in a chimera that was functional in E. coli in vivo and in vitro (Ballal et al., 2002). This result suggested that Anabaena KdpD functions in a manner similar to the N-terminal domain of E. coli KdpD. The Usp domain within the N-terminal domain functions as a binding surface for the universal stress protein UspC, and it was shown to be important for internal protein dynamics, allowing structural alterations within

KdpD upon stimulus perception (Heermann et al., 2009b). Using a ‘domain-swapping’ approach, the Usp domain within KdpD was replaced by KdpD-Usp domains of various bacteria and the six soluble universal stress proteins of E. coli, respectively. In vivo and in vitro analyses of these KdpD chimeras revealed that signaling within KdpD involves alterations of electrostatic interactions.

Chimeras containing UspF or UspG not only prevented kdpFABC expression under salt stress but also under K+ limitation, although these hybrid proteins exhibited kinase and phosphatase activities in vitro (Heermann et al., 2009a). Analysis of the predicted wild-type KdpD-Usp tertiary structure revealed that this domain has a net positively charged surface, while both UspF and UspG are characterized by net negatively charged surfaces (Heermann et al., 2009a). It is proposed that the positively O-methylated flavonoid charged Usp domain interacts with other positively charged residues in KdpD shifting the histidine kinase into the ‘ON’ state by electrostatic repulsion (Fig. 2a). Chimeras containing the negatively charged UspF or UspG remain in the ‘OFF’ state due to electrostatic attraction. A possible explanation as to why KdpD-UspF and KdpD-UspG are active in vitro, but block kdpFABC expression in vivo might be that the stabilization of the phospho-KdpE/DNA complex by the N-terminal domain of KdpD is prevented in the ‘OFF’ state (Fig. 2a) (Heermann et al., 2003a). KdpE belongs to the OmpR/PhoB response regulator family.