, 2005). Indeed, biochemical evidence was obtained that KdpE undergoes a monomer-to-dimer transition upon phosphorylation (Lucassen, 1998). Histidine kinase/response regulator systems are often referred to as ‘two-component systems’ based on the assumption that they consist of only two components. Meanwhile, many systems are known that include accessory proteins responsible for stimulus perception, fine-tuning, cross-talk, or signal integration (Island & Kadner, 1993; Kato & Groisman, 2004; Eguchi et al., 2007; Fleischer et al., 2007; Paul et al., 2008). Accessory

proteins were also identified for BAY 73-4506 concentration the KdpD/KdpE system. The universal stress protein UspC was identified as a scaffolding protein of the KdpD/KdpE signaling cascade by interacting with the Usp domain in KdpD under salt stress (Fig. 2b) (Heermann et al., 2009b). Usp proteins are small soluble proteins that accumulate under diverse stress conditions. They are widespread in living organisms, but their physiological role is poorly understood (Kvint et al., 2003). Scaffolding

proteins are usually known from eukaryotes. These proteins connect proteins and enhance the binding properties in a signaling pathway and thus influence signal transduction (Pawson & Scott, 1997; Garrington & Johnson, 1999; Burack & Shaw, 2000). Under K+-limiting conditions, the Kdp system restores the intracellular K+ concentration, while in response to salt stress, K+ is accumulated far above the normal content TSA HDAC concentration by rapid uptake via Trk. Nevertheless, the Kdp system is induced under salt stress. Because the kinase activity of KdpD is inhibited at high concentrations of K+ (Jung et al., 2000), it has been puzzling how the sensor can be activated in response to salt stress. KdpD has a Usp domain within the N-terminal input domain belonging to the UspA subfamily, and it was hypothesized that KdpD might interact with one or more UspA-subfamily proteins (Heermann et al., 2009b). Escherichia coli encodes Diflunisal three single domain proteins of this subfamily, UspA, UspC, and UspD, and the expression of the corresponding

genes is upregulated under various stress conditions including salt stress (Gustavsson et al., 2002). Among these proteins, only UspC stimulated the in vitro reconstructed signaling cascade (KdpDKdpEDNA), resulting in phosphorylation of KdpE at a K+ concentration that would otherwise almost prevent phosphorylation. In agreement, in a ΔuspC mutant, KdpFABC production was significantly downregulated when cells were exposed to salt stress, but unaffected under K+ limitation. Biochemical studies revealed that UspC specifically interacts with the Usp domain in the stimulus-perceiving N-terminal domain of KdpD. UspC does not influence the enzymatic activities of KdpD, but stabilizes the KdpD/phospho-KdpE/DNA complex. Therefore, UspC can be regarded as one of the rare examples of bacterial scaffolding proteins (Heermann et al., 2009b).