The Rcs (regulator of capsule synthesis) signalling complex comprises the membrane-associated hybrid sensor kinases RcsC and RcsD, the transcriptional regulator RcsB and the two co-inducers RcsA and RcsF. Acting as a global regulatory network, the Rcs phosphorelay controls multiple cellular pathways including capsule synthesis, cell division, motility, biofilm formation and virulence mechanisms. Signal-dependent communication of the individual Rcs domains showing histidine kinase, phosphoreceiver, phosphoryl transfer and DNA-binding activities is characteristic and essential for the modulation of signal transfer. We have analysed the structures of core elements of the Rcs network including the RcsC-PR (phosphoreceiver domain of RcsC) and the RcsD-HPt (histidine phosphotransfer domain of RcsD), and we have started to characterize the dynamics and recognition mechanisms of the proteins. RcsC-PR represents a typical CheY-like α/β/α sandwich fold and it shows a large conformational flexibility near the active-site residue Asp875. NMR analysis revealed that RcsC-PR is able to adopt preferred conformations upon Mg2+ co-ordination, BeF3 activation, phosphate binding and RcsD-HPt recognition. In contrast, the α-helical structure of RcsD-HPt is conformationally stable and contains a recognition area in close vicinity to the active-site His842 residue. Our studies indicate the importance of protein dynamics and conformational exchange for the differential response to the variety of signals perceived by complex regulatory networks.

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