Nuclear Magnetic Resonance Structure and Dynamics of the Response Regulator Sma0114 from Sinorhizobium meliloti

Receiver domains control intracellular responses triggered by signal transduction in bacterial two-component systems. Here, we report the solution nuclear magnetic resonance structure and dynamics of Sma0114 from the bacterium Sinorhizobium meliloti, the first such characterization of a receiver domain from the HWE-kinase family of two-component systems. The structure of Sma0114 adopts a prototypical alpha(5)/beta(5) Rossman fold but has features that set it apart from other receiver domains. The fourth beta-strand of Sma0114 houses a PFxFATGY sequence motif, common to many HWE-kinase-associated receiver domains. This sequence motif in Sma0114 may substitute for the conserved Y-T coupling mechanism, which propagates conformational transitions in the 455 (alpha 4-beta 5-alpha 5) faces of receiver domains, to prime them for binding downstream effectors once they become activated by phosphorylation. In addition, the fourth alpha-helix of the consensus 455 face in Sma0114 is replaced with a segment that shows high flexibility on the pico- to nanosecond time scale by N-15 relaxation data. Secondary structure prediction analysis suggests that the absence of helix alpha 4 may be a conserved property of the HWE-kinase-associated family of receiver domains to which Sma0114 belongs. In spite of these differences, Sma0114 has a conserved active site, binds divalent metal ions such as Mg2+ and Ca2+ that are required for phosphorylation, and exhibits micro- to millisecond active-site dynamics similar to those of other receiver domains. Taken together, our results suggest that Sma0114 has a conserved active site but differs from typical receiver domains in the structure of the 455 face that is used to effect signal transduction following activation.