High-Resolution Solution Structure of the Beryllofluoride-Activated NtrC Receiver Domain^†

Bacterial receiver domains mediate the cellular response to environmental changes through conformational changes induced by phosphorylation of a conserved aspartate residue. While the structures of several activated receiver domains have recently been determined, there is substantial variation in the conformational changes occurring upon activation. Here we present the high-resolution structure of the activated NtrC receiver domain (BeF₃^-−NtrC^r complex) determined using NMR data, including residual dipolar couplings, yielding a family of structures with a backbone rmsd of 0.57 ± 0.08 Å, which is compared with the previous lower-resolution structure of the phosphorylated protein. Both phosphorylation and beryllofluoride addition induce a shift in register and an axial rotation of α-helix 4. In this high-resolution structure, we are able to observe a concerted change in the positions of Thr82 and Tyr101; this correlated change in two conserved residues (termed Y−T coupling) has been considered a general feature of the conformational change in receiver domains upon activation. In NtrC, this correlated side chain shift, leading to the helix reorientation, is distinctly different from the smaller reorganization seen in other activated receiver domains, and involves numerous other residues which do not participate in conformational changes seen in the other systems. Titration of the activated receiver domain with peptides from the NtrC ATPase domain provides direct evidence for interactions on the rearranged face of the receiver domain, which are likely to be responsible for enabling assembly into the active aggregate. Analysis of the active structure also suggests that His84 may play a role in controlling the phosphate hydrolysis rate.