Structure of a transiently phosphorylated switch in bacterial signal transduction

Receiver domains are the dominant molecular switches in bacterial signalling1,2. Although several structures of non-phosphorylated receiver domains have been reported3,4,5,6,7,8, a detailed structural understanding of the activation arising from phosphorylation has been impeded by the very short half-lives of the aspartyl-phosphate linkages. Here we present the first structure of a receiver domain in its active state, the phosphorylated receiver domain of the bacterial enhancer-binding protein NtrC (nitrogen regulatory protein C). Nuclear magnetic resonance spectra were taken during steady-state autophosphorylation/dephosphorylation, and three-dimensional spectra from multiple samples were combined. Phosphorylation induces a large conformational change involving a displacement of β-strands 4 and 5 and α-helices 3 and 4 away from the active site, a register shift and an axial rotation in helix 4. This creates an exposed hydrophobic surface that is likely to transmit the signal to the transcriptional activation domain.