Structure of a transiently phosphorylated switch in bacterial signal transduction

Receiver domains are the dominant molecular switches in bacterial signallin g(1,2), Although several structures of non-phosphorylated receiver domains have been reported(3-8), a detailed structural understanding of the activat ion arising from phosphorylation has been impeded by the very short half-li ves of the aspartyl-phosphate linkages. Here we present the first structure of a receiver domain in its active state, the phosphorylated receiver doma in of the bacterial enhancer-binding protein NtrC (nitrogen regulatory prot ein C), Nuclear magnetic resonance spectra were taken during steady-state a utophosphorylation/dephosphorylation, and three-dimensional spectra from mu ltiple samples were combined. Phosphorylation induces a large conformationa l change involving a displacement of beta-strands 4 and 5 and alpha-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 t ransmit the signal to the transcriptional activation domain.