Structural Analysis of Ligand Stimulation of the Histidine Kinase NarX

Histidine kinase receptors are a large family of membrane-spanning proteins found in many prokaryotes and some eukaryotes. They are a part of two-component signal transduction systems, which each comprise a sensor kinase and a response regulator and are involved with the regulation of many cellular processes. NarX is a histidine kinase receptor that responds to nitrate and nitrite to effect regulation of anaerobic respiration in various bacteria. We present high-resolution X-ray crystal structures of the periplasmic sensor domain from Escherichia coli NarX in a complex with nitrate and in the apo state. Our analysis reveals that nitrate-binding induces conformation changes that result in a piston-type displacement between the N- and C-terminal helices of the periplasmic domain. Such conformational changes might represent a conserved mechanism of signaling in histicline kinases by which ligand binding is communicated across the lipid bilayer.