NMR STRUCTURE OF THE HISTIDINE KINASE DOMAIN OF THE ESCHERICHIA-COLI OSMOSENSOR ENVZ

Bacteria live in capricious environments, in which they must continuou sly sense external conditions in order to adjust their shape, motility and physiology(1). The histidine-aspartate phosphorelay signal-transd uction system (also known as the two-component system) is important in cellular adaptation to environmental changes in both prokaryotes and lower eukaryotes(2,3). In this system, protein histidine kinases funct ion as sensors and signal transducers, The Escherichia coli osmosensor , EnvZ, is a transmembrane protein with histidine kinase activity in i ts cytoplasmic region(2). The cytoplasmic region contains two function al domains(4): domain A (residues 223-289) contains the conserved hist idine residue (H243), a site of autophosphorylation as well as transph osphorylation to the conserved D55 residue of response regulator OmpR, whereas domain B (residues 290-450) encloses several highly conserved regions (G1, G2, F and N boxes) and is able to phosphorylate H243. He re we present the solution structure of domain B, the catalytic core o f EnvZ. This core has a novel protein kinase structure, distinct from the serine/threonine/tyrosine kinase fold, with unanticipated similari ties to both heat-shock protein 90 and DNA gyrase B.