Sensor Domain of Histidine Kinase KinB of Pseudomonas: A HELIX-SWAPPED DIMER

Background: The sensor domain (SD) of histidine kinase (HK) KinB (KinB-SD) receives signals from the environment and induces a transduction cascade. Results: Structures of the KinB-SD were obtained in ligand-free, phosphate-bound, and mutant forms. Conclusion: The unique helix-swapped KinB-SD structure forms a ligand-binding cavity on the dimer interface. Significance: KinB-SD studies provide insights into the signal transduction and identity of potential signaling molecules.The overproduction of polysaccharide alginate is responsible for the formation of mucus in the lungs of cystic fibrosis patients. Histidine kinase KinB of the KinB-AlgB two-component system in Pseudomonas aeruginosa acts as a negative regulator of alginate biosynthesis. The modular architecture of KinB is similar to other histidine kinases. However, its periplasmic signal sensor domain is unique and is found only in the Pseudomonas genus. Here, we present the first crystal structures of the KinB sensor domain. The domain is a dimer in solution, and in the crystal it shows an atypical dimer of a helix-swapped four-helix bundle. A positively charged cavity is formed on the dimer interface and involves several strictly conserved residues, including Arg-60. A phosphate anion is bound asymmetrically in one of the structures. In silico docking identified several monophosphorylated sugars, including -d-fructose 6-phosphate and -d-mannose 6-phosphate, a precursor and an intermediate of alginate synthesis, respectively, as potential KinB ligands. Ligand binding was confirmed experimentally. Conformational transition from a symmetric to an asymmetric structure and decreasing dimer stability caused by ligand binding may be a part of the signal transduction mechanism of the KinB-AlgB two-component system.