| Histidine kinase sensors are a part of a two-component system of protein signaling in prokaryotes and lower eukaryotes that relay an external environmental signal to an adaptive internal cellular response. Signal transduction occurs via phosphotransfer between a sensor protein and a response regulator which interact in tandem. The sensor is usually a transmembrane protein that contains a conserved cytoplasmic histidine kinase transmitter domain and a modular periplasmic sensor domain. The response regulator is cytoplasmic protein that contains a receiver domain that interacts with the histidine kinase, and an output domain that interacts with regulators of transcription or chemotaxis. My work focuses on the X-ray structure determination of a variety of bacterial sensor domains, based on a structural genomics analysis of the entire sensor domain family. Structures of the NarX, DcuS, LisK, and DctB sensor domains have been solved to atomic resolution, some in both ligand-bound and ligand-free states. Two distinct structural folds have been revealed---all-alpha helical and mixed alpha-beta. An analysis of the structures reveals a possible mechanism of transmembrane signaling in histidine kinase sensors as a sliding-piston motion between transmembrane helices. Although there is great diversity in ligand binding, there appears to be a small number of distinct sensor domain folds for which structural representatives of two have been solved. A final synthesis of the structural information with a comprehensive bio-informatics analysis of all histidine kinase sensor domain sequences allows fold prediction for over 400 sensor domains, in a step towards mapping the entire structural landscape of this protein family. |
