Motile bacterial use membrane-embedded chemoreceptor molecules to detect the chemical environment and mediate chemotaxis. Specific compounds are detected at the N-terminal and extracellularligand-binding domain (LBD), and this information is transduced to the cells motility machinery by the intracellular C-terminal signalling domain. The signalling process allows individual cells to bias their swimming behaviour towards potential energy sources and away from toxins. This study used biophysical assays to determine the ligand specificity of LBDs.The current techniques used to determine the binding repertoire of LBDs are slow and laborious. This work utilised a high-throughput screening approach to show that the Escherichia coli Tap chemoreceptor does not bind pyrimidine molecules directly, and to identify three ligands for a novel chemoreceptor of the global kiwifruit pathogen Pseudomonas syringae pv. actinidiae (Psa). Chemotaxis assays revealed that this receptor, Psa_14525, mediates chemotaxis towards each of the three compounds bound by its LBD. Homology modelling of the Psa_14525-LBD and the homologous Pseudomonas aeruginosa PctA-LBD predicted a single variant residue in the predicted binding site of Psa_14525-LBD caused the two proteins to exhibit entirely different ligand-binding specificities. Crystallography trials were conducted to try and understand the molecular details of the specificity of Psa_14525-LBD. The results provide insights into the role that LBD sequence variation plays in ligand detection via the use of high-throughput screen for protein-ligand interactions.
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