Putative sensor domain similar to HisK; the type 2 periplasmic binding fold protein
This group includes periplasmic sensor domain of the histidine kinase receptors (HisK) which are elements of the two-component signal transduction systems commonly found in bacteria and lower eukaryotes. Typically, the two-component system consists of a membrane-spanning histidine kinase sensor and a cytoplasmic response regulator. The two-component systems serve as a stimulus-response coupling mechanism to enable microorganisms to sense and respond to changes in environmental conditions. Extracellular stimuli such as small molecule ligands and ions are detected by the N-terminal periplasmic sensing domain of the sensor kinase receptor, which regulate the catalytic activity of the cytoplasmic kinase domain and promote ATP-dependent autophosphorylation of a conserved histidine residue. The phosphate is then transferred to a conserved aspartate in the response regulator through a phospho-transfer mechanism, and the activity of the response regulator is in turn regulated. The sensor domain belongs to the type 2 periplasmic-binding fold protein (PBP2) superfamily, whose members are involved in chemotaxis and uptake of nutrients and other small molecules from the extracellular space through their function as an initial high-affinity binding component. PBP2 typically comprises of two globular subdomains connected by a flexible hinge and bind their ligand in the cleft between these domains in a manner resembling a Venus flytrap.
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