Structural characterisation of Histidine Kinase 2
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Two-component systems (TCS) are the predominant signal transduction pathways in prokaryotes, being present also in eukaryotic organisms, such as algae, fungi and yeast, and higher plants. TCSs play an important role in environmental signal perception and response, essentially implementing adaptation to the surrounding environment. Histidine Kinase 2 (Hik2) in cyanobacteria is a typical sensor histidine kinase, one component of a TCS, and has been identified to be a homologue protein of Arabidopsis Chloroplast Sensor Kinase (CSK). Previous research has elucidated Hik2 to regulate photosynthetic gene transcription with two response regulators, Rre1 and RppA via phosphorylation. A typical histidine kinase contains a variable sensor domain and a conserved kinase domain. It usually functions as a homodimer. This thesis describes the structural characterisation of Hik2, probing particularly its discovered oligomeric states. Results obtained from size exclusion chromatography, native-PAGE, chemical cross-linking analyses and mass spectrometry, amongst others, have shown a variety of Hik2 structural populations exist, further validated by negative stain transmission electron microscopy coupled to single particle analysis. Hik2 protein exists predominantly as a hexamer in low salt conditions, and adding NaCl dissociates hexamers into tetramers, critical for the autophosphorylation activity of Hik2. Thus, a model is proposed for the constitution change of Hik2 oligomers when salt concentration differs. In addition, the sensor domain is typically responsible for detecting environmental input, however, it is not yet clear how Hik2 and CSK sense signals. In this thesis, the structures of Hik2 and CSK sensor domains were analysed and discussed, to aid our understanding of their mechanism of signal perception and transduction.
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