Please use this identifier to cite or link to this item: http://hdl.handle.net/2440/95877
Type: Thesis
Title: The aryl hydrocarbon receptor: structural analysis and activation mechanisms.
Author: Whelan, Fiona
Issue Date: 2010
School/Discipline: School of Molecular and Biomedical Science
Abstract: The Aryl-hydrocarbon Receptor (AhR) is a basic Helix-Loop-Helix Per-ARNT-Sim (bHLH.PAS) transcription factor (TF) which binds partner protein Aryl hydrocarbon Receptor Nuclear Translocator (ARNT), in order to activate target genes in response to environmental or endogenous stimuli. The PAS region of these TFs consists of two adjacent repeats of the PAS domain, where the PAS repeat defines dimerization specificity and also serves as a primary sensor in exogenous ligand activation of AhR. Active AhR/ARNT heterodimer binds specific DNA sequences, termed Xenobiotic Response Elements (XRE). The molecular detail of interactions that dictate dimerization and DNA binding specificity are unknown for this TF family. In addition, active AhR has recently been shown to function as a recognition component of an E3 ubiquitin ligase. Reports that AhR null mice have poor fertility and defects in liver vasculature are indicative of the potential for a number of endogenous roles. Research into activation of AhR has highlighted that post-translational modifications may affect function by regulating subcellular localization. The complex regulatory outcomes of AhR expression and activation require a number of approaches to elucidate mechanistic information. In this thesis, a structural investigation of heterodimerization and DNA binding has been used to propose a molecular mechanism for target gene recognition and activation following XRE binding. Crystallographic approaches have yielded crystals of bHLH.PAS-A regions of AhR/ARNT heterodimer bound to DNA. Atomic force microscopy and small angle x-ray scattering analyses have illustrated an XRE binding mechanism whereby the DNA is bent, and the PASA region of the dimer flattens around the DNA. A targeted mutagenesis screen of the AhR ligand binding domain (LBD) was performed to investigate polycyclic aromatic hydrocarbon (PAH) and atypical ligand binding specificity. In parallel, mutant AhR proteins were assessed for inducibility by nonexogenous ligand modes of activation, including cell suspension and application of shear stressed serum. This process identified an LBD mutant selectively activated by novel ligand YH439, and completely inactive following PAH, cell suspension and shear stressed serum treatments, inferring the potential for differential ligand binding pocket access by YH439. Finally, given the complex output following expression and activation of AhR, regulation by post-translational modification was investigated as a potential means of subtle regulation of signalling fate. A thorough analysis of untreated AhR has revealed a concert of modifications occurring on functionally relevant regions of the protein that are implicated in regulating subcellular localization, protein: protein interactions, and potentially, protein stability. Preliminary analyses of YH439 and cell suspension treated AhR has additionally indicated the possibility of activation state specific modification patterns. In summary, this thesis describes: Novel approaches to structural characterisation of a bHLH.PAS protein dimer bound to DNA; atypical ligand binding to a novel site of AhR; and an analysis of a proposed AhR PTM code.
Advisor: Whitelaw, Murray Leslie
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Molecular and Biomedical Sciences, Discipline of Biochemistry, 2009
Keywords: aryl hydrocarbon receptor; bHLH; basic Helix Loop Helix; PAS; transcription factor; Dioxin receptor
Provenance: This electronic version is made publicly available by the University of Adelaide in accordance with its open access policy for student theses. Copyright in this thesis remains with the author. This thesis may incorporate third party material which has been used by the author pursuant to Fair Dealing exceptions. If you are the owner of any included third party copyright material you wish to be removed from this electronic version, please complete the take down form located at: http://www.adelaide.edu.au/legals
Appears in Collections:Research Theses

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