Emmas, Catherine Elizabeth; (1992) Structural and functional analysis of the mouse oestrogen receptor. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Steroid receptors are members of a family of ligand-inducible transcription factors which regulate the expression of hormone responsive genes. The mouse oestrogen receptor has been used as a model system to study the structure and function of sequences within the C-terminus. A conserved region has been identified which contains sequences required for steroid binding and dimerisation. The identification of amino acids involved in these two functions was carried out by analysing the functions of an extensive series of point mutations in this region. Dimerisation was affected by mutations between residues 507 and 519. The sequences involved in ligand binding and dimerisation overlap but the two functions are not entirely coincident. Mutant receptors in which amino acids were replaced with hydrophobic or neutral amino acids tended to retain activity whereas charged residues often affected one or more functions quite markedly. The dimerisation interface is rich in hydrophobic residues and does not appear to form an α helix. The effects of point mutations in the oestrogen receptor supported a model for dimerisation based on the structure of uteroglobin, but no direct evidence to support this model was obtained. Oestrogen and the antioestrogens 4-hydroxytamoxifen and ICI 164,384 restored DNA binding activity to mutants defective in dimerisation. This suggested that ligands stabilise dimerisation, possibly by hydrophobic shielding of the charged residues introduced. The controversy surrounding the agonist effects of ICI 164,384 has also been investigated using sequences from the promoter of the oestrogen regulated creatine kinase gene. Although ICI 164,384 induces transcription from this gene in cells, it appears to inhibit DNA binding in a cell-free system. A number of mutant oestrogen receptors were tested for their ability to activate transcription in transient transfections. Mutations which had no effect upon either hormone binding or dimerisation did not interfere with transcriptional activity. Whilst some mutations which disrupt dimerisation in vitro appear to inhibit transcriptional activity in vivo, others do not. Variations in the extent to which oestrogen stabilises dimerisation by mutant receptors in vivo appear to account for this difference. The structure of the oestrogen receptor bound to different hormonal ligands or different DNA binding sites was investigated in a series of proteolytic gel retardation assays. It has been observed that in a gel retardation assay the oestrogen receptor/DNA complex migrates with different mobilities when bound to various hormonal ligands. However, similar proteolytic patterns were generated with no hormone, oestrogen, ICI 164,384 and 4-hydroxytamoxifen. Unfortunately it appears that the entire hormone binding domain is removed before any changes in the cleavage profile can be detected by this method. The oestrogen receptor is capable of binding to a palindromic thyroid response element (TRE), but is unable to induce transcription through this response element, presumably due to the altered conformation it is forced to adopt. Proteolysis was used to compare the conformation of receptor on an ERE and TRE. Differences detected appeared to reflect an inability of oestrogen receptor lacking the ligand binding domain to dimerise on this response element.

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