Mak, Ho Yi; (2000) Molecular mechanism of transcriptional activation by oestrogen receptor alpha. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Oestrogens regulate the transcription of target genes by binding to the oestrogen receptors (ERα and ERβ) which function as ligand inducible transcription factors. Both ERα and ERβ are members of the nuclear receptor superfamily which is characterised by a highly conserved DNA-binding domain. There are two distinct transcriptional activation domains: the ligand independent API at the N-terminus and the ligand dependent AF2 at the C-terminus which is encompassed by the ligand binding domain (LBD). Following sequence specific binding of oestrogen receptors to the promoter of target genes, additional co-factors are recruited in order to remodel the chromatin structure or to aid the docking of the RNA polymerase II holoenzyme. The AF2 activity of ERα is mediated through interaction between the LBD and coactivator proteins upon ligand binding. In order to define the ERα-coactivator interface at a molecular level, systematic mutagenesis was carried out. This led to the identification of a group of conserved hydrophobic residues in the LBD that are required for binding the p160 family of coactivators. Together with helix 12 and lysine 366 at the C-terminal end of helix 3, they form a hydrophobic groove that accommodates an LXXLL motif, which is essential for coactivator binding to the receptor. The presence of endogenous coactivators is a major impediment for studying designated receptor-coactivator pairs in mammalian cells. To circumvent this problem, a yeast genetic screen was conducted to identify suppressor mutant coactivators for a transcriptionally defective ERα. The V380H mutant receptor fails to interact with wild-type p160 coactivators such as SRC1e. However, an altered specificity mutant SRC1e recovered from the screen is able to interact with the mutant receptor, and fully rescues its transcriptional activity in transfected mammalian cells. Remarkably, introduction of the analogous mutation into other p160 coactivator family members confers the ability to suppress the V380H mutation. This suggests that at least in the assays employed, recruitment of a p160 coactivator by ERα is sufficient to activate transcription.

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