Mutational analysis of the human androgen receptor transactivation domain
Mutants were created within the main activation function domain to investigate the structure and function of this region. Structural studies based on limited proteolysis and fluorescence spectroscopy experiments, indicate that though the N-terminus is not as specially structured as either the LBD or DBD, there are regions of specific folding within the activation domain. Results from in silco investigation suggest several possible regions of a helix within the AF-1 domain, and mutants designed to disrupt these regions were less folded than the wild-type protein. Protein-protein interaction studies showed that the four mutants designed to potentially disrupt function rather than structure, had reduced binding to the large subunit of TFIIF - RAP74, but had no effect on binding to the transcriptional co-activator SRC-1a. In a functional assay performed in yeast cells, these four same mutants all showed reduced activity, showing the same trend as binding to RAP74. This could be an indication that the function of the AR is dependent upon binding to the general transcription factor TFIIF. Interestingly one of the mutants that was found to show increased structure over the wild-type protein was previously shown to have a reduced interaction with RAP74. This implies that structure is important for interaction with the transcription machinery. This was confirmed by FTIR experiments which can detect changes in the proportion of secondary structure present in a protein. These data show that the proportion of a helix present in the AF-1 domain increases when it is complexed with RAP74. GST pull-down assays then demonstrated that the complex of AF-1 and RAP74 enhanced the binding of the co-activator SRC-1a. This is the first time that this cooperativity has been demonstrated with nuclear receptors and interacting proteins. Additionally, specific phosphorylation of the AF-1 domain by glycogen synthase kinase 3 also increases the level of binding with SRC-1a. These data together suggest a possible mechanism of action for the androgen receptor and its involvement in regulating transcription.