Mutational analysis of the androgen receptor AF-2 (activation function 2) core domain reveals functional and mechanistic differences of conserved residues compared with other nuclear receptors

A short C-terminal sequence that forms the core of the activation function-2 (AF-2) domain is conserved in members of the nuclear receptor superfamily and is required for their normal biological function. Despite a high degree of sequence similarity, there are differences in the context and structure of AF-2 in different nuclear receptors, To gain deeper insight into these differences, we carried out an extensive mutational analysis of the AF-2 core in the androgen receptor (AR) and compared the changes in transcriptional activity with similar mutations that have previously been generated in other nuclear receptors. Mutagenesis of Met894 to Asp resulted in substantial decreases in both DNA and ligand binding activities and, consequently, a significant drop in ligand-dependent transcriptional activation. In contrast, substitution of Met894 with Ala did not affect DNA or hormone binding, and the transactivation potential was comparable to that of wild-type AR. Mutagenesis of Glu897 either with Val or Ala significantly impaired ligand-dependent activation that was not due to changes in DNA or ligand binding. There are both similarities and distinct differences between these findings compared with previous mutagenesis studies of the corresponding residues in other nuclear receptors. All mutants efficiently interfered with AP-1 activity, indicating that ligand-dependent activation of transcription and interference with AP-1 activity are separable functions in AR. For the Glu897 substitutions, the decrease in ligand-dependent transactivation could partially be reversed by overexpression of GRIP1 (GR-interacting protein 1) or CBP, putative coactivators for AR. However, there was no correlation between ligand-dependent in vitro or in vivo association with coactivators and the ability of the mutants to support ligand-dependent transactivation. This is in contrast to similar mutations in other nuclear receptors that lose interactions with putative coactivators concomitant with their loss of transcriptional activity. However, the Glu897 mutations disrupted the intramolecular interaction between the N-terminal domain and the ligand-binding domain of AR that was recently suggested to be required for normal AR function. We conclude that residues in the AF-2 core domain of AR make distinctly different contributions to its transcriptional activities compared with those of other nuclear receptors studied to date.