ROLE OF HYDROPHOBIC AMINO-ACID CLUSTERS IN THE TRANSACTIVATION ACTIVITY OF THE HUMAN GLUCOCORTICOID RECEPTOR

We have performed a mutagenesis analysis of the 58-amino-acid tau 1-co re peptide, which represents the core transactivation activity of the tau 1 transactivation domain from the glucocorticoid receptor. Mutants with altered activity were identified by phenotypic screening in the yeast Saccharomyces cerevisiae. Most mutants with reduced activity had substitutions of hydrophobic amino acids. Most single-substitution mu tants with reduced activity were localized near the N terminus of the tau 1-core within a segment that has been shown previously to have a p ropensity for alpha-helix conformation, suggesting that this helical r egion is of predominant importance. The particular importance of hydro phobic residues within this region was confirmed by comparing the acti vities of alanine substitutions of the hydrophobic residues in this an d two other helical regions. The hydrophobic residues were shown to be important for the transactivation activity of both the isolated tau 1 -core and the intact glucocorticoid receptor in mammalian cells. Rare mutations in helical regions I and II gave rise to increased transcrip tional activation activity. These mutations increase the hydrophobicit y of hydrophobic patches on each of these helices, suggesting a relati onship between the hydrophobicity of the patches and transactivation a ctivity. However, certain nonhydrophobic residues are also important f or activity. Interestingly, helical region I partially matches a conse nsus motif found in the retinoic acid receptor, VP16, and several othe r activator proteins.