Three amino acid substitutions selectively disrupt the activation but not the repression function of the glucocorticoid receptor N terminus

A 210-amino acid region, termed enh2, near the N terminus of the rat glucocorticoid receptor, is necessary for both transcriptional activation and repression. The mechanism(s) of transcriptional regulation conferred by this region, however, are poorly understood. We screened in Saccharomyces cerevisiae a library of random mutants in the enh2 region of a constitutive glucocorticoid receptor derivative and isolated a series of multiply substituted receptors that are specifically defective in transcriptional activation. Although many substitutions in this area were tolerated, three amino acid substitutions (E219K, F220L, and W234R) within a 16-amino acid region were sufficient to disrupt the enh2 transcriptional activation function both in yeast and in mammalian cells. Although this region is rich in acidic residues, the conserved tryptophan at position 234 appears to be a more critical feature for enh2 activity; hydrophobic but not charged residues were tolerated at this position, Notably, the mutants uncoupled the activation and repression functions of enh2, as the activation defective isolates remained competent for repression of AP-1 at the composite response element plfG.