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We have previously shown that the two transcriptional activation functions (TAF-1 and TAF-2) of the human estrogen receptor (hER) have synergistic properties different from one another and from those of acidic activating domains (AADs). Here we compare the transcriptional interference/squelching properties of the hER TAFs with those of the AADs of yeast GAL4 and chimeric GAL-VP16 activators. Our results indicate that AADs interact with a factor(s) that, while required for activation by AADs, is not essential for activation by hER TAFs. In contrast, hER TAFs appear to interact with factors indispensable for mediating both their activation function and that of AADs. Thus, different classes of trans-activators may interact with different factors. In addition, the synergistic and transcriptional interference/squelching properties of the two TAFs of the human glucocorticoid receptor (hGR) indicate that both are composed of acidic and nonacidic activation functions.

Using GAL4 chimeric receptors, we have reported previously that the hormone-binding domain (HBD) of the human oestrogen receptor (hER) contains an hormone-inducible transcription activation function. We have extended that study here to show that this activation function represents the major activating domain in the hER in HeLa cells. In addition, we have expressed the various exons encoding the hER HBD as GAL4 fusion proteins and have shown that none contain a discrete activation function. Thus the activating domain of the hER HBD appears to be different from the recently characterized 'simple' activating domains, such as acidic 'blob' or amphipathic helix, and more likely corresponds to a protein surface created from dispersed elements and dependent upon the three-dimensional folding of the HBD.