Human TAFII28 promotes transcriptional stimulation by activation function 2 of the retinoid X receptors

Abstract

Transcriptional activation in vitro involves direct interactions of transactivators with the TATA binding protein (TBP) and the TBP-associated factors (TAFIIs) which constitute the TFIID complex. However, the role of TAFIIs in transcriptional regulation in mammalian cells has not been addressed. We show that activation function 2 of the retinoid X receptors (RXR AF-2) does not activate transcription from a minimal promoter in Cos cells. However, coexpression of human (h) TAFII28 promotes a strong ligand-dependent activity of the RXR AF-2 on a minimal promoter and potentiates the ability of the RXRα AF-2 to activate transcription from a complex promoter. The expression of hTAFII28 also potentiated transactivation by several nuclear receptors, notably the oestrogen and vitamin D3 receptors (ER and VDR), whereas other classes of activator were not affected. The effect of hTAFII28 on RXR AF-2 activities did not appear to require direct RXR-TAFII28 interactions, but correlated with the ability of hTAFII28 to interact with TBP. In contrast to Cos cells, the RXR AF-2s had differential abilities to activate transcription from a minimal promoter in HeLa cells, and a lesser increase in their activity was observed upon hTAFII28 coexpression. Moreover, coexpression of hTAFII28 did not increase but rather repressed activation by the ER and VDR AF-2s in HeLa cells. In agreement with these data, showing that TAFII28 is limiting in the AF-2 activation pathway in Cos cells, TAFII28 is selectively depleted in Cos cell TFIID.