HUMAN TAF(PARALLEL-TO)28 PROMOTES TRANSCRIPTIONAL STIMULATION BY ACTIVATION FUNCTION-2 OF THE RETINOID-X RECEPTORS

Transcriptional activation in vitro involves direct interactions of tr ansactivators with the TATA binding protein (TBP) and the TBP-associat ed factors (TAF(II)s) which constitute the TFIID complex, However, the role of TAF(II)s in transcriptional regulation in mammalian cells has not been addressed. We show that activation function 2 of the retinoi d X receptors (RXR AF-2) does not activate transcription from a minima l promoter in Cos cells, However, coexpression of human (h) TAF(II)28 promotes a strong ligand-dependent activity of the RXR AF-2 on a minim al promoter and potentiates the ability of the RXR alpha AF-2 to activ ate transcription from a complex promoter, The expression of hTAF(II)2 8 also potentiated transactivation by several nuclear receptors, notab ly the oestrogen and vitamin D3 receptors (ER and VDR), whereas other classes of activator were not affected. The effect of hTAF(II)28 on RX R AF-2 activities did not appear to require direct RXR-TAF(II)28 inter actions, but correlated with the ability of hTAF(II)28 to interact wit h TBP. In contrast to Cos cells, the RXR AF-2s had differential abilit ies to activate transcription from a minimal promoter in HeLa cells, a nd a lesser increase in their activity was observed upon hTAF(II)28 co expression. Moreover, coexpression of hTAF(II)28 did not increase but rather repressed activation by the ER and VDR AF-2s in HeLa cells. In agreement with these data, showing that TAF(II)28 is limiting in the A F-2 activation pathway in Cos cells, TAF(II)28 is selectively depleted in Cos cell TFIID.