We report here the identification of a novel cofactor, ACTR, that directly binds nuclear receptors and stimulates their transcriptional activities in a hormone-dependent fashion. ACTR also recruits two other nuclear factors, CBP and P/CAF, and thus plays a central role in creating a multisubunit coactivator complex. In addition, and unexpectedly, we show that purified ACTR is a potent histone acetyltransferase and appears to define a distinct evolutionary branch to this recently described family. Thus, hormonal activation by nuclear receptors involves the mutual recruitment of at least three classes of histone acetyltransferases that may act cooperatively as an enzymatic unit to reverse the effects of histone deacetylase shown to be part of the nuclear receptor corepressor complex.

The transcriptional corepressors SMRT and N-CoR function as silencing mediators for retinoid and thyroid hormone receptors. Here we show that SMRT and N-CoR directly interact with mSin3A, a corepressor for the Mad-Max heterodimer and a homolog of the yeast global-transcriptional repressor Sin3p. In addition, we demonstrate that the recently characterized histone deacetylase 1 (HDAC1) interacts with Sin3A and SMRT to form a multisubunit repressor complex. Consistent with this model, we find that HDAC inhibitors synergize with retinoic acid to stimulate hormone-responsive genes and differentiation of myeloid leukemia (HL-60) cells. This work establishes a convergence of repression pathways for bHLH-Zip proteins and nuclear receptors and suggests this type of regulation may be more widely conserved than previously suspected.

The nuclear receptor superfamily includes receptors for steroids, retinoids, thyroid hormone and vitamin D, as well as many related proteins. An important feature of the action of the lipophilic hormones and vitamins is that the maintenance of homeostatic function requires both intrinsic positive and negative regulation. Here we provide in vitro and in vivo evidence that identifies the CREB-binding protein (CBP) and its homologue P300 (refs 6,7) as cofactors mediating nuclear-receptor-activated gene transcription. The role of CBP/P300 in the transcriptional response to cyclic AMP, phorbol esters, serum, the lipophilic hormones and as the target of the E1A oncoprotein suggests they may serve as integrators of extracellular and intracellular signalling pathways.

Acetylation of histones by p300/CBP and PCAF is considered to be a critical step in transcriptional regulation. In order to understand the role of cellular activities that modulate histone acetylation and transcription, we have purified and characterized a multiprotein cellular complex that potently inhibits the histone acetyltransferase activity of p300/CBP and PCAF. We have mapped a novel acetyltransferase-inhibitory domain of this INHAT (inhibitor of acetyltransferases) complex that binds to histones and masks them from being acetyltransferase substrates. Endogenous INHAT subunits, which include the Set/TAF-Ibeta oncoprotein, associate with chromatin in vivo and can block coactivatormediated transcription when transfected in cells. We propose that histone masking by INHAT plays a regulatory role in chromatin modification and serves as a novel mechanism of transcriptional regulation.

Interferon ␥ (IFN-␥) induces rapid tyrosine phosphorylation of the latent cytoplasmic transcription factor, Stat1, which then forms homodimers, translocates to the nucleus and participates in IFN-␥-induced transcription. However, little is known of the interactions between Stat1 and the general transcription machinery during transcriptional activation. We show here that Stat1 can directly interact with the CREB-binding protein (CBP)͞p300 family of transcriptional coactivators. Specifically, two interaction regions were identified: the amino-terminal region of Stat1 interacts with the CREB-binding domain of CBP͞p300 and the carboxylterminal region of Stat1 interacts with the domain of CBP͞ p300 that binds adenovirus E1A protein. Transfection experiments suggest a role for these interactions in IFN-␥-induced transcription. Because CBP͞p300-binding is required for the adenovirus E1A protein to regulate transcription of many genes during viral replication and cellular transformation, it is possible that the anti-viral effect of IFN-␥ is based at least in part on direct competition by nuclear Stat1 with E1A for CBP͞p300 binding.