Role of CBP/P300 in nuclear receptor signalling

Two domains of MyoD mediate transcriptional activation of genes in repressive chromatin: a mechanism for lineage determination in myogenesis

Genetic studies have demonstrated that MyoD and Myf5 establish the skeletal muscle lineage, whereas myogenin mediates terminal differentiation, yet the molecular basis for this distinction is not understood. We show that MyoD can remodel chromatin at binding sites in muscle gene enhancers and activate transcription at previously silent loci. TGF-beta, basic-FGF, and sodium butyrate blocked MyoD-mediated chromatin reorganization and the initiation of transcription. In contrast, TGF-beta and sodium butyrate did not block transcription when added after chromatin remodeling had occurred. MyoD and Myf-5 were 10-fold more efficient than myogenin at activating genes in regions of transcriptionally silent chromatin. Deletion mutagenesis of the MyoD protein demonstrated that the ability to activate endogenous genes depended on two regions: a region rich in cysteine and histidine residues between the acidic activation domain and the bHLH domain, and a second region in the carboxyl terminus of the protein. Neither region has been shown previously to regulate gene transcription and both have domains that are conserved in the Myf5 protein. Our results establish a mechanism for chromatin modeling in the skeletal muscle lineage and define domains of MyoD, independent of the activation domain, that participate in chromatin reorganization.

The phantom ligand effect: allosteric control of transcription by the retinoid X receptor

Regulation of gene expression via allosteric control of transcription is one of the fundamental concepts of molecular biology. Studies in prokaryotes have illustrated that binding of small molecules or ligands to sequence-specific transcription factors can produce conformational changes at a distance from the binding site. These ligand-induced changes can dramatically alter the DNA binding and/or trans-activation abilities of the target transcription factors. In this work, analysis of trans-activation by members of the steroid and thyroid hormone receptor superfamily identifies a unique form of allosteric control, the phantom ligand effect. Binding of a novel ligand (LG100754) to one subunit (RXR) of a heterodimeric transcription factor results in a linked conformational change in the second noncovalently bound subunit of the heterodimer (RAR). This conformational change results in both the dissociation of corepressors and association of coactivators in a fashion mediated by the activation function of the non-liganded subunit. Without occupying the RAR hormone binding pocket, binding of LG100754 to RXR mimics exactly the effects observed when hormone is bound to RAR. Thus, LG100754 behaves as a phantom ligand.

p300 is required for MyoD-dependent cell cycle arrest and muscle-specific gene transcription

The nuclear phosphoprotein p300 is a new member of a family of 'co-activators' (which also includes the CREB binding protein CBP), that directly modulate transcription by interacting with components of the basal transcriptional machinery. Both p300 and CBP are targeted by the adenovirus E1A protein, and binding to p300 is required for E1A to inhibit terminal differentiation in both keratinocytes and myoblasts. Here we demonstrate that, in differentiating skeletal muscle cells, p300 physically interacts with the myogenic basic helix-loop-helix (bHLH) regulatory protein MyoD at its DNA binding sites. During muscle differentiation, MyoD plays a dual role: besides activating muscle-specific transcription, it induces permanent cell cycle arrest by up-regulating the cyclin-dependent kinase inhibitor p21. We show that p300 is involved in both these activities. Indeed, E1A mutants lacking the ability to bind p300 are greatly impaired in the repression of E-box-driven transcription, and p300 overexpression rescues the wild-type E1A-mediated repression. Moreover, p300 potentiates MyoD- and myogenin-dependent activation of transcription from E-box-containing reporter genes. We also provide evidence, obtained by microinjection of anti-p300 antibodies, that p300 is required for MyoD-dependent cell cycle arrest in either myogenic cells induced to differentiate or in MyoD-converted C3H10T1/2 fibroblasts, but is dispensable for maintenance of the postmitotic state of myotubes.

Two contact regions between Stat1 and CBP/p300 in interferon gamma signaling

Interferon gamma (IFN-gamma) induces rapid tyrosine phosphorylation of the latent cytoplasmic transcription factor, Stat1, which then forms homodimers, translocates to the nucleus and participates in IFN-gamma-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 CBP/p300 and the carboxyl-terminal region of Stat1 interacts with the domain of CBP/p300 that binds adenovirus E1A protein. Transfection experiments suggest a role for these interactions in IFN-gamma-induced transcription. Because CBP/p300-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-gamma is based at least in part on direct competition by nuclear Stat1 with E1A for CBP/p300 binding.

The CBP co-activator is a histone acetyltransferase

The CBP protein acts as a transcriptional adaptor for many different transcription factors by directly contacting DNA-bound activators. One mechanism by which CBP is thought to stimulate transcription is by recruiting the histone acetyltransferase (HAT) P/CAF to the promoter. Here we show that CBP has intrinsic HAT activity. The HAT domain of CBP is adjacent to the binding site for the transcriptional activator E1A. Although E1A displaces P/CAF from CBP, it does not disrupt the CBP-associated HAT activity. Thus E1A carries HAT activity when complexed with CBP. Targeting CBP-associated HAT activity to specific promoters may therefore be a mechanism by which E1A acts as a transcriptional activator.

The transcriptional coactivators p300 and CBP are histone acetyltransferases

p300/CBP is a transcriptional adaptor that integrates signals from many sequence-specific activators via direct interactions. Various cellular and viral factors target p300/CBP to modulate transcription and/or cell cycle progression. One such factor, the cellular p300/CBP associated factor (PCAF), possesses intrinsic histone acetyltransferase activity. Here, we demonstrate that p300/CBP is not only a transcriptional adaptor but also a histone acetyltransferase. p300/CBP represents a novel class of acetyltransferases in that it does not have the conserved motif found among various other acetyltransferases. p300/CBP acetylates all four core histones in nucleosomes. These observations suggest that p300/CBP acetylates nucleosomes in concert with PCAF.