PubMed · 9463813
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D M Morgan. 1998. Polyamines. An introduction.. https://doi.org/10.1385/0-89603-448-8%3A3
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CBP/p300 recruitment to enhancer-bound complexes is a key determinant in promoter activation by many transcription factors. We present a novel mechanism of activating such complexes and show that pre-assembled Elk-1-p300 complexes become activated following Elk-1 phosphorylation by changes in Elk-1-p300 interactions rather than recruitment. It is known that Elk-1 binds to promoter in the absence of stimuli. However, it is unclear how activation of Elk-1 by mitogen-acivated protein kinase (MAPK)-mediated phosphorylation leads to targeted gene transactivation. We show that Elk-1 can interact with p300 in vitro and in vivo in the absence of a stimulus through the Elk-1 C-terminus and the p300 N-terminus. Phosphorylation on Ser383 and Ser389 of Elk-1 by MAPK enhances this basal binding but, most importantly, Elk-1 exhibits new interactions with p300. These interaction changes render a strong histone acetyltransferase activity in the Elk-1-associated complex that could play a critical role in chromatin remodeling and gene activation. The pre-assembly mechanism may greatly accelerate transcription activation, which is important in regulation of expression of immediate-early response genes, in particular those involved in stress responses.
p300 is a multifunctional transcriptional coactivator that serves as an adapter for several transcription factors including nuclear steroid hormone receptors. p300 possesses an intrinsic histone acetyltransferase (HAT) activity that may be critical for promoting steroid-dependent transcriptional activation. In osteoblastic cells, transcription of the bone-specific osteocalcin (OC) gene is principally regulated by the Runx2/Cbfa1 transcription factor and is stimulated in response to vitamin D(3) via the vitamin D(3) receptor complex. Therefore, we addressed p300 control of basal and vitamin D(3)-enhanced activity of the OC promoter. We find that transient overexpression of p300 results in a significant dose-dependent increase of both basal and vitamin D(3)-stimulated OC gene activity. This stimulatory effect requires intact Runx2/Cbfa1 binding sites and the vitamin D-responsive element. In addition, by coimmunoprecipitation, we show that the endogenous Runx2/Cbfa1 and p300 proteins are components of the same complexes within osteoblastic cells under physiological concentrations. We also demonstrate by chromatin immunoprecipitation assays that p300, Runx2/Cbfa1, and 1alpha,25-dihydroxyvitamin D(3) receptor interact with the OC promoter in intact osteoblastic cells expressing this gene. The effect of p300 on the OC promoter is independent of its intrinsic HAT activity, as a HAT-deficient p300 mutant protein up-regulates expression and cooperates with P/CAF to the same extent as the wild-type p300. On the basis of these results, we propose that p300 interacts with key transcriptional regulators of the OC gene and bridges distal and proximal OC promoter sequences to facilitate responsiveness to vitamin D(3).
A number of signal-dependent and development-specific transcription factors recruit CREB binding protein (CBP) for their transactivation function. The KIX domain of CBP is a common docking site for many of these transcription factors. We recently determined the solution structure of the KIX domain complexed to one of its targets, the Ser133-phosphorylated kinase inducible transactivation domain (pKID) of the cyclic AMP response element binding protein. The NMR studies have now been extended to a slightly longer KIX construct that, unlike the original KIX construct, is readily amenable to structural analysis in both the free and pKID-bound forms. This addition of six residues (KRRSRL) to the C terminus of the original construct elongates the C-terminal alpha3 helix of KIX by about eight residues. On the basis of the NMR structure of the original KIX construct, residues in the extended helix are predicted to be solvent exposed and thus are not expected to contribute to the hydrophobic core of the domain. Their role appears to be in the stabilization of the alpha3 helix through favorable electrostatic interactions with the helix dipole, which in turn confers stability on the core of the KIX domain. These results have important implications for the identification of novel protein domain boundaries. Chemical shift perturbation mapping firmly establishes a similar mode of pKID binding to the longer KIX construct and rules out any additional intermolecular interactions between residues in the C-terminal extension and pKID.