HATs off to PIC Assembly
2006; Elsevier BV; Volume: 23; Issue: 6 Linguagem: Inglês
10.1016/j.molcel.2006.08.022
ISSN1097-4164
Autores Tópico(s)RNA modifications and cancer
ResumoThere are many regulated steps in the assembly of a transcription preinitiation complex (PIC). In this issue of Molecular Cell, Black et al., 2006Black J.C. Choi J.E. Lombardo S.R. Carey M. Mol. Cell. 2006; 23 (this issue): 809-818Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar reveal a catalytic switch mechanism in which autoacetylation of the HAT p300 triggers its dissociation from a promoter in a manner that is coupled to TFIID association. There are many regulated steps in the assembly of a transcription preinitiation complex (PIC). In this issue of Molecular Cell, Black et al., 2006Black J.C. Choi J.E. Lombardo S.R. Carey M. Mol. Cell. 2006; 23 (this issue): 809-818Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar reveal a catalytic switch mechanism in which autoacetylation of the HAT p300 triggers its dissociation from a promoter in a manner that is coupled to TFIID association. A number of unanswered questions remain at the heart of gene activation. In particular, what are the ordered steps in PIC assembly? What steps control subsequent steps? And what is the physical basis and biological rationale for such control? Carey and colleagues (Black et al., 2006Black J.C. Choi J.E. Lombardo S.R. Carey M. Mol. Cell. 2006; 23 (this issue): 809-818Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar) have made progress on these questions by focusing on the largely enigmatic steps leading up to PIC assembly. How the answers to these questions play out may depend largely on the arrangement of cis-regulatory elements, chromatin architecture, and the regulatory proteins that may already be present. For certain, gene activation is rooted in activator proteins bound to specific DNA sequences in a manner that orchestrates PIC assembly. Any of a number of steps in PIC assembly may be blocked or accelerated by cellular signaling events that include ligand binding and chemical modification of resident proteins. Black et al., 2006Black J.C. Choi J.E. Lombardo S.R. Carey M. Mol. Cell. 2006; 23 (this issue): 809-818Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar set up a system to investigate early steps in gene activation. The setup includes a constitutively strong artificial activator (GAL4-VP16) bound to an immobilized promoter in which chromatin has been assembled. Presumably, this biochemical setup mimics the cellular state, whereby a signaling event has already triggered an activator protein to begin PIC assembly. When Black et al., 2006Black J.C. Choi J.E. Lombardo S.R. Carey M. Mol. Cell. 2006; 23 (this issue): 809-818Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar incubate the activator-promoter complex with a crude nuclear extract derived from HeLa cells, PIC assembly factors and PIC components begin to arrive at the promoter in a temporal order. Two early guests to arrive are Mediator and p300, which is consistent with their in vivo observations. Mediator has been characterized as a multifunctional integrator of many steps during and after PIC assembly (Conaway et al., 2005Conaway R.C. Sato S. Tomomori-Sato C. Yao T. Conaway J.W. Trends Biochem. Sci. 2005; 30: 250-255Abstract Full Text Full Text PDF PubMed Scopus (219) Google Scholar, Kornberg, 2005Kornberg R.D. Trends Biochem. Sci. 2005; 30: 235-239Abstract Full Text Full Text PDF PubMed Scopus (404) Google Scholar, Malik and Roeder, 2005Malik S. Roeder R.G. Trends Biochem. Sci. 2005; 30: 256-263Abstract Full Text Full Text PDF PubMed Scopus (300) Google Scholar), and so its early arrival makes biological sense. Indeed, Black et al., 2006Black J.C. Choi J.E. Lombardo S.R. Carey M. Mol. Cell. 2006; 23 (this issue): 809-818Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar show that Mediator helps recruit p300, and p300 returns the favor by helping Mediator bind. This cooperativity appears to be a consequence of direct interactions and does not depend upon the presence of chromatin. p300 is a protein acetyltransferase (more specifically referred to as a histone acetyltransferase or HAT) that regulates transcription proteins, including activators, coactivators, corepressors, chromatin proteins, and itself via acetylation (Roth et al., 2001Roth S.Y. Denu J.M. Allis C.D. Annu. Rev. Biochem. 2001; 70: 81-120Crossref PubMed Scopus (1494) Google Scholar). The acetyltransferase activity of p300 is intrinsically weak (Thompson et al., 2004Thompson P.R. Wang D. Wang L. Fulco M. Pediconi N. Zhang D. An W. Ge Q. Roeder R.G. Wong J. et al.Nat. Struct. Mol. Biol. 2004; 11: 308-315Crossref PubMed Scopus (300) Google Scholar). However, autoacetylation of p300 (or possibly acetylation via another protein acetyltransferase) activates its acetyltransferase domain (Thompson et al., 2004Thompson P.R. Wang D. Wang L. Fulco M. Pediconi N. Zhang D. An W. Ge Q. Roeder R.G. Wong J. et al.Nat. Struct. Mol. Biol. 2004; 11: 308-315Crossref PubMed Scopus (300) Google Scholar). Deacetylases such as HDAC1 or other proteins such as E6 and p53 might associate with p300 to keep it in a catalytically inactive state (Qiu et al., 2006Qiu Y. Zhao Y. Becker M. John S. Parekh B.S. Huang S. Hendarwanto A. Martinez E.D. Chen Y. Lu H. et al.Mol. Cell. 2006; 22: 669-679Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar, Thomas and Chiang, 2005Thomas M.C. Chiang C.M. Mol. Cell. 2005; 17: 251-264Abstract Full Text Full Text PDF PubMed Scopus (162) Google Scholar). Conceivably, p300 could arrive at a promoter in either an active or inactive state. Recently, Qiu et al., 2006Qiu Y. Zhao Y. Becker M. John S. Parekh B.S. Huang S. Hendarwanto A. Martinez E.D. Chen Y. Lu H. et al.Mol. Cell. 2006; 22: 669-679Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar have shown that p300 can inactivate HDAC1 via acetylation. Thus, two positive feedback loops (activation through autoacetylation and inhibition of an inhibitor) ensure maximal p300 activity. Presumably, a biological function of promoter-bound p300 is to acetylate nucleosomes, resulting in a more accessible chromatin structure. A second biological role of acetylation might be to provide docking sites for bromodomain-containing transcriptional regulators and PIC components. Thus, the early arrival of p300 also makes sense. At this point the Black et al., 2006Black J.C. Choi J.E. Lombardo S.R. Carey M. Mol. Cell. 2006; 23 (this issue): 809-818Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar study takes an interesting turn. First, they noticed that p300's tenure at the promoter during in vitro PIC assembly was short lived, arriving early but departing when other PIC components began to arrive. Using purified components, they show that departure is linked to autoacetylation. A catalytically dead mutant of p300 failed to depart. At face value this observation raises an apparent paradox in that autoacetylation makes p300 catalytically active while at the same time banishing it from its chromatin substrate. However, Black et al., 2006Black J.C. Choi J.E. Lombardo S.R. Carey M. Mol. Cell. 2006; 23 (this issue): 809-818Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar go on to report their second interesting find: that banishment is only made complete when TFIID is present. In fact, they show that TFIID and p300 compete for binding to Mediator at the promoter, with p300 autoacetylation tipping the balance in favor of TFIID. TFIID is a general transcription factor, which in cooperation with Mediator directs rapid and efficient PIC assembly. Thus, the positive feedback loops associated with p300 activation sow the seeds of its own demise by promoting a negative feedback loop that removes p300 from its targets. Taken together, the results establish a biological motive and physical basis for some of the ordered events that lead up to PIC assembly. At certain promoters, cooperative assembly of Mediator and underacetylated p300 may be preferred over cooperative assembly of Mediator and TFIID. Why? Perhaps downstream events, possibly involving Mediator-TFIID cooperativity, are dependent upon the acetylation state of chromatin or other factors. At this point, p300's ability to block entry of TFIID could serve as a checkpoint in which acetylation of p300 targets, including itself, must be completed before TFIID is allowed to enter. Black et al., 2006Black J.C. Choi J.E. Lombardo S.R. Carey M. Mol. Cell. 2006; 23 (this issue): 809-818Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar refer to the replacement of p300 by TFIID as a coactivator switch and the autoacetylation event that precipitates it as a catalytic switch. The Black et al., 2006Black J.C. Choi J.E. Lombardo S.R. Carey M. Mol. Cell. 2006; 23 (this issue): 809-818Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar study takes us further down the road toward understanding the mechanistic events that lead to PIC assembly and thus puts us in position to consider new questions. It will be of interest to know whether histone acetylation by p300 contributes to the coactivator switch by promoting TFIID binding, possibly through the acetyl-lysine binding bromodomains of TFIID. It would also be of interest to know whether TFIID is the only direct beneficiary of p300's catalytic switch. Mediator is associated with the CDK8 inhibitory kinase (Conaway et al., 2005Conaway R.C. Sato S. Tomomori-Sato C. Yao T. Conaway J.W. Trends Biochem. Sci. 2005; 30: 250-255Abstract Full Text Full Text PDF PubMed Scopus (219) Google Scholar, Kornberg, 2005Kornberg R.D. Trends Biochem. Sci. 2005; 30: 235-239Abstract Full Text Full Text PDF PubMed Scopus (404) Google Scholar, Malik and Roeder, 2005Malik S. Roeder R.G. Trends Biochem. Sci. 2005; 30: 256-263Abstract Full Text Full Text PDF PubMed Scopus (300) Google Scholar), and this too could serve as a checkpoint, requiring its removal for efficient PIC assembly. The departure of p300 upon PIC assembly might not be the end of the line for p300, as it has been proposed to function in elongation as well (Guermah et al., 2006Guermah M. Palhan V.B. Tackett A.J. Chait B.T. Roeder R.G. Cell. 2006; 125: 275-286Abstract Full Text Full Text PDF PubMed Scopus (74) Google Scholar). Conceivably, components of the PIC or elongating polymerase could recruit p300 in a manner and context that differs from its original presence at the promoter. Could this be the reason for kicking out p300 prior to PIC assembly? A Mechanism for Coordinating Chromatin Modification and Preinitiation Complex AssemblyBlack et al.Molecular CellSeptember 15, 2006In BriefTranscription of eukaryotic genes within a chromatin environment requires the sequential recruitment of histone modification enzymes and the general transcription factors (GTFs) by activators. However, it is unknown how preinitiation complex assembly is coordinated with chromatin modification. Here, we show that the model activator GAL4-VP16 directs the ordered assembly of Mediator, histone acetyltransferases (HATs), and GTFs onto immobilized chromatin and naked DNA templates in vitro. Using purified proteins, we found that the Mediator regulates this assembly process by binding to p300 and TFIID. Full-Text PDF Open Archive
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