The c-Myc oncoprotein (Myc) controls cell fate by regulating gene transcription in association with a DNA-binding partner, Max. While Max lacks a transcription regulatory domain, the N terminus of Myc contains a transcription activation domain (TAD) that recruits cofactor complexes containing the histone acetyltransferases (HATs) GCN5 and Tip60. Here, we report a novel functional interaction between Myc TAD and the p300 coactivator-acetyltransferase. We show that p300 associates with Myc in mammalian cells and in vitro through direct interactions with Myc TAD residues 1 to 110 and acetylates Myc in a TAD-dependent manner in vivo at several lysine residues located between the TAD and DNA-binding domain. Moreover, the Myc:Max complex is differentially acetylated by p300 and GCN5 and is not acetylated by Tip60 in vitro, suggesting distinct functions for these acetyltransferases. Whereas p300 and CBP can stabilize Myc independently of acetylation, p300-mediated acetylation results in increased Myc turnover. In addition, p300 functions as a coactivator that is recruited by Myc to the promoter of the human telomerase reverse transcriptase gene, and p300/CBP stimulates Myc TAD-dependent transcription in a HAT domain-dependent manner. Our results suggest dual roles for p300/CBP in Myc regulation: as a Myc coactivator that stabilizes Myc and as an inducer of Myc instability via direct Myc acetylation.The c-Myc oncoprotein (Myc) is the ubiquitous member of a small family of highly related DNA-binding transcription factors (including L-Myc and N-Myc) that regulate a wide variety of genes involved in the control of cell growth, proliferation, differentiation, and apoptotic cell death. Myc is essential for embryonic development and both Myc expression and activity are tightly regulated by mitogens and other physiological stimuli in normal somatic cells. Notably, unregulated Myc expression is tumorigenic in mice and has been associated with most types of cancer in humans. Myc binds to E-box DNA elements having the core consensus sequence CACGTG as a heterodimer with an obligatory partner protein called Max. Myc and Max dimerize and bind DNA via their respective basic-helix-loop-helix-leucine zipper (bHLHZip) domains. While Max does not have a transcription regulatory domain, Myc has a phylogenetically conserved N-terminal transcription activation domain (TAD) that is also essential for oncogenic cellular transformation (reviewed in reference 10).Several proteins have been shown to interact with Myc N-terminal TAD and are potential regulators or mediators of Myc transactivating and transforming activities (10,30). Among these, the TRRAP protein has been shown to contribute to the transformation activity of Myc through interactions with the conserved Myc box 1 (MB1) and MB2 regions within the TAD (23) and is a subunit of various transcription regulatory cofactors complexes that have histone acetyltransferase (HAT) activity. These TRRAP-HAT complexes include the GCN5 HAT-containing complexes STAGA (21, 22) and TFTC (3), the rela...

MYC is an oncogenic DNA-binding transcription activator of many genes and is often upregulated in human cancers. MYC has an N-terminal transcription activation domain (TAD) that is also required for cell transformation. Various MYC TAD-interacting coactivators have been identified, including the transcription/transformation-associated protein (TRRAP), a subunit of different histone acetyltransferase (HAT) complexes such as the human “SPT3-TAF9-GCN5 Acetyltransferase” (STAGA) complex involved in MYC transactivation of the TERT gene. However, it remains unclear whether TRRAP and/or other subunits are directly contacted by MYC within these macromolecular complexes. Here, we characterize the interactions of MYC TAD with the STAGA complex. By protein crosslinking we identify both TRRAP and the GCN5 acetyltransferase as MYC TAD-interacting subunits within native STAGA. We show that purified GCN5 binds to an N-terminal sub-domain of MYC TAD (residues 21-108) and that the interaction of GCN5 and STAGA with this sub-domain is dependent on two related sequence motifs: M2 within the conserved MYC homology box I (MBI), and M3 located between residues 100-106. Interestingly, specific substitutions within the M2/3 motifs that only moderately reduce the intracellular MYC-STAGA interaction and do not influence dimerization of MYC with its DNA-binding partner MAX, strongly inhibit MYC acetylation by GCN5 and reduce MYC binding and transactivation of the GCN5-dependent TERT promoter in vivo. Hence, we propose that MYC associates with STAGA through extended interactions of the TAD with both TRRAP and GCN5 and that the TAD-GCN5 interaction is important for MYC acetylation and MYC binding to certain chromatin loci.