UC Riverside

One of the most heavily researched oncoproteins is the MYC oncoprotein and this oncogenic transcription factor can regulate a large fraction of cellular genes. MYC overexpression becomes oncogenic and has been shown to be observed in many types of human cancers. GCN5, a histone acetyltransferase (HAT), can acetylate MYC via the STAGA complex at K323, but MYC can also be acetylated by different HATs. For example, p300, another HAT, can preferentially acetylate MYC at K149 and K158. However, p300-mediated acetylation increases MYC turnover, whereas GCN5-mediated acetylation stabilizes MYC. However, the significance of MYC acetylation and the functions of these three acetylated residues have yet to be elucidated. In vertebrates, there is a paralogous gene called PCAF, which codes for an enzyme much like GCN5. Due to their almost nearly identical sequences, it is thought that PCAF and GCN5 have largely redundant functions. PCAF has been reported to have ubiquitin E3 ligase activity within the N-terminal PCAF homology domain and this domain is highly conserved in GCN5. However, it is unknown whether GCN5 has any intrinsic E3 ligase activity or whether it recruits an E3 ligase for its surprising role in ubiquitination. Here we show that these three main lysine residues of MYC are reversibly acetylated in various immortalized non-tumorigenic and malignant cells. Furthermore, acetylation of MYC at different lysine residues differentially affects its stability in a cell type-dependent manner and all acetylated lysine (AcK) residues are required for anchorage independent growth in vitro. We show that each individual AcK site has gene-specific functions in controlling select MYC-regulated processes and are required for the oncogenic transformation of MYC. In this study, we have also reported that not only can GCN5 directly acetylate MYC at K323, but it can also stimulate MYC ubiquitination in a GCN5-dependent manner. Furthermore, the PCAF homology domain of GCN5, described earlier, does not contribute to GCN5-dependent MYC ubiquitination, but a direct interaction between MYC and GCN5 is required for the stimulation of MYC ubiquitination. Altogether, we have established a novel and unexpected function of GCN5, and we propose a new model for the regulation of MYC by HATs. MYC binds to HATs, brings them to promoter DNA and recruits components of the preinitiation complex, and then becomes ubiquitinated to allow release of RNAPII for transcription elongation and allow for a new round of transcription. This model suggests that HAT complexes and associated acetylation or E3 ligase pathways could eventually lead to the development of new therapeutic targets to inhibit MYC-dependent cancers.