UC Riverside

MYC is a transcription factor that is overexpressed in a wide range of human cancers, and MYC is known to play a role in many important biological processes such as cell death and cell proliferation. Heterodimerizing with the obligatory partner protein, MAX, the MYC and MAX dimer binds a specific DNA sequence, called an E box (CACGTG), through its basic helix-loop-helix zipper domain. MYC N-terminal transcription activation domain (TAD) has been shown to interact with proteins that are potential regulators of MYC transactivation and transformation activities. Among them, the TRRAP protein has been shown to contribute to the transformation activity of MYC through associations with the MYC box (MB) II and MBI-containing sequences. Importantly, TRRAP is also a subunit of several multiprotein complexes that possess histone acetyltransferase (HAT) activity, such as the STAGA (SPT-TAF-GCN5 Acetylase) coactivator complex. GCN5, another subunit of STAGA, is a HAT that acetylates lysine residues in histone H3 and non-histone proteins. MYC has been also identified as a target of GCN5, although the functional consequences of MYC acetylation by GCN5 remain to be investigated. In this work, we identify MYC K323 as the major site of acetylation by GCN5 and then demonstrate that GCN5 stabilizes MYC in a K323- and HAT domain-dependent manner. Interestingly, we also find that GCN5 enhances MYC ubiquitination in a HAT domain/activity-independent manner. Subsequently, we investigate which subunits of the STAGA complex directly interact with MYC, and then analyze the role of the MYC-STAGA interactions in the biological functions of MYC. We identify GCN5 as another STAGA subunit that directly associates with MYC. The direct interaction of MYC with GCN5 facilitates the interaction of MYC with the STAGA complex, acetylation of MYC, binding of MYC to the human TERT promoter, and activation of human TERT transcription. Lastly, we unexpectedly find a physical interaction between MYC and the tumor suppressor p53 that has not been reported previously, despite decades of intense research investigation on their functions. Based on our preliminary results, we suggest a prospective model in which MYC and p53 influence each other`s transcription regulatory activities via a direct physical interaction. These results further deepen our understanding of MYC biological functions and its role in cancer, and could eventually lead to the development of new therapeutic targets against cancer.