DNA binding by the Myc oncoproteins.

The c-Myc protein is a potential activator of transcription, with the ability to bind in a heterodimer form with Max to DNA sequences containing the core hexanucleotide sequence CAC(G/A)TG. These properties are shared with L-Myc, a homologous oncoprotein expressed in small cell lung carcinoma cells; with N-Myc, expressed in neuroblastoma cells; and with avian v-Myc, the c-Myc homolog expressed by a chicken retrovirus. The c-Myc, and probably v-Myc, proteins also have nonspecific DNA binding function, which may improve the kinetics of specific DNA binding. Curiously, this domain appears not to be conserved in L-Myc or N-Myc [22]. The data that have accumulated to date are consistent with a model in which a c-Myc/Max heterodimer positively regulates the transcription of growth-related genes, with Max homodimer functioning as a negative regulator of the same genes (Fig. 4) [55]. Max is expressed constitutively at low levels, whereas c-Myc is expressed at low levels in quiescent cells, but high levels of c-Myc are induced by mitogenic stimulation [56]. Thus, in proliferating cells c-Myc/Max heterodimers might bind to the regulatory elements of growth-related genes, where the c-Myc TAD might stimulate transcription. Conversely, in quiescent cells with little c-Myc present, Max homodimers might predominate. They might bind to exactly the same regulatory elements, but due to the apparent absence of a TAD in Max [36], transcription might be repressed. Validation of this model will require the demonstration of clear regulation of a physiological promoter of a growth-related gene by c-Myc/Max. Although it is widely believed that Myc proteins function as transcriptional activators, this hypothesis has only been conclusively supported recently [57, 58]. A theory that c-Myc plays a role in DNA replication is not as well substantiated at this point. It is even possible that Myc might be involved in both transcription and replication. Although the function of these fascinating proteins has been enigmatic for a decade, the rate of progress in our understanding of Myc function is accelerating. Such progress will undoubtedly lead to a deeper appreciation of this protein, which lies at the crossroads of cellular proliferation and oncogenesis.