The Role of the Transactivation Domain in c-Myc Mediated Cell Cycle Progression and Transformation
Dissertation under the direction of Professor Stephen R. Hann c-Myc is a transcription factor whose deregulation has been implicated in numerous cancers. The biological responses resulting from Myc deregulation include hyperproliferation, apoptosis upon serum withdrawal, changes in cellular size, blocking of cellular differentiation, and transformation in conjunction with constitutively activated Ras. c-Myc contains three highly conserved regions within the N-terminal transcriptional transregulatory domain, termed Myc Box I, Myc Box II and Myc Box III. MycS, which is missing the first 100 amino acids, retains several biological functions; however it is unable to transform primary fibroblasts or induce cell cycle entry from quiescence. This suggests that the first 100 amino acids are necessary for some biological functions. It also indicates that specific Myc boxes can each mediate separable and distinct functions through distinct cofactor interactions. I hypothesize that the first 100 amino acids have a role in primary cell transformation and cell cycle progression. Dissection of the transregulatory domain has uncovered a region spanning the first 62 amino acids containing Myc Box I that has the highest transactivation activity. The transactivation activity can be inhibited by interaction with the p19ARF tumor suppressor and be modulated by post-translational modification. When the transactivation domain was fused to the c-Myc C-terminal DNA binding and heterodimerization domain, the resulting protein induced hyperproliferation of immortalized cells, but not Myc-mediated apoptosis. Further assessment of the transactivation activity revealed that a smaller region containing only the first 46 amino acids had substantial transactivation activity. Sequence analysis of this region compared to other Myc proteins uncovered a small sequence of high sequence similarity within the first 46 amino acids, termed MB0. Deletion of this new Myc box demonstrated that this domain is critical for transactivation, cotransformation of primary rat fibroblasts and upregulation of the target genes examined.