Functional Interaction of the c-Myc Transactivation Domain with the TATA Binding Protein: Evidence for an Induced Fit Model of Transactivation Domain Folding

Abstract

C-Myc is a member of a family of sequence specific-DNA binding proteins that are thought to regulate the transcription of genes involved in normal cell growth, differentiation, and apoptosis. In order to understand how human c-myc functions as a transcription factor, we have studied the mechanism of action and structure of the N-terminal transactivation domain, amino acids 1−143. In a protein interaction assay, c-myc_1-143 bound selectively to two basal transcription factors, the TATA binding protein (TBP) and the RAP74 subunit of TFIIF. Furthermore, the isolated c-myc transactivation domain competed for limiting factors required for the assembly of a functional preinitiation complex. This squelching of basal transcription was reversed in a dose-dependent manner by recombinant TBP. Taken together, these results identify TBP as an important target for the c-myc transactivation domain, during transcriptional initiation. Similar to other transactivation domains, the c-myc_1-143 polypeptide showed little or no evidence of secondary structure, when measured by circular dichroism spectroscopy (CD) in aqueous solution. However, significant α-helical conformation was observed in the presence of the hydrophobic solvent trifluoroethanol. Strikingly, addition of TBP caused changes in the CD spectra consistent with induction of protein conformation in c-myc_1-143 during interaction with the target factor. This change was specific for TBP as a similar effect was not observed in the presence of TFIIB. These data support a model in which target factors induce or stabilize a structural conformation in activator proteins during transcriptional transactivation.