FUNCTIONAL INTERACTION OF THE C-MYC TRANSACTIVATION DOMAIN WITH THE TATA-BINDING PROTEIN - EVIDENCE FOR AN INDUCED FIT MODEL OF TRANSACTIVATION DOMAIN FOLDING

c-Myc is a member of a family of sequence specific-DNA binding protein s that are thought to regulate tile transcription of genes involved in normal cell growth, differentiation, and apoptosis. In order to under stand how human c-myc functions as a transcription factor, we have stu died the mechanism of action and structure of the N-terminal transacti vation domain, amino acids 1-143. In a protein interaction assay-, c-m yc(1-143) bound selectively to two basal transcription factors, the TA TA binding protein (TBP) and the RAP74 subunit of TFIIF. Furthermore, the isolated c-myc transactivation domain competed for limiting factor s required the assembly of a functional preinitiation complex. This sq uelching of basal transcription was reversed in a dose,dependent manne r by recombinant TBP. Taken together, these results identify TBP as an important target for the c-myc transactivation domain, during transcr iptional initiation. Similar to other transactivation domains, the c-m yc(1-143) polypeptide showed little or no evidence of secondary struct ure, when measured by circular dichroism spectroscopy (CD) in aqueous solution. However, significant alpha-helical conformation was observed in the presence of the hydrophobic solvent trifluoroethanol. Striking ly, addition of TBP caused changes in the CD spectra consistent with i nduction of protein conformation in c-myc(1-143) during interaction wi th the target factor. This change was specific for TBP as a similar ef fect was not observed in the presence of TFIIB, These data support a m odel in which target factors induce or stabilize a structural conforma tion in activator proteins during transcriptional transactivation.