Characterization of the DNA-Binding and Dominant Negative Activity of v-erbA Homodimers

The oncoprotein v-erbA is a mutated form of thyroid hormone receptor alpha1 that is virtually incapable of binding T3. V-erbA is a dominant repressor of transcription induced by thyroid hormone receptors and retinoic acid receptors; however, the genetic targets of v-erbA that lead to oncogenesis are not known. Although v-erbA can bind as monomers and dimers to DNA containing the consensus sequence AGGTCA arranged as direct, inverted, or everted repeats, it is not known which sequence represents the optimal v-erbA-binding site. Determination of the DNA recognition properties of v-erbA would allow a better understanding of the repressor activity of this oncoprotein. The current studies, by using a random DNA selection strategy, have determined that the imperfect everted repeat 5'-TGACC(T/C)NT(A/G)AGGTCAC is the optimal v-erbA homodimer-binding site, where N represents any di- or trinucleotide. Functional studies show that everted repeats containing this sequence are substantially more potent v-erbA response elements than direct or inverted repeats, even though many classic T3 response elements are direct repeats. Thus, v-erbA represses only a subset of T3 response elements. In a similar fashion, v-erbA was found to repress a subset of vitamin D response elements. Of general interest, the data indicate that the two molecules of a transcription factor homodimer do not necessarily have identical DNA-binding specificities.