CHARACTERIZATION OF THE DNA-BINDING AND DOMINANT-NEGATIVE ACTIVITY OFV-ERBA HOMODIMERS

The oncoprotein v-erbA is a mutated form of thyroid hormone receptor a lpha 1 that is virtually incapable of binding T-3. V-erbA is a dominan t repressor of transcription induced by thyroid hormone receptors and retinoic acid receptors; however, the genetic targets of v-erbA that l ead to oncogenesis are not known. Although v-erbA can bind as monomers and dimers to DNA containing the consensus sequence AGGTCA arranged a s 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 u sing a random DNA selection strategy, have determined that the imperfe ct everted repeat 5'-TGACC(T/C)NT(A/G)AGGTCAC is the optimal v-erbA ho modimer-binding site, where N represents any di- or trinucleotide. Fun ctional studies show that everted repeats containing this sequence are substantially more potent v-erbA response elements than direct or inv erted repeats, even though many classic T-3 response elements are dire ct repeats. Thus, v-erbA represses only a subset of T-3 response eleme nts. In a similar fashion, v-erbA was found to repress a subset of vit amin D response elements. Of general interest, the data indicate that the two molecules of a transcription factor homodimer do not necessari ly have identical DNA-binding specificities.