THE RETINOIC ACID RECEPTOR-BETA-2 CONTAINS 2 SEPARATE CELL-SPECIFIC TRANSACTIVATION DOMAINS, AT THE N-TERMINUS AND IN THE LIGAND-BINDING DOMAIN

In contrast to other members of the steroid/thyroid hormone superfamily, not much is known about the regions involved in transactivation of the receptors for retinoic acid. To determine the transactivation function of RARs, fusion proteins between the DNA-binding domain of the yeast transcription factor GAL4 and retinoic acid receptor-alpha (RARalpha) or RARbeta were made. Transfection of these constructs resulted in RA-induced activation of a GAL4-responsive element-containing promoter. Deletion analysis revealed that RARbeta-2 has two transcription activation functions (TAFs). TAF-1 activates transcription constitutively and was mapped to the first 32 amino acids of the A-region. TAF-2 is located in the ligand-binding domain between amino acids 137 and 410 and activated transcription only in the presence of RA. The presence of two TAFs was confirmed by cotransfection of RARbeta deletion constructs with the human RARbeta-2 promoter as reporter, showing that the absence of RARbeta TAF-1 causes a decrease in transactivation, whereas truncation of TAF-2 completely blocks this function. Internal deletions in the ligand-binding domain in both GAL-RARbeta and RARbeta expression constructs resulted in a nonfunctional receptor, indicating that the complete ligand-binding domain is required for its transactivation function. Furthermore, we have shown that the contribution of the two TAFs in transcription activation varies among different cell lines, suggesting that they act in a cell-specific manner.