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 superfamil y, not much is known about the regions involved in transactivation of the receptors for retinoic acid. To determine the transactivation func tion of RARs, fusion proteins between the DNA-binding domain of the ye ast transcription factor GAL4 and retinoic acid receptor-alpha (RARalp ha) or RARbeta were made. Transfection of these constructs resulted in RA-induced activation of a GAL4-responsive element-containing promote r. Deletion analysis revealed that RARbeta-2 has two transcription act ivation 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 a nd activated transcription only in the presence of RA. The presence of two TAFs was confirmed by cotransfection of RARbeta deletion construc ts with the human RARbeta-2 promoter as reporter, showing that the abs ence of RARbeta TAF-1 causes a decrease in transactivation, whereas tr uncation of TAF-2 completely blocks this function. Internal deletions in the ligand-binding domain in both GAL-RARbeta and RARbeta expressio n constructs resulted in a nonfunctional receptor, indicating that the complete ligand-binding domain is required for its transactivation fu nction. Furthermore, we have shown that the contribution of the two TA Fs in transcription activation varies among different cell lines, sugg esting that they act in a cell-specific manner.