TIF2 mediates the synergy between RARalpha 1 activation functions AF-1 and AF-2.

Nuclear receptors recruit coregulator complexes through both their AF-1 and AF-2 activation domains. Here we demonstrate that TIF2, a p160 coactivator, is able to bridge the two activation domains of the retinoic acid (RA) receptor isotype RARalpha1, resulting in synergistic activation of transcription. Bridging requires the presence of motifs in region A of RARalpha1 and in the activation domain AD1 of TIF2. Notably, only RARalpha1 exerted this interaction, which requires additional unknown factors. This is the first observation of a RAR isotype-selective coactivator interaction. Because another p160 coactivator, SRC-1, has no effect, this is also the first demonstration of a difference between the members of this coactivator family.