LIGAND-DEPENDENT INTERACTION OF NUCLEAR RECEPTORS WITH POTENTIAL TRANSCRIPTIONAL INTERMEDIARY FACTORS (MEDIATORS)

The activity of the ligand-inducible activation function 2 (AF-2) cont ained in the ligand binding domain (LBD) of nuclear receptors (NRs) is thought to be mediated by transcriptional intermediary factors (TIFs) . We have recently reported the isolation and characterization of two novel mouse proteins, designated TIF1 and mSUG1, that interact in a li gand-dependent fashion with the LBD (region E) of several NRs in vivo as well as in vitro. Remarkably, these interactions require the conser ved core motif of the AF-2 activating domain (AF-2 AD) and can be bloc ked by AF-2 antagonists. TIF1 and mSUG1 might therefore represent TIFs /mediators for the ligand-dependent AF-2 of NRs. By comparing the inte raction properties of these two putative TIFs with different NRs inclu ding the oestrogen (ER); thyroid hormone (TR), vitamin D3 (VDR), retin oic acid (RAR alpha) and retinoid X (RXR) receptors, we demonstrate th at: (i) RXR alpha efficiently interacts with TIF1, but not with mSUG1, whereas TR alpha interacts much more efficiently with mSUG1 than with TIF1, and RAR alpha, VDR and ER efficiently interact with both TIF1 a nd mSUG1; (ii) the amphipathic alpha helix core of AF-2 AD is differen tially involved in the interactions of RAR alpha with TIF1 and mSUG1; and (iii) the AF-2 AD cores of RAR alpha and ER are similarly involved in their interaction with TIF1, but not with mSUG1. Thus the interact ion interfaces between the various NRs and either TIF1 or mSUG1 may va ry depending on the nature of both the receptor and the putative media tor of its AF-2 function. We discuss the possible roles of TIF1 and mS UG1 as mediators of the transcriptional activity of the AF-2 of NRs.