DIFFERENTIAL LIGAND-DEPENDENT INTERACTIONS BETWEEN THE AF-2 ACTIVATING DOMAIN OF NUCLEAR RECEPTORS AND THE PUTATIVE TRANSCRIPTIONAL INTERMEDIARY FACTORS MSUG1 AND TIF1

Using a yeast two-hybrid system we report the isolation of a novel mou se protein, mSUG1, that interacts with retinoic acid receptor alpha (R AR alpha) both in yeast cells and in vitro in a ligand- and AF-2 activ ating domain (AF-2 AD)-dependent manner and show that it is a structur al and functional homologue of the essential yeast protein SUG1, mSUG1 also efficiently interacts with other nuclear receptors, including oe strogen (ER), thyroid hormone (TR), Vitamin D3 (VDR) and retinoid X (R XR) receptors, By comparing the interaction properties of these recept ors with mSUG1 and TIF1, we demonstrate that: (i) RXR alpha efficientl y interacts with TIF1, but not with mSUG1, whereas TR alpha interacts much more efficiently with mSUG1 than with TIF1, and RAR alpha, VDR an d ER efficiently interact with mSUG1 and TIF1; (ii) the amphipathic al pha-helix core of the AF-2 AD is differentially involved in interactio ns of RAR alpha with mSUG1 and TIF1; (iii) the AF-2 AD cores of RAR al pha and ER are similarly involved in their interaction with TIF1, but not with mSUG1, Thus, the interaction interfaces between the different receptors and either mSUG1 or TIF1 may vary depending on the nature o f the receptor and the putative mediator of its AF-2 function, We disc uss the possibility that mSUG1 and TIF1 may mediate the transcriptiona l activity of the AF-2 of nuclear receptors through different mechanis ms.