THE YEAST ADA COMPLEX MEDIATES THE LIGAND-DEPENDENT ACTIVATION FUNCTION AF-2 OF RETINOID-X AND ESTROGEN-RECEPTORS

Nuclear receptors can function as ligand-inducible transregulators in both mammalian and yeast cells, indicating that important features of control of transcription have been conserved throughout evolution. Her e, we report the isolation and characterization of a yeast protein tha t exhibits properties expected for a coactivator/mediator of the ligan d-dependent activation function AF-2 present in the ligand-binding dom ain (LBD, region E) of the retinoid X (RXR alpha) and estrogen (ER alp ha) receptors. This protein is identical to Ada3, a component of the y east Ada coactivator complex. We demonstrate that: (1) the region enco mpassing residues 347-702 of Ada3 interacts with the LBD of RXR alpha and ER alpha in a ligand-dependent manner in yeast; (2) this interacti on corresponds to a direct binding and requires the integrity of the c ore of the AF-2 activating domain (AF-2 AD) of both RXR alpha and ER a lpha; (3) Ada3 as well as Ada2 and Gcn5, two other components of the A da complex, are required for maximal AF-2 activity in yeast; and (4) A da3 is able to enhance the AF-2 activity of RXR alpha and ER alpha whe n overexpressed in yeast and mammalian cells. Taken together, these da ta indicate that ligand-dependent transactivation by RXR alpha and ER alpha in yeast is mediated at least in part by the Ada complex, in whi ch the Ada3 subunit directly binds to the holoreceptor LED.