Role of the essential yeast protein PSU1 in transcriptional enhancement bythe ligand-dependent activation function AF-2 of nuclear receptors

Nuclear receptors (NRs) can function as ligand-inducible transregulators in both mammalian and yeast cells, indicating that important features of tran scriptional control have been conserved throughout evolution. We report her e the isolation and characterization of an essential yeast protein of unkno wn function, PSU1, which exhibits properties expected for a co-activator/me diator of the ligand-dependent activation function AF-2 present in the liga nd-binding domain (LBD, region E) of NRs, PSU1 interacts in a ligand-depend ent manner with the LED of several NRs, including retinoic acid (RAR alpha) , retinoid X (RXR alpha), thyroid hormone (TR alpha), vitamin D3 (VDR) and oestrogen (ER alpha) receptors, Importantly, both in yeast and in vitro, th ese interactions require the integrity of the AF-2 activating domain. When tethered to a heterologous DNA-binding domain, PSU1 can activate transcript ion on its own. By using yeast reporter cells that express PSU1 conditional ly, we show that PSU1 is required for transactivation by the AF-2 of ER alp ha, Taken together these data suggest that in yeast, PSU1 is involved in li gand-dependent transactivation by NRs, Sequence analysis revealed that in a ddition to a highly conserved moth found in a family of MutT-related protei ns, PSU1 contains several a-helical leucine-rich motifs sharing the consens us sequence LLx Phi L (x, any amino acid; Phi, hydrophobic amino acid) in r egions that elicit either transactivation or NR-binding activity.