Crystal structures of the yeast prion Ure2p functional region in complex with glutathione and related compounds

The [URE3] phenotype in yeast Saccharomyces cerevisiae is due to an altered prion form of Ure2p, a protein involved in nitrogen catabolism. To underst and possible conformational changes at the origin of prion propagation, we previously solved the crystal structure of the Ure2p functional region [Bou sset et al. (2001) Structure 9; 39-46]. We showed the protein to have a fol d similar to that of the beta class of glutathione S-transferases (GSTs). H ere we report crystal structures of the Ure2p functional region (extending from residues 95-354) in complex with glutathione (GSH), the substrate of a ll GSTs, and two widely used GST inhibitors, namely, S-hexylglutathione and S-p-nitrobenzylglutathione. In a manner similar to what is observed in man y GM, ligand binding is not accompanied by a significant change in the conf ormation of the protein. We identify one GSH and one hydrophobic electrophi le binding site per monomer as observed in all other GSTs. The sulfur group of GSH, that conjugates electrophiles, is located near the amide group of Asn124, allowing a hydrogen bond to be formed. Biochemical data indicate th at GSH binds to Ure2p with high affinity. Its binding affects Ure2p oligome rization but has no effect on the assembly of the protein into amyloid fibr ils. Despite results indicating that Ure2p lacks GST activity, we propose t hat Ure2p is a member of the GST superfamily that may describe a novel GST class. Our data bring new insights into the function of the Ure2p active re gion.