INTERACTION BETWEEN YEAST SUP45P (ERF1) AND SUP35P (ERF3) POLYPEPTIDE-CHAIN RELEASE FACTORS - IMPLICATIONS FOR PRION-DEPENDENT REGULATION

The SUP45 and SUP35 genes of Saccharomyces cerevisiae encode polypepti de chain release factors eRF1 and eRF3, respectively, It has been sugg ested that the Sup35 protein (Sup35p) is subject to a heritable confor mational switch, similar to mammalian prions, thus giving rise to the non-Mendelian [PSI+] nonsense suppressor determinant, In a [PSI+] stat e, Sup35p forms high-molecular-weight aggregates which may inhibit Sup 35p activity, leading to the [PSI+] phenotype, Sup35p is composed of t he N-terminal domain (N) required for [PSI+] maintenance, the presumab ly nonfunctional middle region (M), and the C-terminal domain (C) esse ntial for translation termination, In this study, we observed that the N domain, alone or as a part of larger fragments, can form aggregates in [PSI+] cells, Two sites for Sup35p binding were found within Sup35 p: one is formed by the N and M domains, and the other is located with in the C domain, Similarly to Sup35p, in [PSI+] cells Sup45p was found in aggregates, The aggregation of Sup45p is caused by its binding to Sup35p and was not observed when the aggregated Sup35p fragments did n ot contain sites for Sup45p binding, The incorporation of Sup35p into the aggregates should inhibit its activity, The N domain of Sup35p, re sponsible for its aggregation in [PSI+] cells, may thus act as a repre ssor of another polypeptide chain release factor, Sup45p, This phenome non represents a novel mechanism of regulation of gene expression at t he posttranslational level.