MECHANISM OF INHIBITION OF PSI(-TERMINUS OF THE YEAST SUP35 PROTEIN()PRION DETERMINANT PROPAGATION BY A MUTATION OF THE N)

The SUP35 gene of Saccharomyces cerevisiae encodes the polypeptide cha in release factor eRF3. This protein (also called Sup35p) is thought t o be able to undergo a heritable conformational switch, similarly to m ammalian prions, giving rise to the cytoplasmically inherited Psi(+) d eterminant, A dominant mutation (PNM2 allele) in the SUP35 gene causin g a Gly58-->Asp change in the Sup35p N-terminal domain eliminates Y+. Here we observed that the mutant Sup35p can be converted to the prion- like form in vitro, but such conversion proceeds slower than that of w ild-type Sup35p. The overexpression of mutant Sup35p induced the de no vo appearance of Psi(+) cells containing the prion-like form of mutant Sup35p, which was able to transmit its properties to wild-type Sup35p both in vitro and in vivo. Our data indicate that this Psi(+)-elimina ting mutation does not alter the initial binding of Sup35p molecules t o the Sup35p Psi(+)-specific aggregates, but rather inhibits its subse quent prion-like rearrangement and/or binding of the next Sup35p molec ule to the growing prion-like Sup35p aggregate.