Mechanism of inhibition of Psi + prion determinant propagation by a mutation of the N-terminus of the yeast Sup35 protein

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Abstract
The SUP35 gene of Saccharomyces cerevisiae encodes the polypeptide chain release factor eRF3. This protein (also called Sup35p) is thought to be able to undergo a heritable conformational switch, similarly to mammalian prions, giving rise to the cytoplasmically inherited Ψ+ determinant. A dominant mutation (PNM2 allele) in the SUP35 gene causing a Gly58→Asp change in the Sup35p N‐terminal domain eliminates Ψ+. 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 wild‐type Sup35p. The overexpression of mutant Sup35p induced the de novo appearance of Ψ+ 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 Ψ+‐eliminating mutation does not alter the initial binding of Sup35p molecules to the Sup35p Ψ+‐specific aggregates, but rather inhibits its subsequent prion‐like rearrangement and/or binding of the next Sup35p molecule to the growing prion‐like Sup35p aggregate.