Yeast [PSI⁺] "prions" that are crosstransmissible and susceptible beyond a species barrier through a quasi-prion state

The yeast [PSI⁺] element represents an aggregated form of release factor Sup35p and is inherited by a prion mechanism. A "species barrier" prevents crosstransmission of the [PSI⁺] state between heterotypic Sup35p "prions." Kluyveromyces lactis and Yarrowia lipolytica Sup35 proteins, however, show interspecies [PSI⁺] transmissibility and susceptibility and a high spontaneous propagation rate. Cross-seeding was visualized by coaggregation of differential fluorescence probes fused to heterotypic Sup35 proteins. This coaggregation state, referred to as a "quasi-prion" state, can be stably maintained as a heritable [PSI⁺] element composed of heterologous Sup35 proteins. K. lactis Sup35p was capable of forming [PSI⁺] elements not only in S. cerevisiae but in K. lactis. These two Sup35 proteins contain unique multiple imperfect oligopeptide repeats responsible for crosstransmission and high spontaneous propagation of novel [PSI⁺] elements.