Heterologous cross-seeding mimics cross-species prion conversion in a yeast model

BACKGROUND: Prions are self-perpetuating, infectious, aggregated proteins that are associated with several neurodegenerative diseases in mammals and heritable traits in yeast. Sup35p, the protein determinant of the yeast prion [PSI(+)], has a conserved C terminal domain that performs the Sup35p function and a prion domain that is highly divergent. Prions formed by chimeras of the prion domain of various species fused to the C domain of Saccharomyces cerevisiae exhibit a 'species barrier', a phenomenon first observed in mammals, and often fail to transmit the prion state to chimeras with prion domains of other species. RESULTS: We focus on the chimera containing the prion domain of Pichia methanolica and examine how tight the 'species barrier' is between the chimera and S. cerevisiae. Although either of two Q/N-rich prions, [PSI(+)] or [PIN(+)], enhances the formation of the chimeric prion, [CHI(+)(PM)], neither a non-Q/N-rich prion nor a non-prion Q-rich aggregate promotes the formation of [CHI(+)(PM)]. [CHI(+)(PM)] has many features characteristic of yeast prions: aggregation, cytoplasmic transmission and a two-level protein structure. [CHI(+)(PM)] formed in the presence of [PSI(+)] can propagate independently of [PSI(+)] and forms at least two different variants of the prion, suggesting the generation and not transmission of new prion seeds. CONCLUSION: Although the sequence similarity between the S. cerevisiae Q/N-rich prion determinants and the P. methanolica prion domain is low, we find that the chimera containing the prion domain of P. methanolica can occasionally be cross-seeded by [PSI(+)] to mimic crossing the species barrier, to form the [CHI(+)(PM)] prion. Our data suggests that crossing the barrier occurs by a de novo formation of the foreign chimeric prion. Thus, the species barrier appears to be crossed by a heterologous seeding mechanism, wherein the infected prion protein uses the pre-existing seed as an inefficient template.