Strain-specific sequences required for yeast [PSI+] prion propagation

Unraveling infectious structures, strain variants and species barriers for the yeast prion [PSI+]

Prions are proteins that can access multiple conformations, at least one of which is beta-sheet rich, infectious and self-perpetuating in nature. These infectious proteins show several remarkable biological activities, including the ability to form multiple infectious prion conformations, also known as strains or variants, encoding unique biological phenotypes, and to establish and overcome prion species (transmission) barriers. In this Perspective, we highlight recent studies of the yeast prion [PSI(+)], using various biochemical and structural methods, that have begun to illuminate the molecular mechanisms by which self-perpetuating prions encipher such biological activities. We also discuss several aspects of prion conformational change and structure that remain either unknown or controversial, and we propose approaches to accelerate the understanding of these enigmatic, infectious conformers.

Heterologous prion interactions are altered by mutations in the prion protein Rnq1p

Prions in the yeast Saccharomyces cerevisiae show a surprising degree of interdependence. Specifically, the rate of appearance of the [PSI+] prion, which is thought to be an important mechanism to respond to changing environmental conditions, is greatly increased by another prion, [RNQ+]. While the domains of the Rnq1 protein important for formation of the [RNQ+] prion have been defined, the specific residues required remain unknown. Furthermore, residues in Rnq1p that mediate the interaction between [PSI+] and [RNQ+] are unknown. To identify residues important for prion protein interactions, we created a mutant library of Rnq1p clones in the context of a chimera that serves as proxy for [RNQ+] aggregates. Several of the mutant Rnq1p proteins showed structural differences in the aggregates they formed, as revealed by semi-denaturing detergent agarose gel electrophoresis. Additionally, several of the mutants showed a striking defect in the ability to promote [PSI+] induction. These data indicate that the mutants formed strain variants of [RNQ+]. By dissecting the mutations in the isolated clones, we found five single mutations that caused [PSI+] induction defects, S223P, F184S, Q239R, N297S, and Q298R. These are the first specific mutations characterized in Rnq1p that alter [PSI+] induction. Additionally, we have identified a region important for the propagation of certain strain variants of [RNQ+]. Deletion of this region (amino acids 284-317) affected propagation of the high variant but not medium or low [RNQ+] strain variants. Furthermore, when the low [RNQ+] strain variant was propagated by Delta284-317, [PSI+] induction was greatly increased. These data suggest that this region is important in defining the structure of the [RNQ+] strain variants. These data are consistent with a model of [PSI+] induction caused by physical interactions between Rnq1p and Sup35p.

Prion variants and species barriers among Saccharomyces Ure2 proteins

As hamster scrapie cannot infect mice, due to sequence differences in their PrP proteins, we find "species barriers" to transmission of the [URE3] prion in Saccharomyces cerevisiae among Ure2 proteins of S. cerevisiae, paradoxus, bayanus, cariocanus, and mikatae on the basis of differences among their Ure2p prion domain sequences. The rapid variation of the N-terminal Ure2p prion domains results in protection against the detrimental effects of infection by a prion, just as the PrP residue 129 Met/Val polymorphism may have arisen to protect humans from the effects of cannibalism. Just as spread of bovine spongiform encephalopathy prion variant is less impaired by species barriers than is sheep scrapie, we find that some [URE3] prion variants are infectious to another yeast species while other variants (with the identical amino acid sequence) are not. The species barrier is thus prion variant dependent as in mammals. [URE3] prion variant characteristics are maintained even on passage through the Ure2p of another species. Ure2p of Saccharomyces castelli has an N-terminal Q/N-rich "prion domain" but does not form prions (in S. cerevisiae) and is not infected with [URE3] from Ure2p of other Saccharomyces. This implies that conservation of its prion domain is not for the purpose of forming prions. Indeed the Ure2p prion domain has been shown to be important, though not essential, for the nitrogen catabolism regulatory role of the protein.

Protein inheritance (prions) based on parallel in-register beta-sheet amyloid structures

Most prions (infectious proteins) are self-propagating amyloids (filamentous protein multimers), and have been found in both mammals and fungal species. The prions [URE3] and [PSI+] of yeast are disease agents of Saccharomyces cerevisiae while [Het-s] of Podospora anserina may serve a normal cellular function. The parallel in-register beta-sheet structure shown by prion amyloids makes possible a templating action at the end of filaments which explains the faithful transmission of variant differences in these molecules. This property of self-reproduction, in turn, allows these proteins to act as de facto genes, encoding heritable information.