Prion folding produces strains

In the decades since prions were discovered by Stanley Prusiner, scientists have become comfortable with the idea that they cause disease. But how an infectious agent made solely of protein can produce varying strains has remained unclear.In the March 18 Nature, two papers based on studies of prion infections of yeast report that the conformational differences in the infecting prions determine strain variation. “This has been a major mystery of the whole prion field,” said Jonathan Weissman, professor at the University of California, San Francisco, and senior author of the first paper. “For example, people are used to there being different viral strains, because every time you get a new mutation, you get a new strain out. But if you have an infectious protein and protein is the only component, how could you have strain differences?” he told The Scientist.To answer this question, both teams used the prion form of the Sup35p protein—in Saccharomyces cerevisiae, Sup35p is required by ribosomes to terminate polypeptide production. Conversion of Sup35p to its prion form produces a defective termination phase, generating [PSI +] strains that can be readily screened by cell color and protein analysis. “We wanted to explore whether it was really possible for the different strains to be encoded within the conformation of this protein,” Weissman said.It was the design of a high-efficiency protocol for infecting yeast with preformed prion particles that enabled Weissman and his colleagues to generate different infectious prion amyloids—composed of a fragment of the Sup35p protein (Sup-NM)—at different temperatures. The infection of yeast with these different amyloids lead to different [PSI +] strains, inferring that Sup-NM conformation determined prion strain variation.In the second paper, Chih-Yen King and Ruben Diaz-Avalos from Florida State University adopted a similar approach. They used a fragment of the Sup35p protein fused to green fluorescent protein and an affinity tag to purify infectious fibrous aggregates from different prion strains. These aggregates were then be used as “seeds” to generate new aggregates in vitro from a bacterially expressed fragment of Sup35p. When these in vitro aggregates were sheared and introduced back into yeast, they produced a strain-specific [PSI] infectivity based on the original seed strain, demonstrating that the strain-specifying information was encoded in originally isolated protein fragment. “We now have a definite demonstration that proteins alone can also act as genetic material as long as they fold differently and faithfully transmit this folding pattern,” said King. The impact of this is that “it removes a psychological barrier, in that if it happens in yeast, there is no reason to think it won't happen in mammals. So people are more ready to accept the protein-only hypothesis,” he said.Susan Liebman, professor at the University of Illinois who was not involved in the study, agreed. “I think they are the critical experiments and that they prove that prions are protein based only. At least for yeast prions this should take the [protein-only] prion story out of realm of conjecture to very solid ground,” she told The Scientist. “A number of experiments over the last decade have firmly pointed to a protein-only mechanism, but there has always been one or more caveats associated with these experiments. These new results are the coup de grace and provide what I feel is unquestionable proof of the hypothesis—at least for yeast prions,” said Mick Tuite, professor at the University of Kent and author of an accompanying New and Views article.Tuite does add that although there are “very good parallels between prion behavior in yeast and animal cells,” some important differences, for example that “yeast prions require a novel cell protein (called Hsp104),” may exist.Nevertheless, “this is a very important development… [and] it is our feeling that the underlying mechanism by which prions are replicated is similar between yeast and animals,” Tuite told The Scientist.

Prion folding produces strains

Yeast prion strain differences directly linked to conformation of original infecting prion

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