Susan W. Liebman

Protein as the hereditary material - a yeast model for prion diseases in mammals

Certain neurodegenerative diseases, such as ‘mad cow’ disease, are transmitted in an unusual way-- so unusual that it challenges the central dogma. Indeed, the infectious agent for these diseases appears to be the PrP protein without any nucleic acid. Infectivity depends upon the shape into which the PrP protein is folded: when some PrP is in its disease-causing (‘prion’) conformation, it converts normal PrP into that form too. Several genetic traits in yeast are propagated by this unusual ‘protein only’ mechanism, and although they involve proteins distinct from PrP, the term prion has been expanded to include them. We are using yeast to elucidate the factors that influence prion inheritance and to look for new prions.

One yeast prion, [PSI⁺], is caused by an altered shape of a protein called eRF3 (a translational release factor). When eRF3 is in the prion shape it aggregates and loses its normal activity and this causes the [PSI⁺] phenotype. An important piece of evidence for this prion hypothesis was our finding that a transient excess of eRF3 protein induces the permanent appearance of [PSI⁺]. Presumably this occurs because the excess eRF3 increases the chance that some eRF3 molecules will accidentally fold into the prion shape. Our finding that an intermediate level of the Hsp104 chaperone is required for the propagation of [PSI⁺] also provides dramatic support for the prion hypothesis since the only function of Hsp104 is to facilitate protein folding. This finding has spawned considerable interest in the role of other chaperones in the maintenance of various prions.

We have identified nine prion candidate genes from a genetic screen and are now characterizing these putative prion proteins. We are also examining the interactions between prions and find that different prions influence the appearance and propagation of other prions.

Can prion proteins with the identical amino acid sequence take on more then one heritable prion shape? It appears that PrP can! These different prions have been called ‘strains’. The relationship between different prion strains and their effects on prion infectivity are of great interest. This can now be studied in yeast because we found [PSI⁺] ‘strains’ and [PIN⁺] ‘strains’ with distinct heritable phenotypes. We are currently examining the biochemical differences and genetic interactions between these prion ‘strains’.

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