Evidence for Two Prions in Yeast: [URE3] and [PSI]

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A prion is an infectious protein, a concept that has its origins in studies of scrapie of sheep, kuru, and Creutzfeldt-Jakob disease of humans and similar spongiform encephalopathies of other mammals (Alper etal. 1967; Prusiner 1982; Prusiner and McKinley 1987). A prion is believed to be an altered form of a normal cellular protein that may have lost its normal function but has acquired the ability to convert the normal form of the protein to this altered (prion) form. If either the absence of the normal protein function or the accumulation of the abnormal form is detrimental to the organism, a disease (or phenotype) is seen. Although still somewhat controversial, an impressive body of evidence supporting this idea has built up, including the UV resistance of the scrapie agent, the absence in purified scrapie agent preparations of an identifiable nucleic acid genome, epidemiological studies, and studies using transgenic mice that strongly point to this idea. These studies are well summarized in the other contributions to this volume and elsewhere (Brown and Gajdusek 1991; Prusiner 1994). Most recently, the in vitro conversion of the normal form of PrP to the prion form, induced by the presence of the prion form, has been demonstrated (Kocisko et al. 1994).