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The only identified component of the scrapie prion is PrPSc, a glycosylinositol phospholipid (GPI)-linked protein that is derived from the cellular isoform (PrPC) by an as yet unknown posttranslational event. Analysis of the PrPSc GPI has revealed six different glycoforms, three of which are unprecedented. Two of the glycoforms contain N-acetylneuraminic acid, which has not been previously reported as a component of any GPI. The largest form of the GPI is proposed to have a glycan core consisting of Man alpha-Man alpha-Man-(NeuAc-Gal-GalNAc-)Man-GlcN-Ino. Identical PrPSc GPI structures were found for two distinct isolates or "strains" of prions which specify different incubation times, neuropathology, and PrPSc distribution in brains of Syrian hamsters. Limited analysis of the PrPC GPI reveals that it also has sialylated glycoforms, arguing that the presence of this monosaccharide does not distinguish PrPC from PrPSc.

Studies with transgenic mice expressing foreign and mutant prion protein (PrP) genes have provided a wealth of new knowledge about the structure of the prion particle, the pathogenesis of prion diseases, and the events that feature in the replication of prions (Scott et al. 1989;Hsiao et al. 1990;Prusiner et al. 1990;Westaway et al. 1991). Investigations of transgenic (Syrian hamster PrP) [Tg(SHaPrP)] mice expressing SHaPrP demonstrated that PrP sc in the inoculum dictates whether mouse prions (Mo prions) or Syrian hamster prions (SHa prions) will be produced. Furthermore, the scrapie incubation times and neuropathology are determined by the particular prion synthesized. Although no physical evidence is available, it seems likely that PrP c and PrP sc transiently form a complex during the synthesis of nascent PrP sc molecules Prusiner 1991).Early studies of Dickinson and colleagues not only identified inbred strains of mice with short or long incubation times, but also distinct isolates of the scrapie agent distinguishable by their different incubation times and neuropathologic lesions Dickinson 1968, 1973; Dickinson and Meikle 4present address:

The cellular prion protein (PrP') is encoded by a chromosomal gene, and its scrapie isoform (PrP") features in all aspects of the prion diseases. Prior to the studies reported here, purification of PrPC has only been accomplished using immunoaffinity chromatography yielding small amounts of protein. Brain homogenates contain two PrPC forms designated PrPC-I and -11. These proteins were purified from a microsomal fraction by detergent extraction and separated by immobilized Cu2+ ion affinity chromatography. PrPC-I1 appears to be generated from PrPC-I by limited proteolysis of the N-terminus. Fractions enriched for PrPC-I were purified further by cationexchange chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Greater than 90% of the final product migrated as a broad band of M, 33-35 kDa as judged by silver staining after SDS-PAGE. Digestion of PrPC-I with peptide-N-glycosidase (PNGase) compressed the band and shifted its mobility giving an M, of 27 kDa. The protocol described should be amenable to large-scale preparation of PrPC, enabling physical comparisons of PrPC and PrP''.