Neurodegenerative diseases of animals and humans including scrapie, bovine spongiform encephalopathy, and Creutzfeldt-Jakob disease are caused by unusual infectious pathogens called prions. There is no evidence for a nucleic acid in the prion, but diverse experimental results indicate that a host-derived protein called PrPSc is a component of the infectious particle. Experiments with scrapie-infected cultured cells show that PrPSc is derived from a normal cellular protein called PrPC through an unknown posttranslational process. We have analyzed the amino acid sequence and posttranslational modifications of PrPSc and its proteolytically truncated core PrP 27-30 to identify potential candidate modifications that could distinguish PrPSc from PrPC. The amino acid sequence of PrP 27-30 corresponds to that predicted from the gene and cDNA. Mass spectrometry of peptides derived from PrPSc has revealed numerous modifications including two N-linked carbohydrate moieties, removal of an amino-terminal signal sequen...