C, the cellular isoform of prion protein, is widely expressed in most tissues. Despite its involvement in several bioprocesses it still has no apparent physiological role. During propagation of Transmissible Spongiform Encephalopathies, PrP
C is converted to the pathological isoform, PrP
Sc, in a process believed to be mediated by unknown host factors. PrP
Sc has altered biochemical properties and forms amyloid aggregates that display infectious characteristics. PrP
Sc is also the major component in biochemically enriched infectious samples. Other molecules co-purify with it, but the protein content of these aggregates remains unknown. The goal of this project was to identify other host molecules with high affinity for PrP
Sc. Here, we present the identification of protein molecules that co-purify with PrP
Sc isolated from naturally scrapie-infected ovine brain tissue. The infectious preparations were analyzed by two-dimensional gel electrophoresis and unknown proteins were identified by LC-MS/MS. These proteins may prove to be strategic targets for prevention and therapy of prion diseases.