Protease-resistant and detergent-insoluble prion protein is not necessarily associated with prion infectivity

PrP(Sc), an abnormal isoform of PrP(C), is the only known component of the prion, an agent causing fatal neurodegenerative disorders such as bovine spongiform encephalopathy (BSE) and Creutzfeldt-Jakob disease (CJD). It has been postulated that prion diseases propagate by the conversion of detergent- soluble and protease-sensitive PrP(C) molecules into protease-resistant and insoluble PrP(Sc) molecules by a mechanism in which PrP(Sc) serves as a template. We show here that the chemical chaperone dimethyl sulfoxide (Me₂SO) can partially inhibit the aggregation of either PrP(Sc) or that of its protease-resistant core PrP27-30. Following Me₂SO removal by methanol precipitation, solubilized PrP27-30 molecules aggregated into small and amorphous structures that did not resemble the rod configuration observed when scrapie brain membranes were extracted with Sarkosyl and digested with proteinase K. Interestingly, aggregates derived from Me₂SO-solubilized PrP27-30 presented less than 1% of the prion infectivity obtained when the same amount of PrP27-30 in rods was inoculated into hamsters. These results suggest that the conversion of PrP(C) into protease-resistant and detergent- insoluble PrP molecules is not the only crucial step in prion replication. Whether an additional requirement is the aggregation of newly formed proteinase K-resistant PrP molecules into uniquely structured aggregates remains to be established.