PK-sensitive PrP^Sc is infectious and shares basic structural features with PK-resistant PrP^Sc

One of the main characteristics of the transmissible isoform of the prion protein (PrP^Sc) is its partial resistance to proteinase K (PK) digestion. Diagnosis of prion disease typically relies upon immunodetection of PK-digested PrP^Sc following Western blot or ELISA. More recently, researchers determined that there is a sizeable fraction of PrP^Sc that is sensitive to PK hydrolysis (sPrP^Sc). Our group has previously reported a method to isolate this fraction by centrifugation and showed that it has protein misfolding cyclic amplification (PMCA) converting activity. We compared the infectivity of the sPrP^Sc versus the PK-resistant (rPrP^Sc) fractions of PrP^Sc and analyzed the biochemical characteristics of these fractions under conditions of limited proteolysis. Our results show that sPrP^Sc and rPrP^Sc fractions have comparable degrees of infectivity and that although they contain different sized multimers, these multimers share similar structural properties. Furthermore, the PK-sensitive fractions of two hamster strains, 263K and Drowsy (Dy), showed strain-dependent differences in the ratios of the sPrP^Sc to the rPrP^Sc forms of PrP^Sc. Although the sPrP^Sc and rPrP^Sc fractions have different resistance to PK-digestion, and have previously been shown to sediment differently, and have a different distribution of multimers, they share a common structure and phenotype.