Characterization of prion protein (PrP)-derived peptides that discriminate full-length PrPSc from PrPC

On our initial discovery that prion protein (PrP)-derived peptides were capable of capturing the pathogenic prion protein (PrPSc), we have been interested in how these peptides interact with PrPSc. After screening peptides from the entire human PrP sequence, we found two peptides (PrP₁₉₋₃₀ and PrP₁₀₀₋₁₁₁) capable of binding full-length PrPSc in plasma, a medium containing a complex mixture of other proteins including a vast excess of the normal prion protein (PrPC). The limit of detection for captured PrPSc was calculated to be 8 amol from a [almost equal to]10⁵-fold dilution of 10% (wt/vol) human variant Creutzfeldt-Jakob disease brain homogenate, with >3,800-fold binding specificity to PrPSc over PrPC. Through extensive analyses, we show that positively charged amino acids play an important, but not exclusive, role in the interaction between the peptides and PrPSc. Neither hydrophobic nor polar interactions appear to correlate with binding activity. The peptide-PrPSc interaction was not sequence-specific, but amino acid composition affected binding. Binding occurs through a conformational domain that is only present in PrPSc, is species-independent, and is not affected by proteinase K digestion. These and other findings suggest a mechanism by which cationic domains of PrPC may play a role in the recruitment of PrPC to PrPSc.