The protease-sensitive N-terminal polybasic region of prion protein modulates its conversion to the pathogenic prion conformer

Conversion of normal prion protein (PrPC) to the pathogenic PrPSc conformer is central to prion diseases such as Creutzfeldt-Jakob disease and scrapie; however, the detailed mechanism of this conversion remains obscure. To investigate how the N-terminal polybasic region of PrP (NPR) influences the PrPC-to-PrPSc conversion, we analyzed two PrP mutants: Delta N6 (deletion of all six amino acids in NPR) and Met4-1 (replacement of four positively charged amino acids in NPR with methionine). We found that Delta N6 and Met4-1 differentially impacted the binding of recombinant PrP (recPrP) to the negatively charged phospholipid 1-palmitoyl-2-oleoylphosphatidylglycerol, a nonprotein cofactor that facilitates PrP conversion. Both mutant recPrPs were able to form recombinant prion (recPrP(Sc)) in vitro, but the convertibility was greatly reduced, with Delta N6 displaying the lowest convertibility. Prion infection assays in mammalian RK13 cells expressing WT or NPR-mutant PrPs confirmed these differences in convertibility, indicating that the NPR affects the conversion of both bacterially expressed recPrP and posttranslationally modified PrP in eukaryotic cells. We also found that both WT and mutant recPrP(Sc) conformers caused prion disease in WT mice with a 100% attack rate, but the incubation times and neuropathological changes caused by two recPrP(Sc) mutants were significantly different from each other and from that of WT recPrP(Sc). Together, our results support that the NPR greatly influences PrPC-to-PrPSc conversion, but it is not essential for the generation of PrPSc. Moreover, the significant differences between Delta N6 and Met4-1 suggest that not only charge but also the identity of amino acids in NPR is important to PrP conversion.