Structural evidence for the critical role of the prion protein hydrophobic region in forming an infectious prion

Prion or PrPSc is the proteinaceous infectious agent causing prion diseases in various mammalian species. Despite decades of research, the structural basis for PrPSc formation and prion infectivity remains elusive. To understand the role of the hydrophobic region in forming infectious prion at the molecular level, we report X-ray crystal structures of mouse (Mo) prion protein (PrP) (residues 89-230) in complex with a nanobody (Nb484). Using the recombinant prion propagation system, we show that the binding of Nb484 to the hydrophobic region of MoPrP efficiently inhibits the propagation of proteinase K resistant PrPSc and prion infectivity. In addition, when added to cultured mouse brain slices in high concentrations, Nb484 exhibits no neurotoxicity, which is drastically different from other neurotoxic anti-PrP antibodies, suggesting that the Nb484 can be a potential therapeutic agent against prion disease. In summary, our data provides the first structure-function evidence supporting a crucial role of the hydrophobic region of PrP in forming an infectious prion. Author summary Prion disease is caused by the misfolding of normal prion protein (PrP (c)) to its pathogenic isoform, termed PrPSc. The underlying mechanism for such fatal misfolding is still unknown, which greatly impedes the development of efficacious therapeutic strategies against these currently incurable diseases. Previously, we reported the crystal structure of human PrP in complex with a nanobody (Nb484), which could inhibit mouse prion propagation in a cell culture model. To understand the molecular mechanism for the inhibitory action of Nb484, in this study, firstly we determine the crystal structures of mouse PrP-Nb484 complexes and show that Nb484 binds to and stabilizes the hydrophobic region of PrP. Then in our newly developed recombinant prion propagation system, we show that Nb484 inhibits prion propagation through the competitive inhibition mechanism. We further demonstrate that, contrary to certain neurotoxic anti-PrP antibodies, the inhibitory Nb484 exhibits no neurotoxicity on cultured organotypic brain slices, indicating the great therapeutic potential of Nb484. In summary, our current study provides the first structure-function evidence supporting a critical role of the hydrophobic region in forming the infectious prion and highlights the therapeutic potential of Nb484.