A prerequisite for understanding Prion Diseases is unraveling the molecular mechanism leading to the conversion process wherein the α-helical motifs of the cellular prion protein (PrPC) are replaced by β-sheets in the disease-causing form (PrPSc). Importantly, most point mutations linked to inherited prion diseases are clustered in the C-terminal domain region of PrPC and cause spontaneous conversion to PrPSc. Structural studies with PrPC variants promise new clues regarding the proposed conversion mechanism and may help identify “hot spots” in PrPC involved in the pathogenic conversion. These investigations may also shed light on the early structural rearrangements occurring in some PrPC epitopes thought to be involved in modulating prion susceptibility.
Our solution-state NMR studies on human prion protein carrying pathological point mutations revealed structural disorders of the β2-α2 loop. This, together with the increased spacing between this loop and the C-terminal part of α3 helix are key pathological features. The disruption of these interactions and the consequent exposure of the hydrophobic core to the solvent led to the suggestion that the early stage of prion conversion possibly involves the critical epitope formed by the β2-α2 loop and the α3 helix [1-7].
Additionally, we crystallized the full-length HuPrP in complex with a nanobody (Nb484) that inhibits prion propagation. Our X-ray structures revealed that the palindromic motif arranges into a novel β-strand, denoted β0 (residues 118–122), which folds into a three-stranded antiparallel β-sheet with β1 and β2. The implications of these findings are remarkable, as we provide a first atomic structural view of the palindromic region adopting a well-defined β-sheet conformation [8].
By using synchrotron-based X-ray absorption fine structure (XAFS) techniques, we have investigated how copper coordination in one of the copper binding sites located in the N-terminus may explain the different susceptibility to prion diseases observed in different mammalian species including human, sheep, bank vole and opossum [9, 10].
Finally, our recent solution-state NMR studies revealed a new role of PrPC N-terminus as a dynamic and functional element responsible for protein-protein interaction. Among the different PrPC protein interactors, we provide the first structural evidence that the PrPC N-terminus interacts with the neuronal cell adhesion molecule (NCAM) [11].
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8. Abskharon RN., et al., Probing the N-terminal β-sheet conversion in the crystal structure of the human prion protein bound to a nanobody. JACS, 2014. 136(3):937-44.
9. D'Angelo, P., et al., Effects of the pathological Q212P mutation on human prion protein non-octarepeat copper-binding site. Biochemistry, 2012. 51(31): p. 6068-79.
10. Giachin, G., et al., The non-octarepeat copper binding site of the prion protein is a key regulator of prion conversion. Sci Rep, 2015. 5, 15253; doi: 10.1038/srep15253
11. Slapšak U., et al., Prion Protein N-terminus Mediates Functional Interactions with NCAM Fibronectin Domain. J. Biol. Chem., 2016. 291, 21857-21868.
