Abstract
A self-perpetuating conformational conversion of the prion protein (PrP) is believed to underlie pathology and transmission of prion diseases. Here we explore the effects of N-terminal pathogenic mutations (P102L, P105L, A117V) and the residue 129 polymorphism on amyloid fibril formation by the human PrP fragment 23-144, an in vitro conversion model that can reproduce certain characteristics of prion replication such as strains and species barriers. We find that these amino acid substitutions neither affect PrP23-144 amyloidogenicity nor introduce barriers to cross-seeding of soluble protein. However, the polymorphism strongly influences the conformation of the amyloid fibrils, as determined by infrared spectroscopy. Intriguingly, unlike conformational features governed by the critical amyloidogenic region of PrP23-144 (residues 138-139), the structural features distinguishing Met-129 and Val-129 PrP23-144 amyloid fibrils are not transmissible by cross-seeding. While based only on in vitro data, these findings provide fundamental insight into the mechanism of prion-based conformational transmission, indicating that only conformational features controlling seeding specificity (e.g. those in critical intermolecular contact sites of amyloid fibrils) are necessarily transmissible by cross-seeding; conformational traits in other parts of the PrP molecule may not be "heritable" from the amyloid template.
Highlights
One line of support for the protein-only hypothesis is the existence of familial human prion diseases linked to mutations in the PRNP gene
Pathogenic Mutations and the Residue 129 Polymorphism Do Not Alter PrP23–144 Conversion Kinetics and Seeding Specificity—Since Gerstmann-Straussler-Scheinker disease (GSS)-associated mutations outside the folded domain of PrP conformer (PrPC) do not affect thermodynamic properties of the protein [12,13,14], it has been postulated that these mutations may facilitate the PrPC3 PrPSc conversion by increasing the amyloidogenic potential of the N-terminal region [14]
Pathogenic Mutations and PrP23–144 Fibrillogenesis—While the link between prion protein (PrP) mutations and familial prion diseases provides strong support for the protein-only model, the mechanism by which these mutations facilitate the pathogenic process remains unclear. Intriguing in this context are GSS-associated mutations at residues 102, 105, and 117. Because these residues lie within the unstructured region of PrP, their effect cannot be rationalized by a model based on an increase in the population of partially folded intermediates, as proposed for mutations within the C-terminal domain [14]
Summary
One line of support for the protein-only hypothesis is the existence of familial human prion diseases linked to mutations in the PRNP gene. We have shown that substitution of a single amino acid in a critical region encompassing residues 138 and 139 may have a dramatic effect on the properties of PrP23–144 fibrils, resulting in cross-seeding barriers [22].
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