The prion protein fragment PrP23-144 as a minimal in vitro model of prion-based inheritance
The transmissible spongiform encephalopathies (prion diseases) are a group of neurodegenerative disorders of mammals. These debilitating conditions may arise by infection, genetic mutation, or sporadically, suggesting a novel character of the disease-causing agent. It is now believed that the pathogen in these disorders consists of self-replicating protein particles (prions), which lack nucleic acid. These particles are composed of an abnormal conformer of a chromosomally-encoded protein, the prion protein (PrP). Prions are believed to replicate by an autocatalytic mechanism involving association of the harmful "scrapie" PrP conformer (PrPSc) with the benign, "cellular" conformer (PrPC) in vivo. This so-called protein-only hypothesis explains many aspects of prion disease, but some features of prion infectivity and inheritance are difficult to reconcile with this model. For example, cross-species prion transmission may be highly inefficient, reflecting a "barrier" to cross-infection. Moreover, prion diseases often display differing phenotypes and transmissibility within a single species (i.e., a single PrP sequence). Precisely how these features arise from pathogens lacking nucleic acid has remained a persistent challenge to the protein-only hypothesis.
Our laboratory is engaged in unraveling the in vitro conformational conversion of recombinant prion proteins in an effort to elucidate mechanistic aspects of the PrPC to PrPSc transition. This work is focused on the disease-associated, truncated PrP mutant PrP23-144, which converts to an amyloid state in vitro by a nucleation-dependent process. Previous work has shown that this protein can model cross-species prion transmission barriers and strain adaptation. The conformational properties of species-and strain-specific PrP23-144 amyloid fibrils were examined to correlate structural aspects of prion fibrils with conversion activity, using microscopy and spectroscopy. The results demonstrate that both seeding barriers and strain adaptation depend upon two factors: one, the conformational preference of a particular PrP23-144 sequence; and two, the range of conformers accessible to that sequence. Moreover, disease-associated PrP mutations do not increase the amyloidogenicity of PrP23-144, and that a "core" sequence (residues 113-120), proposed as a nucleation site for PrP conversion, is nonessential to PrP23-144 fibrillogenesis. These findings suggest a general mechanism exists for the structural propagation of prions and for amyloids of other proteins.