The transmissible spongiform encephalopathies (prion diseases) are a group of neurodegenerative diseases. The infectious agents (prions) have been widely accepted as aggregates of a misfolded normal protein, prion protein (PrP). Prions, or the scrapie form of PrP (PrPSc), replicate by associating with the cellular PrP (PrPC) and catalyzing the refolding of the latter to the former. Such a propagation, independent of nucleic acid, suggests that the pathogenic features of prions are all encoded in the structure of PrPSc. Currently, the only well-agreed structural feature of PrPSc is that they are amyloid fibrils with a cross-β core of ~140 residues. However, this feature on its own cannot fully determine prion infectivity. Furthermore, a major piece of missing information about prion structure is the basic folding motifs.
Recently, the development of protein misfolding cyclic amplification (PMCA) allowed for in vitro generation of large quantities of synthetic prions from bacterially expressed recombinant PrP, providing suitable materials for structural studies. In this work, structural features of one type of synthetic prions were compared with those of a biochemically similar yet non-infectious PrP conformer, using mass spectrometry-based methods. The results suggest the infectivity of these synthetic prions is associated with more specific structural features, particularly, a highly ordered region within residues ~126-167 as well as unique side chain packing arrangements.
To further understand the structure of synthetic prions, we employed the method of site-specific crosslinking to test a recently proposed four-rung β-solenoid structural architecture. This method probes the intermolecular interactions within protein aggregates and provides information about the basic folding motif of monomers. The results suggest that the proposed four-rung β-solenoid structure model is not fully adequate to describe the structure of the synthetic prions we studied.
Finally, amyloid fibrils generated from the C-terminal fragment (CTF) of TDP-43 that can propagate by prion-like mechanism were studied at high resolution by cryoEM. These fibrils are related to neurodegenerative diseases. The reconstructed 4-5 Å resolution map provided an initial insight into the structure of these amyloid fibrils. Even though additional improvements in sample quality and data processing are needed to obtain atomic-resolution structure of TDP-43 CTF fibrils.