The Positively Charged Cluster in the N-terminal Disordered Region may Affect Prion Protein Misfolding: Cryo-EM Structure of Hamster PrP(23–144) Fibrils

Abstract

Prions, the misfolding form of prion proteins, are contagious proteinaceous macromolecules. Recent studies have shown that infectious prion fibrils formed in the brain and non-infectious fibrils formed from recombinant prion protein in a partially denaturing condition have distinct structures. The amyloid core of the in vitro-prepared non-infectious fibrils starts at about residue 160, while that of infectious prion fibrils formed in the brain involves a longer sequence (residues ∼90–230) of structural conversion. The C-terminal truncated prion protein PrP(23–144) can form infectious fibrils under certain conditions and cause disease in animals. In this study, we used cryogenic electron microscopy (cryo-EM) to resolve the structure of hamster sHaPrP(23-144) fibrils prepared at pH 3.7. This 2.88 Å cryo-EM structure has an amyloid core covering residues 94–144. It comprises two protofilaments, each containing five β-strands arranged as a long hairpin plus an N-terminal β-strand. This N-terminal β-strand resides in a positively charged cluster region (named PCC2; sequence 96–111), which interacts with the turn region of the opposite protofilaments’ hairpin to stabilize the fibril structure. Interestingly, this sHaPrP(23–144) fibril structure differs from a recently reported structure formed by the human or mouse counterpart at pH 6.5. Moreover, sHaPrP(23–144) fibrils have many structural features in common with infectious prions. Whether this structure is infectious remains to be determined. More importantly, the sHaPrP(23–144) structure is different from the sHaPrP(108–144) fibrils prepared in the same fibrillization buffer, indicating that the N-terminal disordered region, possibly the positively charged cluster, influences the misfolding pathway of the prion protein.