The Polybasic N-Terminal Region of the Prion Protein Controls the Physical Properties of Both the Cellular and Fibrillar Forms of PrP

Individual variations in structure and morphology of amyloid fibrils produced from a single polypeptide are likely to underlie the molecular origin of prion strains and control the efficiency of the species barrier in the transmission of prions. Previously, we observed that the shape of amyloid fibrils produced from full-length prion protein (PrP 23–231) varied substantially for different batches of purified recombinant PrP. Variations in fibril morphology were also observed for different fractions that corresponded to the highly pure PrP peak collected at the last step of purification. A series of biochemical experiments revealed that the variation in fibril morphology was attributable to the presence of miniscule amounts of N-terminally truncated PrPs, where a PrP encompassing residue 31–231 was the most abundant of the truncated polypeptides. Subsequent experiments showed that the presence of small amounts of recombinant PrP 31–231 (0.1–1%) in mixtures with full-length PrP 23–231 had a dramatic impact on fibril morphology and conformation. Furthermore, the deletion of the short polybasic N-terminal region 23–30 was found to reduce the folding efficiency to the native α-helical forms and the conformational stability of α-PrP. These findings are very surprising considering that residues 23–30 are very distant from the C-terminal globular folded domain in α-PrP and from the prion folding domain in the fibrillar form. However, our studies suggest that the N-terminal polybasic region 23–30 is essential for effective folding of PrP to its native cellular conformation. This work also suggests that this region could regulate diversity of prion strains or subtypes despite its remote location from the prion folding domain.