Alanine replacements in the structured C-terminal domain of the prion protein reveal conformationally variable regions as major determinants for prion propagation

Mutational analysis of the cellular prion protein (PrPC) has revealed various regions of the protein that modulate prion propagation. However, most approaches involve deletions, insertions, or replacements in the presence of the wild-type cellular protein, which may mask the true phenotype. Here, site-directed alanine mutagenesis of PrPC was conducted to identify sites particularly a surface patch of the protein pertinent to prion propagation in the absence of the wild-type prion protein. Mutations were targeted to the helical, sheet and loop regions of PrPC, or a combination thereof and the mutated proteins expressed in PK1 cells in which endogenous PrPC had been silenced. PK1 cells are a clone of mouse neuroblastoma cells that are highly susceptible to Rocky Mountain Laboratory mouse prions. Using the scrapie cell assay, a highly sensitive cell culture-based bioassay for quantifying infectious titres of mouse prions, we found that all mutations within the structured 121-230 domain, irrespective of secondary structure, severely reduced prion propagation. The reduction was most pronounced for mutations within conformationally variable regions of the protein (G123A.L124A.G125A and V188A.T191A.T192A) and those neighbouring or within helix 1 (S134A.R135A.M153A and H139A.G141A.D146A). While mutations G123A and G125A would likely disrupt the structure of the prion fibril, the other mutations are unlikely to cause disruption. Our data therefore suggests that conformationally variable regions within the structured domain of PrPC are the major determinants of prion propagation efficacy.