The G126V Mutation in the Mouse Prion Protein Hinders Nucleation-Dependent Fibril Formation by Slowing Initial Fibril Growth and by Increasing the Critical Concentration.

The middle disordered hydrophobic region of the prion protein plays a critical role in conformational conversion of the protein, with pathogenic as well as protective mutations being localized to this region. In particular, it has been shown that the G127V mutation in this region of the human prion protein (huPrP) is protective against the spread of prion disease, but the mechanism of protection remains unknown. In this study, quantitative analyses of the kinetics of fibril formation by wild-type mouse prion protein (moPrP) and G126V moPrP (equivalent to G127V huPrP) reveal important differences: the critical concentration is higher, the lag phase is longer, and the initial effective rate constant of fibril growth is slower for the mutant variant. The study offers a simple biophysical explanation for why the G127V mutation in huPrP would be protective in humans: the ∼5-fold increase in critical concentration caused by the mutation likely results in the critical concentration (below which fibril formation cannot occur) being higher that the concentration of the protein present in and on cells in vivo.