Amyloidogenic unfolding intermediates differentiate sheep prion protein variants

Sheep is a unique example among mammalian species to present a strong correlation between genotype and prion disease susceptibility phenotype. Indeed a well-defined set of PrP polymorphisms at positions 136, 154 and 171 (sheep numbering) govern scrapie susceptibility, ranging from very high susceptibility for V136-R154-Q171 variant (VRQ) to resistance for A136-R154-R171 variant (ARR). To get better insight into the molecular mechanisms of scrapie susceptibility/resistance, the unfolding pathways of the different full-length recombinant sheep prion protein variants were analysed by differential scanning calorimetry in a wide range of pH. In the pH range 4.5-6.0, thermal unfolding occurs through a reversible one-step process while at pH <4.5 and >6.0 unfolding intermediates are formed, which are stable in the temperature range 65-80 degrees C. While these general behaviours are shared by all variants, VRQ and ARQ (susceptibility variants) show higher thermal stability than AHQ and ARR (resistance variants) and the formation of their unfolding intermediates requires higher activation energy than in the case of AHQ and ARR. Furthermore, secondary structures of the unfolding intermediates differentiate variants: ARR unfolding intermediate exhibits random coil structure, contrasting with the beta-sheet structure of VRQ and ARQ unfolding intermediates. The rate of the unfolding intermediate formation allows us to classify genetic variants along increasing scrapie susceptibility at pH 4.0, VRQ and ARQ rates being the highest. Rather poor correlation is observed at pH 7.2. Upon cooling, these intermediates refold into stable species, which are rich in beta-type secondary structures and, as revealed by thioflavin T fluorescence and electron microscopy, share amyloid characteristics. These results highlight the prion protein plasticity genetically modulated in sheep, and might provide a molecular basis for sheep predisposition to scrapie taking into account both thermodynamic stability and transconformation rate of prion protein.