| When prions are transmitted between species, there can be a delay in pathogenesis due to a phenomenon referred to as the transmission barrier. Some species also show very low susceptibility to prion disease. In this study I hypothesized that the susceptibility of species to prion disease is proportional to the tendency of their endogenous prion protein, PrP, to adopt the beta-state, an oligomeric form of misfolded recombinant PrP that is rich in beta sheet.;Using a novel method of two-wavelength CD analysis, it could be shown that recombinant PrP from prion-susceptible species have a higher propensity to refold to the beta-state than resistant species. The crystal structure of rabbit PrPC 121-230 revealed a helix-cap motif at the N-terminus of helix-2 that contributes to the reduced beta-state propensity of rabbit PrP.;Single amino acid changes in the sequence of PrP can lead to a transmission barrier and/or resistance in species. Mutating single residues in rabbit PrPC to those found in corresponding positions in hamster PrPC, ablated the helix-cap observed in the wild-type and caused an increase in the beta-state propensity of rabbit PrP. Conversely, a decrease in beta-state propensity was observed when rabbit mutations were introduced into PrP of hamster, a susceptible species.;A dimeric association is hypothesized to be involved in the function of PrP and/or the conversion mechanism to infectious prion. In the structures of the wild-type and mutant rabbit PrPCs a dimeric arrangement was observed in the asymmetric unit of the crystals. Using 1-ethyl- 3-[3-dimethylaminopropyl]carbodiimide hydrochloride (EDC), a dimer of rabbit and hamster PrPC was crosslinked in solution. The dimer crosslink was specific and dependent on the tertiary structure of PrPC. Crosslinking of the beta-state octamer with EDC showed that similar contacts may be present in this oligomeric form.;Together these data provide strong evidence that species susceptibility is linked to beta-state propensity. |
