Figure 4. Prion variants & species barrier.

Sheep scrapie shows limited infectivity for goats, a phenomenon called the species barrier. The overlap of conformations that donor and recipient proteins can assume determine the strength of the species barrier⁷⁶. As species B and C have prion proteins able to assume many similar amyloid conformations, there will be little species barrier between them. Prion protein of species A and C have few common conformations and so will have a high species barrier. A prion variant (such as bovine spongiform encephalopathy) due to an amyloid conformation that can be assumed by the protein sequence of many species will have a broad host range.