Prion-related diseases include scrapie in sheep, bovine spongiform enc
ephalopathy in cattle and Creutzfeldt-Jakob disease in humans. The inf
ectious agent for these diseases surprisingly contains no nucleic acid
, but is a protein (PrP) which exists in two conformations, PrPC and P
rPSc. The infectious PrPSc form has a higher beta-sheet and lower alph
a-helix content than PrPC. The structures of PrP and models for how Pr
PSc is able to replicate by converting PrPC to PrPSc are discussed.