Molecular Biology of Prion Propagation

Abstract

Prions are novel pathogens and are distinct from both viroids and viruses. They are composed largely, if not entirely, of the scrapie isoform of the prion protein (PrP) designated PrP^Sc. Prions cause neurodegenerative diseases including scrapie of sheep, mad cow disease, and Creutzfeldt-Jakob disease (CJD) of humans. That prion diseases are manifest as genetic, infectious and sporadic illnesses is unprecedented. The conversion of PrP^C into PrP^Sc is a post-translational process involving a profound conformational change which is the fundamental event underlying the propagation of prions. PrP^C contains −40% α-helix and virtually no β-sheet; in contrast, PrP^Sc has −30% α-helix and −40% β-sheet. These data argue that the conversion of α-helices into β-sheets underlies the formation of PrP^Sc. Efficient formation of PrP^Sc requires targeting PrP^C by glycosylphosphatidyl inositol (GPI) anchor to a caveolae-like membrane domain (CLD) which is detergent insoluble and enriched for cholesterol and glycosphingolipids. Redirecting PrP^C to clathrin-coated pits by creating chimeric PrP molecules with four different C-terminal targeting domains prevented the formation of PrP^Sc. To determine if these C-terminal transmembrane segments prevented PrP^C from refolding into PrP^Sc by altering the structure of the polypeptide, we fused the 28 amino acid C-termini from the Qa protein. Two C-terminal Qa segments differing by a single residue direct the trans-membrane protein to clathrin coated pits or the BPI form to CLDs. The CLD targeted PrP^C was converted into PrP^Sc while the transmembrane PrP^C was not. Transgenic (Tg) mice expressing human (Hu) prion protein (PrP) and chimeric Hu/mouse (Mo) PrP genes were inoculated with brain extracts from humans with inherited or sporadic prion disease.