EVIDENCE FOR PROTEIN-X BINDING TO A DISCONTINUOUS EPITOPE ON THE CELLULAR PRION PROTEIN DURING SCRAPIE PRION PROPAGATION

Studies on the transmission of human (Hu) prions to transgenic (Tg) mi ce suggested that another molecule provisionally designated protein X participates in the formation of nascent scrapie isoform of prion prot ein (PrPSc), We report the identification of the site at which protein X binds to the cellular isoform of PrP (PrPC) using scrapie-infected mouse (Mo) neuroblastoma cells transfected with chimeric Hu/MoPrP gene s even though protein X has not yet been isolated, Substitution of a H u residue at position 214 or 218 prevented PrPSc formation. The side c hains of these residues protrude from the same surface of the C-termin al alpha-helix and form a discontinuous epitope with residues 167 and 171 in an adjacent loop. Substitution of a basic residue at positions 167, 171, or 218 also prevented PrPSc formation: at a mechanistic leve l, these mutant PrPs appear to act as ''dominant negatives'' by bindin g protein X and rendering it unavailable for prion propagation. Our fi ndings seem to explain the protective effects of basic polymorphic res idues in PrP of humans and sheep and suggest therapeutic and prophylac tic approaches to prion diseases.