SOLUTION STRUCTURE OF A 142-RESIDUE RECOMBINANT PRION PROTEIN CORRESPONDING TO THE INFECTIOUS FRAGMENT OF THE SCRAPIE ISOFORM

The scrapie prion protein (PrPSc) is the major, and possibly the only, component of the infectious prion; it is generated from the cellular isoform (PrPC) by a conformational change. N-terminal truncation of Pr PSc by limited proteolysis produces a protein of approximate to 142 re sidues designated PrP 27-30, which retains infectivity, A recombinant protein (rPrP) corresponding to Syrian hamster PrP 27-30 was expressed in Escherichia coli and purified, After refolding rPrP into an alpha- helical form resembling PrPC, the structure was solved by multidimensi onal heteronuclear NMR, revealing many structural features of rPrP tha t were not found in two shorter PrP fragments studied previously, Exte nsive side-chain interactions for residues 113-125 characterize a hydr ophobic cluster, which packs against an irregular beta-sheet, whereas residues 90-112 exhibit little defined structure. Although identifiabl e secondary structure is largely lacking in the N terminus of rPrP, pa radoxically this N terminus increases the amount of secondary structur e in the remainder of rPrP, The surface of a long helix (residues 200- 227) and a structured loop (residues 165-171) form a discontinuous epi tope for binding of a protein that facilitates PrPSc formation, Polymo rphic residues within this epitope seem to modulate susceptibility of sheep and humans to prion disease. Conformational heterogeneity of rPr P at the N terminus may be key to the transformation of PrPC into PrPS c, whereas the discontinuous epitope near the C terminus controls this transition.