Role of polysaccharide and lipid in lipopolysaccharide induced prion protein conversion

Conversion of native cellular prion protein (PrP(c)) from an α-helical structure to a toxic and infectious β-sheet structure (PrP(Sc)) is a critical step in the development of prion disease. There are some indications that the formation of PrP(Sc) is preceded by a β-sheet rich PrP (PrP(β)) form which is non-infectious, but is an intermediate in the formation of infectious PrP(Sc). Furthermore the presence of lipid cofactors is thought to be critical in the formation of both intermediate-PrP(β) and lethal, infectious PrP(Sc). We previously discovered that the endotoxin, lipopolysaccharide (LPS), interacts with recombinant PrP(c) and induces rapid conformational change to a β-sheet rich structure. This LPS induced PrP(β) structure exhibits PrP(Sc)-like features including proteinase K (PK) resistance and the capacity to form large oligomers and rod-like fibrils. LPS is a large, complex molecule with lipid, polysaccharide, 2-keto-3-deoxyoctonate (Kdo) and glucosamine components. To learn more about which LPS chemical constituents are critical for binding PrP(c) and inducing β-sheet conversion we systematically investigated which chemical components of LPS either bind or induce PrP conversion to PrP(β). We analyzed this PrP conversion using resolution enhanced native acidic gel electrophoresis (RENAGE), tryptophan fluorescence, circular dichroism, electron microscopy and PK resistance. Our results indicate that a minimal version of LPS (called detoxified and partially de-acylated LPS or dLPS) containing a portion of the polysaccharide and a portion of the lipid component is sufficient for PrP conversion. Lipid components, alone, and saccharide components, alone, are insufficient for conversion.