Ultrastructures and strain comparison of under-glycosylated scrapie prion fibrils

Prions, composed primarily of misfolded, often fibrillar, polymers of prion protein, have poorly understood structures. Heavy surface glycosylation may obscure visualization of their fibrillar cores, so we purified severely under-glycosylated prion protein fibrils from scrapie-infected transgenic mice expressing anchorless prion protein. Using electron and atomic force microscopy, we obtained dimensions and morphological information about prion protein core protofilaments which variably intertwined to form scrapie fibrils. Occasional isolated protofilaments were observed, suggesting that the lateral association of protofilaments is neither essential nor invariant in prion protein polymerization. Strain comparisons suggested basic structural differences; ME7 and 22L fibrils contained thinner protofilaments, 22L fibrils preferred left-handed twists, and 22L fibril periodicities averaged 106 nm per half-turn, compared with 64 and 66 nm for RML and ME7 fibrils, respectively. The strains displayed overlapping fibril morphologies, providing evidence that prion fibril morphology is influenced, but not dictated, by strain-dependent differences in protofilament structure. These measurements of the amyloid core of scrapie fibrils should aid development of models of prion structure and strain determination.