X-Ray Fiber Diffraction Reveals Major Structural Differences Between Brain-Derived Prions and Recombinant Prion Protein Amyloid

X-ray fiber diffraction was used to study the structure of brain-derived prions and recombinant prion protein amyloid. Partially oriented, dried fibers were prepared from brain-derived PrP 27-30 and recombinant PrP amyloid. Fiber diffraction patterns were analyzed and used to interrogate models for the structure of the infectious prion.Fiber diffraction patterns of recombinant PrP amyloid displayed characteristic, meridional reflections at ∼4.8 A and equatorial reflections at ∼10 A. These patterns were similar to those of other amyloids and are consistent with a basic cross-β architecture. In contrast, diffraction patterns from brain-derived PrP 27-30 displayed meridional reflections at ∼9.6, ∼6.4, and ∼4.8 A, which correspond to the 2nd, 3rd, and 4th order of a ∼19.2-A repeating unit, suggesting that PrP 27-30 contains four β-strands in a cross-β architecture. Furthermore, PrP 27-30 lacked the typical, equatorial reflection at ∼10 A, but instead produced equatorial reflections characterizing the diameter of the amyloid fiber and of individual protofilaments. Therefore, PrP 27-30 seems to have a structure consistent with a β-helix or β-solenoid, not unlike the model that was proposed earlier (Govaerts et al., 2004). This interpretation was also supported by extensive modeling, simulation of diffraction, electron microscopy, and FTIR.In a previous study, recombinant PrP amyloid induced a transmissible prion disease in transgenic mice overexpressing PrP, and was thus termed a "synthetic prion" (Legname et al., 2004). Serially transmitted, synthetic prions were purified from mouse brains and analyzed by fiber diffraction. These brain-derived, synthetic prions showed the same structural characteristics as natural prion isolates and not those of its recombinant protein precursor. The relationship between structural differences and prion infectivity can be explained by several hypotheses. It remains to be determined which one, if any, is correct.