Stacked Sheets and Solenoids: Implications of Polymorphic Amyloid Structures of the Fungal Prion HET-s

Abstract

Prions are aberrantly folded infectious proteins that propagate by inducing the refolding of normal proteins. Prions generally form amyloids: chemically homogeneous, fibrillar protein aggregates. Amyloids are associated with many diseases including Alzheimer's, type II diabetes, and more specifically to prions, Creutzfeldt-Jakob disease and bovine spongiform encephalopathy (“mad cow disease”). Despite this variety, all amyloids share a common cross-β structural motif, β-strands running perpendicular to a central fiber axis. Data from tissue extracted amyloids and studies of short amyloidogenic peptides have led some to hypothesize that all amyloids have a generic amyloid fold consisting of stacks of β-sheets. However, structural models of Alzheimer's related Aβ-amyloid and diabetes related IAPP amyloid indicate that amyloid structure is more diverse. Studies of brain-derived and recombinant prion protein, PrP, show that the generic amyloid fold shows only marginal biological activity while a β-solenoid fold is highly pathogenic.We have looked at HET-s(218-289), the prion forming domain of a functional prion in the fungus Podospora anserina. It has been determined by solid state NMR that under physiological conditions, HET-s(218-289) fibrilizes into a β-solenoid fold. Others have shown that when fibrilized under low pH conditions, a non-functional polymorph is formed. We have determined by X-ray fiber diffraction that at low pH, proteolysis leads to the formation of stacked β-sheet amyloids. These amyloids can propagate the generic fold onto undegraded HET-s(218-289), though only under non-physiological conditions. These results indicate that the biological activity of HET-s(218-289) is intimately tied to its specific amyloid structure and that short amyloidogenic fragments may not adequately reproduce the interactions of larger prion domains. Supported by NIH grants P01-AG002132 and T32-GM008320-21.