title: Structural dependence of HET-s amyloid fibril infectivity assessed by cryoelectron microscopy.
abstract: HET-s is a prion protein of the fungus Podospora anserina which, in the prion state, is active in a self/nonself recognition process called heterokaryon incompatibility. Its prionogenic properties reside in the C-terminal "prion domain." The HET-s prion domain polymerizes in vitro into amyloid fibrils whose properties depend on the pH of assembly; above pH 3, infectious singlet fibrils are produced, and below pH 3, noninfectious triplet fibrils. To investigate the correlation between structure and infectivity, we performed cryo-EM analyses. Singlet fibrils have a helical pitch of approximately 410 Å and a left-handed twist. Triplet fibrils have three protofibrils whose lateral dimensions (36 × 25 Å) and axial packing (one subunit per 9.4 Å) match those of singlets but differ in their supercoiling. At 8.5-Å resolution, the cross-section of the singlet fibril reconstruction is largely consistent with that of a β-solenoid model previously determined by solid-state NMR. Reconstructions of the triplet fibrils show three protofibrils coiling around a common axis and packed less tightly at pH 3 than at pH 2, eventually peeling off. Taken together with the earlier observation that fragmentation of triplet fibrils by sonication does not increase infectivity, these observations suggest a novel mechanism for self-propagation, whereby daughter fibrils nucleate on the lateral surface of singlet fibrils. In triplets, this surface is occluded, blocking nucleation and thereby explaining their lack of infectivity.
authors.affiliation: Laboratory of Structural Biology, National Institute of Arthritis, Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
authors.forename: Naoko
authors.initials: N
authors.lastname: Mizuno
authors.forename: Ulrich
authors.initials: U
authors.lastname: Baxa
authors.forename: Alasdair C
authors.initials: AC
authors.lastname: Steven
mesh_terms: D000682:Amyloid / Q000737:chemistry*; D020285:Cryoelectron Microscopy; D005656:Fungal Proteins / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007239:Infections / Q000209:etiology; D009682:Magnetic Resonance Spectroscopy; D048468:Podospora / Q000737:chemistry; D011328:Prions / Q000737:chemistry*
chemical_list: D000682:Amyloid; D005656:Fungal Proteins; D011328:Prions