Structural dependence of HET-s amyloid fibril infectivity assessed by cryoelectron microscopy

Proc Natl Acad Sci U S A. 2011 Feb 22;108(8):3252-7. doi: 10.1073/pnas.1011342108. Epub 2011 Feb 7.


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.

Publication types

  • Research Support, N.I.H., Intramural

MeSH terms

  • Amyloid / chemistry*
  • Cryoelectron Microscopy*
  • Fungal Proteins / chemistry
  • Hydrogen-Ion Concentration
  • Infections / etiology
  • Magnetic Resonance Spectroscopy
  • Podospora / chemistry
  • Prions / chemistry*


  • Amyloid
  • Fungal Proteins
  • Prions