Abstract
Fungal prions are infectious filamentous polymers of proteins that are soluble in uninfected cells. In its prion form, the HET-s protein of Podospora anserina participates in a fungal self/non-self recognition phenomenon called heterokaryon incompatibility. Like other prion proteins, HET-s has a so-called "prion domain" (its C-terminal region, HET-s-(218-289)) that is responsible for induction and propagation of the prion in vivo and for fibril formation in vitro. Prion fibrils are thought to have amyloid backbones of polymerized prion domains. A relatively detailed model has been proposed for prion domain fibrils of HET-s based on a variety of experimental constraints (Ritter, C., Maddelein, M. L., Siemer, A. B., Luhrs, T., Ernst, M., Meier, B. H., Saupe, S. J., and Riek, R. (2005) Nature 435, 844-848). To test specific predictions of this model, which envisages axial stacking of beta-solenoids with two coils per subunit, we examined fibrils by electron microscopy. Electron diffraction gave a prominent meridional reflection at (0.47 nm)(-1), indicative of cross-beta structure, as predicted. STEM (scanning transmission electron microscopy) mass-per-unit-length measurements yielded 1.02 +/- 0.16 subunits per 0.94 nm, in agreement with the model prediction (1 subunit per 0.94 nm). This is half the packing density of approximately 1 subunit per 0.47 nm previously obtained for fibrils of the yeast prion proteins, Ure2p and Sup35p, whence it follows that the respective amyloid architectures are basically different.
Highlights
When mycelia of filamentous fungi fuse, various het loci screen this process for genetic similarity, a process termed heterokaryon incompatibility [1]
To test specific predictions of this model, which envisages axial stacking of -solenoids with two coils per subunit, we examined fibrils by electron microscopy
scanning transmission electron microscopy (STEM) mass-per-unit-length measurements yielded 1.02 ؎ 0.16 subunits per 0.94 nm, in agreement with the model prediction (1 subunit per 0.94 nm). This is half the packing density of ϳ1 subunit per 0.47 nm previously obtained for fibrils of the yeast prion proteins, Ure2p and Sup35p, whence it follows that the respective amyloid architectures are basically different
Summary
When mycelia of filamentous fungi fuse, various het loci screen this process for genetic similarity (i.e. self-recognition), a process termed heterokaryon incompatibility [1]. STEM (scanning transmission electron microscopy) mass-per-unit-length measurements yielded 1.02 ؎ 0.16 subunits per 0.94 nm, in agreement with the model prediction (1 subunit per 0.94 nm). HET-s Prion Fibril Structure each subdomain forms a coil of the solenoid, and the experimental determination that each subdomain has two -strands led to the proposed -roll fold (Fig. 1a).
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