The mechanism of toxicity in HET-S/HET-s prion incompatibility

Heterogeneous seeding of HET-s(218-289) and the mutability of prion structures

One fundamental property of prions is the formation of strains-prions that have distinct biological effects, despite a common amino acid sequence. The strain phenomenon is thought to be caused by the formation of different molecular structures, each encoding for a particular biological activity. While the precise mechanism of the formation of strains is unknown, they tend to arise following environmental changes, such as passage between different species. One possible mechanism discussed here is heterogeneous seeding; the formation of a prion nucleated by a different molecular structure. While heterogeneous seeding is not the only mechanism of prion mutation, it is consistent with some observations on species adaptation and drug resistance. Heterogeneous seeding provides a useful framework to understand how prions can adapt to new environmental conditions and change biological phenotypes.

Wild type and mutants of the HET-s(218-289) prion show different flexibility at fibrillar ends: a simulation study

The C-terminal segment (residues 218-289) of the HET-s protein of the filamentous fungus Podosporina anserina is a prion-forming domain. The structural model of the HET-s(218-289) amyloid fibril based on solid-state nuclear magnetic resonance (NMR) restraints shows a β solenoid topology which is comprised of a β-sheet core and interconnecting loops. For the single-point mutants Phe286Ala and Trp287Ala, slower aggregation rates in vitro and loss of prionic infectivity have been reported recently. Here we have used molecular dynamics to compare the flexibility of the mutants and wild type. The simulations, initiated from a trimeric aggregate extracted from the NMR structural model, show structural stability on a 100-ns time scale for wild type and mutants. Analysis of the fluctuations along the simulations reveals that the mutants are less flexible than the wild type in the C-terminal segment at only one of the two external monomers. Analysis of interaction energy and buried accessible surface indicates that residue Phe286 in particular is stabilized in the Trp287Ala mutant. The simulation results provide an atomistic explanation of the suggestion (based on indirect experimental evidence) that flexibility at the protofibril end(s) is required for fibril elongation. Moreover, they provide further evidence that the growth of the HET-s amyloid fibril is directional.

PAIN with and without PAR: variants for third-spin assisted heteronuclear polarization transfer

In this article, we describe third-spin assisted heteronuclear recoupling experiments, which play an increasingly important role in measuring long-range heteronuclear couplings, in particular (15)N-(13)C, in proteins. In the proton-assisted insensitive nuclei cross polarization (PAIN-CP) experiment (de Paëpe et al. in J Chem Phys 134:095101, 2011), heteronuclear polarization transfer is always accompanied by homonuclear transfer of the proton-assisted recoupling (PAR) type. We present a phase-alternating experiment that promotes heteronuclear (e.g. (15)N → (13)C) polarization transfer while simultaneously minimizing homonuclear (e.g.(13)C → (13)C) transfer (PAIN without PAR). This minimization of homonuclear polarization transfer is based on the principle of the resonant second-order transfer (RESORT) recoupling scheme where the passive proton spins are irradiated by a phase-alternating sequence and the modulation frequency is matched to an integer multiple of the spinning frequency. The similarities and differences between the PAIN-CP and this het-RESORT experiment are discussed here.

The transcriptional response to nonself in the fungus Podospora anserina

In fungi, heterokaryon incompatibility is a nonself recognition process occurring when filaments of different isolates of the same species fuse. Compatibility is controlled by so-called het loci and fusion of strains of unlike het genotype triggers a complex incompatibility reaction that leads to the death of the fusion cell. Herein, we analyze the transcriptional changes during the incompatibility reaction in Podospora anserina. The incompatibility response was found to be associated with a massive transcriptional reprogramming: 2231 genes were up-regulated by a factor 2 or more during incompatibility. In turn, 2441 genes were down-regulated. HET, NACHT, and HeLo domains previously found to be involved in the control of heterokaryon incompatibility were enriched in the up-regulated gene set. In addition, incompatibility was characterized by an up-regulation of proteolytic and other hydrolytic activities, of secondary metabolism clusters and toxins and effector-like proteins. The up-regulated set was found to be enriched for proteins lacking orthologs in other species and chromosomal distribution of the up-regulated genes was uneven with up-regulated genes residing preferentially in genomic islands and on chromosomes IV and V. There was a significant overlap between regulated genes during incompatibility in P. anserina and Neurospora crassa, indicating similarities in the incompatibility responses in these two species. Globally, this study illustrates that the expression changes occurring during cell fusion incompatibility in P. anserina are in several aspects reminiscent of those described in host-pathogen or symbiotic interactions in other fungal species.

Functional amyloidogenesis and cytotoxicity-insights into biology and pathology

Prions are self-templating protein structures that can be transferred from organism to organism. The [Het-s] prion propagates as a functional amyloid aggregate in the filamentous fungi Podospora anserina, and is involved in mediating heterokaryon incompatibility. Fusion of a P. anserina strain harboring the [Het-s] prion with another strain expressing the soluble Het-S protein results in cell death. The mechanism of Het-s/Het-S-mediated cell death has now been revealed in a paper just published in PLOS Biology. The study shows that Het-s and Het-S C-terminal domain co-amyloidogenesis induces a profound conformational rearrangement in the N-terminal Het-S HeLo domain, resulting in the exposure of a nascent transmembrane helix. Oligomerization of these helices leads to pore formation, leakage of the cytosolic contents, and subsequent cell death. Thus, Het-s amyloid plays a major role in the life cycle of P. anserina by orchestrating a complex conformational change in the Het-S protein, resulting in cytotoxicity by compromising membrane integrity. This ability of Het-s functional amyloid to initiate programmed cytotoxicity by mediating a conformational change in another protein significantly expands the functional repertoire of amyloid. Moreover, the mechanism of Het-S cell killing may be similar to the mechanism by which some pathological amyloid proteins lead to the demise of post-mitotic tissue.