In vivo aggregation of the HET-s prion protein of the fungus Podospora anserina

Amyloid aggregates of the HET-s prion protein are infectious

The [Het-s] infectious element of the filamentous fungus Podospora anserina is a prion. We have recently reported that recombinant HET-s protein aggregates in vitro into amyloid fibers. In vivo, the protein aggregates specifically in the [Het-s] prion strains. Here, we show that biolistic introduction of aggregated recombinant HET-s protein into fungal cells induces emergence of the [Het-s] prion with a high frequency. Thus, we demonstrate that prion infectivity can be created de novo, in vitro from recombinant protein in this system. Although the amyloid filaments formed from HET-s could transmit [Het-s] efficiently, neither the soluble form of the protein nor amorphous aggregates would do so. In addition, we have found that (i) [Het-s] infectivity correlates with the ability to convert HET-s to amyloids in vitro, (ii) [Het-s] infectivity is resistant to proteinase K digestion, and (iii) HET-s aggregates formed in vivo in [Het-s] strains have the ability to convert the recombinant protein to aggregates. Together, our data designate the HET-s amyloids as the molecular basis of [Het-s] prion propagation.

The HET-s prion protein of the filamentous fungus Podospora anserina aggregates in vitro into amyloid-like fibrils

The HET-s protein of Podospora anserina is a fungal prion. This protein behaves as an infectious cytoplasmic element that is transmitted horizontally from one strain to another. Under the prion form, the HET-s protein forms aggregates in vivo. The specificity of this prion model compared with the yeast prions resides in the fact that under the prion form HET-s causes a growth inhibition and cell death reaction when co-expressed with the HET-S protein from which it differs by 13 residues. Herein we describe the purification and initial characterization of recombinant HET-s protein expressed in Escherichia coli. The HET-s protein self-associates over time into high molecular weight aggregates. These aggregates greatly accelerate precipitation of the soluble form. HET-s aggregates appear as amyloid-like fibrils using electron microscopy. They bind Congo Red and show birefringence under polarized light. In the aggregated form, a HET-s fragment of approximately 7 kDa is resistant to proteinase K digestion. CD and FTIR analyses indicate that upon transition to the aggregated state, the HET-s protein undergoes a structural rearrangement characterized by an increase in antiparallel beta-sheet structure content. These results suggest that the [Het-s] prion element propagates in vivo as an infectious amyloid.