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

Abstract

SummaryWe have proposed that the [Het-s] infectious cytoplasmic element of the filamentous fungus Podospora anserina is the prion form of the HET-s protein. The HET-s protein is involved in a cellular recognition phenomenon characteristic of filamentous fungi and known as heterokaryon incompatibility. Under the prion form, the HET-s protein causes a cell death reaction when co-expressed with the HET-S protein, from which it differs by only 13 amino acid residues. We show here that the HET-s protein can exist as two alternative states, a soluble and an aggregated form in vivo. As shown for the yeast prions, transition to the infectious prion form leads to aggregation of a HETs-green fluorescent protein (GFP) fusion protein. The HET-s protein is aggregated in vivo when highly expressed. However, we could not demonstrate HET-s aggregation at wild-type expression levels, which could indicate that only a small fraction of the HET-s protein is in its aggregated form in vivo in wild-type [Het-s] strains. The antagonistic HET-S form is soluble even at high expression level. A double amino acid substitution in HET-s (D23A P33H), which abolishes prion infectivity, suppresses in vivo aggregation of the GFP fusion. Together, these results further support the model that the [Het-s] element corresponds to an abnormal self-perpetuating aggregated form of the HET-s protein.

Chaperone-dependent amyloid assembly protects cells from prion toxicity

“…Similarly, in yeast, the toxicity of Huntington exon 1 depends on the [RNQ ϩ ] prion state (16,22). This concept may even extend to the heterokaryon incompatibility mediated by the [Het-s] prion in Podospora anserina (34). HET-s in its prion form only leads to cell death when coexpressed with the HET-S allele that cannot form amyloid.…”

Sexual transmission of the [Het-s] prion leads to meiotic drive in Podospora anserina

“…This result is presumably also what happens during the sexual stage. One proposed model explaining this toxicity would be that HET-S interferes with HET-s amyloid aggregation, which leads to accumulation of toxic oligomeric aggregates (17).…”

“…Het-s and het-S heterokaryon incompatibility alleles were crossed into this strain S yielding the [Het-s*] and [Het-S] strains. The knockout mutant and several transformants were as described by Coustou et al (11,17,23). The list of abbreviations and Tables 1 and 2 display an overview of all different het-s͞S genotypes and phenotypes mentioned in this paper.…”

“…Furthermore, looking at the GFP signal in Fig. 3 A and B, the condensed, intense, and particle-like nature of the signal indicates that the HET-s-GFP protein was in its prion form (17). (Tables 1 and 2).…”

“…This transformation reflects the cytoplasmic infectious character of the prion form of HET-s (11). [Het-s] was proposed to propagate as a self-perpetuating amyloid aggregated form of the HET-s protein (17)(18)(19) (20).…”