The het-s locus of Podospora anserina is a heterokaryon incompatibility locus. The coexpression of the antagonistic het-s and het-S alleles triggers a lethal reaction that prevents the formation of viable heterokaryons. Strains that contain the het-s allele can display two different phenotypes, [Het-s] The het-s locus of the filamentous fungus Podospora anserina is one of the nine known loci controlling heterokaryon incompatibility in that species (for review, see ref. 1). Coexpression of the antagonistic het-s and het-S alleles in the same cytoplasm triggers an adverse reaction that prevents the formation of viable heterokaryotic cells between strains that contain the incompatible alleles (2). This locus encodes a 289-aa protein that is not essential for cell viability or completion of the life cycle of the fungus (3, 4)

The het-c l contc a different alleles that elicit nelic vegetative incompatibility through specific interactions with alleles of the unlinked loci het-e and het-d.

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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.