The <i>P. anserina</i> Het-s/Het-S locus mediates heterokaryon incompatibility.

<p>When two <i>P. anserina</i> mycelia fuse to form a heterokaryon, the Het-s/Het-S heterokaryon incompatibility system dictates whether the heterokaryon will be viable. The <i>P. anserina</i> Het-s locus has two alleles, <i>Het-s</i> and <i>Het-S</i>, meaning that a given mycelium expresses either the Het-s or Het-S protein. The Het-s protein generally adopts a prion structure and this form of Het-s is denoted [Het-s], whereas the soluble form is denoted [Het-s*]. Thus, there are three possible combinations of Het-s with Het-S. (A) A [Het-s*] strain can fuse with a [Het-s] strain to produce a viable heterokaryon in which the prion phenotype, [Het-s], is transmitted. (B) A [Het-s*] strain can fuse with a [Het-S] strain to produce a viable heterokaryon. (C) A [Het-s] strain can fuse with a [Het-S] strain to produce a nonviable heterokaryon. Het-s amyloid induced Het-S amyloid formation (indicated by red squares) leads to a conformational change in the Het-S HeLo domain that triggers intermolecular Het-S pore formation (indicated by red triangles).