Reactivity in vegetative incompatibility of the HET-E protein of the fungus Podospora anserina is dependent on GTP-binding activity and a WD40 repeated domain

The het-e gene of the filamentous fungus Podospora anserina is involved in vegetative incompatibility. Go-expression of antagonistic alleles of the unlinked loci het-e and het-e triggers a cell death reaction that prevents the formation of viable heterokaryons between strains that contain incompatible combinations of het-c and het-e alleles. The het-el(A) gene encodes a polypeptide that contains a putative GTP-binding site and WD40 repeats, The role of these two domains in the reactivity of the HET-E protein in incompatibility was analyzed. An in vitro assay confirmed that the first domain is functional and can bind GTP and not ATP. suggesting that GTP-binding is essential for triggering the incompatibility reaction. The relationship between the number of WD40 repeats and the reactivity of the protein in incompatibility was investigated by estimating this number in different wild-type and mutant het-e alleles. It was deduced that reactive alleles contain a minimal number of ten WD40 repeats. These results demonstrate that the reactivity of the HET-E protein depends on two functional elements, a GTP-binding domain and several WD40 repeats. These motifs are present in separate polypeptides in trimeric G proteins, suggesting that HET-E polypeptides are also involved in signal transduction. Disruption of the het-e locus does not impair the phenotype of strains but DNA hybridization analyses revealed that her-e may belong to a multigenic family.