Novel requirements for HAP2-mediated gamete fusion in Tetrahymena.

Abstract The ancestral gamete fusion protein, HAP2/GCS1 catalyzes sperm-egg fusion in a broad range of taxa dating to the last eukaryotic common ancestor. Remarkably, HAP2/GCS1 orthologs are structurally related to the class II fusogens of modern-day viruses, and recent studies make clear that these proteins utilize similar mechanisms to achieve membrane merger. To identify factors that may regulate HAP2/GCS1 activity, we screened mutants of the ciliate Tetrahymena thermophila for behaviors that mimic Δhap2/gcs1 knockout phenotypes in this species. Using this approach, we identified two new genes, GFU1 and GFU2, whose products are necessary for the formation of membrane pores during fertilization and show that the product of a third gene, namely ZFR1, may be involved in pore maintenance and/or expansion. Finally, we propose a model that explains cooperativity between the fusion machinery on apposed membranes of mating cells and accounts for successful fertilization in T. thermophila’s multiple mating type system.