During fertilization, required for the formation of intercellular membrane pores and subsequent exchange of gametic pronuclei between cells. Probably initiates the formation of intercellular membrane pores by inserting part of its extracellular domain into the cell membrane of the adjoining cell in the mating pair. Mating requires the presence of HAP2 on at least one of the two cells. Mating efficiency is high when HAP2 is present on both cells, and is strongly reduced when HAP2 is present on only one of the two cells. Detected at the mating junction. Detected in all seven mating types. The region that is important for membrane fusion binds to lipids and can insert itself into lipid membranes. It is unstructured in an aqueous environment and assumes a more ordered, beta-stranded conformation in the presence of lipid membranes. HAP2/GCS1 family members mediate membrane fusion between gametes in a broad range of eukaryotes, ranging from algae and higher plants to protozoans and cnidaria, suggesting they are derived from an ancestral gamete fusogen (PubMed:20080406). They function similar to viral fusogens, by inserting part of their extracellular domain into the lipid bilayer of an adjoining cell (PubMed:28238660). Belongs to the HAP2/GCS1 family.