During fertilization, required on male (minus) gametes for their fusion with female (plus) gametes (PubMed:18367645, PubMed:20335357, PubMed:25655701, PubMed:28235200). Required for membrane fusion, but not for the initial adhesion between gametes (PubMed:18367645, PubMed:25655701, PubMed:28235200). Inserts (via its extracellular domain) into lipid membranes (in vitro) (PubMed:28235200). Probably initiates the fusion of gamete cell membranes by inserting its extracellular domain into the cell membrane of a female gamete (PubMed:28235200). Monomer. Homotrimer. Membrane contact and insertion (via its extracellular domain) into a lipid membrane probably triggers trimerization. In male gametes, detected both in cytoplasmic vesicles and on cell projections called mating structures at the cell membrane. Detected on male (minus) gametes (at protein level) (PubMed:20335357, PubMed:25655701, PubMed:28235200). Detected at low levels in vegetative cells. Highly expressed in male (minus) gametes, with low expression in female (plus) gametes (PubMed:16378100). Both the cytoplasmic and the extracellular domain are required for normal trafficking to the cell surface, and thus for fertilization (PubMed:25655701). The extracellular domain can insert itself into lipid membranes, probably as a trimer (PubMed:28235200). The extracellular domain has structural similarity to class II viral fusion proteins. The protein present at the cell membrane is rapidly degraded after fusion between male (minus) and female (plus) gametes, contrary to the protein present in intracellular pools (PubMed:20335357). This may represent a mechanism to avoid fusion of several male gametes with a single female gamete (Probable). N-glycosylated. No effect on normal adhesion of male (minus) gametes and female (plus) gametes, but the gametes fail to fuse and to give rise to quadriflagellated zygotes. 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:28235200). Belongs to the HAP2/GCS1 family. Becoming one - Issue 193 of July 2017