During fertilization, required on male gametes for their fusion with female gametes, and for subsequent ookinete formation in the host (PubMed:18403203, PubMed:18367645, PubMed:21209845, PubMed:29212032). Thereby, required for mosquito-mediated transmission to other animals (PubMed:18403203, PubMed:18367645, PubMed:29212032). Probably initiates the fusion of gamete cell membranes by inserting part of its extracellular domain into the cell membrane of a female gamete (PubMed:20080406). Detected in male gametocytes and gametes, but not in female gametes, nor in the asexual stages present in host erythrocytes (at protein level). The N-terminal extracellular domain is important for gamete fusion. No effect on gametogenesis, but abolishes the ability of male gametes to merge with female gates. As a consequence, fertilization cannot occur, the normal life cycle is disrupted and mosquito-mediated transmission is abolished. Potential candidate for the development of parasite sexual stage vaccines. In vitro and in vivo, neutralizing antibodies against the cd loop prevent gamete fertilization and thus transmission. 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. They function similar to viral fusogens, by inserting part of their extracellular domain into the lipid bilayer of an adjoining cell. Belongs to the HAP2/GCS1 family.