Izumo sperm-egg fusion protein 1 (IZUMO1) is an essential sperm cell-surface protein required for fertilisation by acting as a ligand for IZUMO1R/JUNO receptor on eggs []. It is a member of an immunoglobulin superfamily and possesses a well-conserved putative N-glycosylation site. However, glycosylation is not essential for the function of IZUMO, but has a role in protecting it from fragmentation in cauda epididymis [].
This is the immunoglobulin domain on Izumo proteins from higher eukaryotes. Izumo is a typical type I membrane glycoprotein with one immunoglobulin-like domain and a putative N-glycoside link motif - glycosylation site. The full-length human IZUMO1 protein is a molecule with a single immunoglobulin (Ig) domain. It is thought that Izumo proteins bind to putative Izumo receptors on the oocyte. Izumo is not detectable on the surface of fresh sperm but becomes exposed only after an exocytotic process, the acrosome reaction, has occurred. Studies have shown that knock-out mice (Izumo-/- males) were sterile despite normal mating behaviour and ejaculation, indicating the importance of the protein in fertilisation []. There are cysteine residues thought to form a disulphide bridge. Izumo is a typical type I membrane glycoprotein with one immunoglobulin-like domain and a putative N-glycoside link motif (Asn 204) []. There is a conserved GCL sequence motif. Izumo expression has been found to be testis-specific [, ].
