Essential sperm cell-surface protein required for fertilization by acting as a ligand for IZUMO1R/JUNO receptor on egg (PubMed:15759005, PubMed:24739963, PubMed:27309808). The IZUMO1:IZUMO1R/JUNO interaction is a necessary adhesion event between sperm and egg that is required for fertilization but is not sufficient for cell fusion (PubMed:15759005, PubMed:24739963, PubMed:27309808). The ligand-receptor interaction probably does not act as a membrane 'fusogen' (PubMed:15759005, PubMed:24739963, PubMed:27309808). Monomer, homodimer and homooligomer; depending on the context (PubMed:19658160, PubMed:26568141, PubMed:29954238). Interacts with IZUMO1R/JUNO (PubMed:25209248, PubMed:24739963, PubMed:27309808, PubMed:32484434). IZUMO1 and IZUMO1R/JUNO form a complex with 1:1 stoichiometry (By similarity). In gamete recognition, IZUMO1R/JUNO first binds to monomeric IZUMO1 (PubMed:26568141, PubMed:29954238). The weak, but specific interaction with IZUMO1R/JUNO induces IZUMO1 homodimerization (PubMed:26568141, PubMed:29954238). The process follows a tight binding phase where IZUMO1 bends the entire structure towards the sperm membrane side through a thiol-disulfide exchange reaction (PubMed:26568141, PubMed:29954238). The molecule no longer binds to IZUMO1R/JUNO and instead binds to a putative second oocyte receptor (PubMed:26568141, PubMed:29954238). Interacts with ACE3 (PubMed:20421979). Localizes initially to the acrosome membrane of the sperm head (both outer and inner acrosomal membranes). During the acrosome reaction, translocates to the plasma membrane. Sperm-specific (at protein level) (PubMed:15759005, PubMed:19658160, PubMed:20421979, PubMed:32484434). Detectable on sperm surface only after the acrosome reaction (PubMed:15759005). The extracellular domain assumes a distinct boomerang shape when not bound to IZUMO1R/JUNO (By similarity). Interaction with IZUMO1R/JUNO triggers a conformation change, so that the IZUMO1 extracellular domain assumes an upright conformation (By similarity). The cytoplasmic C-terminus region is not essential for fertilization (PubMed:27624483). It is however required for protein stability (PubMed:27624483). N-glycosylated. Glycosylation is not essential for fusion and for proper protein trafficking in sperm. Phosphorylated (PubMed:19658160, PubMed:27624483). The cytoplasmic C-terminus is phosphorylated and undergoes phosphorylation changes during epididymal transit (PubMed:27624483). Mice are healthy but the males are sterile (PubMed:15759005). They produce morphologically normal sperm that can bind to and penetrate the zona pellucida but that are incapable of fusing with eggs (PubMed:15759005). Izumo is the name of a Japanese shrine to marriage. Belongs to the Izumo family.