Essential sperm cell-surface protein required for fertilization by acting as a ligand for IZUMO1R/JUNO receptor on egg (PubMed:15759005). 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). The ligand-receptor interaction probably does not act as a membrane 'fusogen' (PubMed:15759005). Monomer (PubMed:27309808, PubMed:27309818). Monomer, homodimer and homooligomer; depending on the context (By similarity). Interacts with IZUMO1R/JUNO. IZUMO1 and IZUMO1R/JUNO form a complex with 1:1 stoichiometry (PubMed:27309808, PubMed:27309818). In gamete recognition, IZUMO1R/JUNO first binds to monomeric IZUMO1 (By similarity). The weak, but specific interaction with IZUMO1R/JUNO induces IZUMO1 homodimerization (By similarity). The process follows a tight binding phase where IZUMO1 bends the entire structure towards the sperm membrane side through a thiol-disulfide exchange reaction (By similarity). The molecule no longer binds to IZUMO1R/JUNO and instead binds to a putative second oocyte receptor (By similarity). Interacts with ACE3 (By similarity). Localizes initially to the acrosome membrane of the sperm head (both outer and inner acrosomal membranes) (By similarity). During the acrosome reaction, translocates to the plasma membrane (PubMed:15759005). Sperm-specific (at protein level) (PubMed:15759005). 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 (PubMed:27309818). Interaction with IZUMO1R/JUNO triggers a conformation change, so that the IZUMO1 extracellular domain assumes an upright conformation (PubMed:27309818). The cytoplasmic C-terminus region is not essential for fertilization. It is however required for protein stability. N-glycosylated. Glycosylation is not essential for fusion and for proper protein trafficking in sperm. Phosphorylated. The cytoplasmic C-terminus is phosphorylated and undergoes phosphorylation changes during epididymal transit. Izumo is the name of a Japanese shrine to marriage. Belongs to the Izumo family.