Izumo-Ig 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 protein, eg Swiss:Q8IYV9, 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 [1]. There is a conserved GCL sequence motif. Izumo expression has been found to be testis-specific [1,2].

1: The molecular players of sperm-egg fusion in mammals. Rubinstein E, Ziyyat A, Wolf JP, Le Naour F, Boucheix C; Semin Cell Dev Biol. 2006;17:254-263. PMID:16574441

2: The immunoglobulin superfamily protein Izumo is required for sperm to fuse with eggs. Inoue N, Ikawa M, Isotani A, Okabe M; Nature. 2005;434:234-238. PMID:15759005