Izumo Sperm Egg Fusion 4 (IZUMO4)

Izumo is essential for gamete fusion in the mouse. It has an Immunoglobulin (Ig) domain and an N-terminal domain for which neither the functions nor homologous sequences are known. Izumo 1-3 are transmembrane proteins expressed specifically in the testis, and Izumo 4 is a soluble protein expressed in the testis and in other tissues. Electrophoresis under mildly denaturing conditions, followed by Western blot analysis, showed that Izumo 1, 3, and 4 formed protein complexes on sperm, Izumo 1 forming several larger complexes and Izumo 3 and 4 forming a single larger complex. Studies using different recombinant Izumo constructs suggested the Izumo domain possesses the ability to form dimers, whereas the transmembrane domain or the cytoplasmic domain or both of Izumo 1 are required for the formation of multimers of higher order.