Sperm-egg fusion protein Juno (JUNO)

Sperm-egg fusion protein Juno (JUNO), also called the IZUMO1 receptor, is a glycosylphosphatidylinositol (GPI)-anchored receptor expressed on the surface of mammalian oocytes. It was originally identified as folate receptor 4 (FOLR4/FR-delta), but unlike other folate receptors, it does not bind folate due to the lack of a conserved folate-binding pocket. Instead, JUNO is essential for mammalian fertilization: it specifically mediates adhesion of sperm to the egg by binding to the sperm-specific protein IZUMO1, forming a unique and indispensable ligand-receptor pair for gamete recognition. This interaction is required for the initial physical contact (adhesion) between gametes; disruption or genetic deletion of either partner (JUNO or IZUMO1) in mouse models causes complete infertility due to failure of sperm-egg fusion, although it does not result in broader developmental or somatic abnormalities[2][4]. JUNO displays structural similarity with the folate receptor family but has evolved a fertilization-specific adhesive function[2][3][4]. There are no known drugs targeting JUNO, and its only well-established disease relevance is infertility caused by genetic deficiency or dysfunction[4].

Other names

IZUMO1 receptorIZUMO1RFolate receptor 4FOLR4FR-deltaFolbp3Probable folate receptor deltaFertilization factor on oocyte membraneEgg IZUMO receptor

Not applicable (No drugs directly target JUNO; functionally, it mediates the binding of IZUMO1 on sperm to enable gamete adhesion and subsequent fertilization)