Juno also known as folate receptor 4, folate receptor delta or IZUMO1R is a
protein that in humans is encoded by the FOLR4gene.[5] Juno is a member of the
folate receptor family[6] and is
GPI-anchored to the
plasmalemma of the mammalian
egg cell that recognizes its
sperm-riding counterpart,
IZUMO1, and facilitates
fertilization. The protein was named after
Juno, the Roman goddess of fertility and marriage.[7]
After the initial fertilisation stage, a sudden decrease of Juno from the egg cell surface occurs and Juno becomes virtually undetectable after just 40 minutes.[7][8] Still, after fertilization via
intracytoplasmic sperm injection, the egg cell does not lose cell-surface expression of Juno, which suggests that Juno contributes to the prevention of
polyspermy.[8] Mice lacking Juno on the surface of their egg cells are
infertile because their egg cells do not fuse with normal sperm, demonstrating Juno's essential role in the fertility of female mice.[7]
Discovery
Based on a sequence homology search for genes relate to the
folate receptor, the gene for folate receptor 4 was first identified in mice and humans in 2000 at the
University of Nebraska.[6]
In 2014, the function of folate receptor 4 was discovered by the researchers of the
Wellcome Trust Sanger Institute who also proposed that the protein be renamed as Juno.[8] Juno was initially found in murine
oocytes, but its interaction with Izumo was subsequently found in other mammalian species, including humans.[8][9][10][11] Being previously elusive, Juno was discovered nine years after its male counterpart, Izumo1.[7]