Sperm-egg fusion is believed to be mediated via specific molecular interact
ions. Integrin alpha 6 beta 1 is a strong candidate for a sperm receptor on
the egg plasma membrane. However, the ability of the egg integrin alpha 6
beta 1 to interact with molecules on intact sperm has not yet been proven.
In this report, possible involvement of integrin alpha 6 beta 1 in sperm-eg
g interactions was examined by biochemical and immunocytochemical analyses.
To identify egg molecules that specifically interact with sperm, we first
incubated sperm with biotin-labeled egg surface proteins. Under this condit
ion, solubilized proteins from eggs inhibited sperm-egg fusion. Western blo
t analysis under reducing conditions indicated that a major-labeled band of
135 kDa bound to sperm. An immunodepletion experiment using the anti-integ
rin alpha 6 antibody GoH3 indicated that the 135 kDa egg surface molecule t
hat bound to sperm was the integrin alpha 6 subunit. To investigate the pot
ential involvement of integrin alpha 6 beta 1 in sperm-egg fusion, we next
examined the localization of integrin a6 and beta 1 subunits before and aft
er fertilization by confocal laser microscopy. At an early stage of sperm-e
gg fusion, the integrin alpha 6 and beta 1 subunits were accumulated at the
sperm binding site. The frequency of cluster formation was closely related
to that of sperm-egg fusion, indicating that integrin receptors are accumu
lated by sperm destined for fusion. Taken together, these results strongly
suggest that the integrin alpha 6 beta 1 is involved in sperm-egg binding l
eading to fusion via direct association of the integrin alpha 6 with sperm.
Mol. Reprod. Dev. 56:412-423, 2000. (C) 2000 Wiley-Liss, Inc.