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* Molecular and Cellular Biology and Sperm-egg plasma membrane fusion is preceded by sperm adhesion to the egg plasma membrane.
Cell-cell adhesion frequently involves multiple adhesion molecules on the adhering cells. One sperm
surface protein with a role in sperm-egg plasma
membrane adhesion is fertilin, a transmembrane heterodimer (
Department of Cell Biology and Human Anatomy, School of Medicine, University of
California, Davis, California 95616; and § Cell Biology Program, University of Connecticut Health Center, Farmington,
Connecticut 06030
and
subunits). Fertilin
and
are the
first identified members of a new family of membrane proteins that each has the following domains: pro-, metalloprotease, disintegrin, cysteine-rich, EGF-like, transmembrane, and cytoplasmic domain. This protein family has been named ADAM because all members contain a disintegrin and metalloprotease domain. Previous studies indicate that the disintegrin domain of fertilin
functions in sperm-egg adhesion leading to fusion. Full length cDNA clones have been isolated for
five ADAMs expressed in mouse testis: fertilin
, fertilin
, cyritestin, ADAM 4, and ADAM 5. The presence
of the disintegrin domain, a known integrin ligand, suggests that like fertilin
, other testis ADAMs could be
involved in sperm adhesion to the egg membrane. We
tested peptide mimetics from the predicted binding
sites in the disintegrin domains of the five testis-expressed
ADAMs in a sperm-egg plasma membrane adhesion and fusion assay. The active site peptide from cyritestin
strongly inhibited (80-90%) sperm adhesion and fusion
and was a more potent inhibitor than the fertilin
active site peptide. Antibodies generated against the active site region of either cyritestin or fertilin
also
strongly inhibited (80-90%) both sperm-egg adhesion
and fusion. Characterization of these two ADAM family members showed that they are both processed during sperm maturation and present on mature sperm. Indirect immunofluorescence on live, acrosome-reacted
sperm using antibodies against either cyritestin or fertilin
showed staining of the equatorial region, a region of the sperm membrane that participates in the early
steps of membrane fusion. Collectively, these data indicate that a second ADAM family member, cyritestin,
functions with fertilin
in sperm-egg plasma membrane adhesion leading to fusion.
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