Characterization of the binding of recombinant mouse sperm fertilin α subunit to mouse eggs: Evidence for function as a cell adhesion molecule in sperm-egg binding

Fertilin (previously known as PH-30) is a sperm protein that is a candidate molecule for mediating the binding and fusion of the sperm and egg plasma membranes. Fertilin is a heterodimer, with a β subunit that has a region of homology to the disintegrin family of integrin ligands and an α subunit that has a region of homology to viral fusion peptides. it has been hypothesized that fertilin β and α asubunits mediate the interactions between sperm and egg plasma membranes, namely, binding and fusion, respectively. To address this hypothesis and to examine specifically the role of fertilin α in fertilization, we have expressed the predicted extracellular domain of mouse fertilin α as a bacterial fusion protein with maltose-binding protein. This fusion protein (hereafter referred to as recombinant fertilin α-Ec) binds to the microvillar region of zona pellucida (ZP)-free eggs, the region of the membrane to which sperm bind. This binding is reduced in the absence of divalent cations and is supported by Ca²⁺, Mg²⁺, or Mn²⁺. Eggs that have been treated with chymotrypsin bind less recombinant fertilin α-EC than do untreated eggs, suggesting that a chymotrypsin-sensitive binding site for recombinant fertilin α-EC is present on egg surfaces. Binding to eggs is also affected by the method used to remove the ZP. Finally, recombinant fertilin α-EC inhibitors the binding of sperm to eggs during in vitro fertilization of ZP-free eggs. These data are the first evidence to suggest that fertilin α can function as a cell adhesion molecule during fertilization, mediating the binding of sperm and egg plasma membranes.