Mouse sperm-egg plasma membrane interactions: Analysis of roles of egg integrins and the mouse sperm homologue of PH-30 (fertilin) β

The guinea pig sperm protein, PH-30 (also known as fertilin), is postulated to participate in the interaction between the sperm and egg plasma membranes. The β subunit of guinea pig PH-30 (gpPH-30β) contains a domain with homology to disintegrins, snake venom proteins that bind to integrins via an integrin-binding domain containing the tripeptide RGD. This raises the question of whether an egg integrin serves as a receptor for PH-30. Although mouse eggs express integrin subunits, their role in mouse fertilization is unresolved. Therefore, we examined fertilization for two different hallmarks of integrin function, namely, dependence of ligand binding on divalent cations and the ability to inhibit ligand binding with RGD peptides. We demonstrate that sperm binding to zona pellucida-free eggs is supported by Ca²⁺, Mg²⁺, or Mn²⁺. Ca²⁺ was necessary and sufficient for sperm-egg fusion, with 2.5 mM Ca²⁺ being the most effective concentration. In addition, fertilization could be partially inhibited with various RGD peptides, which caused a decrease in sperm-egg fusion by 30-58%. This partial inhibition of fusion with RGD peptides prompted the cloning of the mouse homologue of gpPH-30β (hereafter referred to as mPH-30β) to determine if it possessed the tripeptide RGD or a different amino acid sequence in its disintegrin domain. mPH-30β, which is expressed during meiotic and post-meiotic phases of spermatogenesis, shares significant similarities to gpPH-30β throughout the length of the molecule, from the signal sequence to the cytoplasmic tail. The full-length deduced amino acid sequence of mPH-30β is 55% identical and 72% homologous to gpPH-30β. The disintegrin domain of mPH-30β has the tripeptide QDE (instead of RGD) in its cell recognition region. Peptides containing this QDE sequence decrease the binding and fusion of sperm with zona pellucida-free eggs by approximately 70%, suggesting that the disintegrin domain of mPH-30β participates in the interaction between sperm and egg membranes.