Analysis of Mouse Fertilin in Wild-Type and Fertilin β−/− Sperm: Evidence for C-terminal Modification, α/β Dimerization, and Lack of Essential Role of Fertilin α in Sperm–Egg Fusion

Abstract

The sperm surface protein fertilin functions in sperm–egg interaction. On guinea pig and bovine sperm, fertilin is a heterodimer of α and β subunits. Both subunits are initially synthesized as precursors and then proteolytically processed by removing N-terminal domains. Since the mouse is currently the main mammalian species in which fertilization is studied, in the present report, we analyzed the structure, processing, and expression of fertilin in mouse. We found that the processing of mouse fertilin β occurs during epididymal maturation and involves changes in the cytoplasmic tail domain as well as the N-terminal domains. Although we (R. Yuan et al., 1997, J. Cell Biol. 137, 105–112) and others (M. S. Chen et al., 1999, J. Cell Biol. 144, 549–561) have previously reported that mature fertilin β is 55–57 kDa, here we show that 55 kDa is an unrelated protein in the sperm extract which cross-reacts with an antibody that recognizes precursor, but not mature, fertilin β. Comparison of Western blots of wild-type and fertilin β knockout sperm revealed that authentic, mature fertilin β is 45 kDa. We also obtained direct evidence that mouse fertilin α and β exist as a heterodimer. In addition, we found that in mice lacking the fertilin β subunit, fertilin α is absent from mature sperm. A widely proposed model for sperm–egg fusion suggests that fertilin α is the sperm component that promotes membrane fusion by undergoing a conformational change that exposes a virus-like, hydrophobic fusion peptide. Because sperm lacking fertilin α and fertilin β can fuse with eggs at 50% the wild-type rate, this model is called into question. The results suggest instead that other gamete surface molecules act to promote membrane fusion and that fertilin's role in gamete fusion is in sperm–egg plasma membrane adhesion.