Analysis of mouse fertilin in wild-type and fertilin beta(-/-) sperm: Evidence for C-terminal modification, alpha/beta dimerization, and lack of essential role of fertilin alpha in sperm-egg fusion

The sperm surface protein fertilin functions in sperm-egg interaction. On g uinea pig and bovine sperm, fertilin is a heterodimer of alpha and beta sub units. Both subunits are initially synthesized as precursors and then prote olytically processed by removing N-terminal domains. Since the mouse is cur rently the main mammalian species in which fertilization is studied, in the present report, we analyzed the structure, processing, and expression of f ertilin in mouse. We found that the processing of mouse fertilin beta occur s during epididymal maturation and involves changes in the cytoplasmic tail domain as well as the N-terminal domains. Although we (R. Yuan et al., 199 7, 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 beta is 55-57 kDa, here we show that 55 kDa is an unrelated protein in the sperm ex tract which cross-reacts with an antibody that recognizes precursor, but no t mature, fertilin beta. Comparison of Western blots of wild-type and ferti lin beta knockout sperm revealed that authentic, mature fertilin beta is 45 kDa. We also obtained direct evidence that mouse fertilin alpha and beta e xist as a heterodimer. In addition, we found that in mice lacking the ferti lin beta subunit, fertilin alpha is absent from mature sperm. A widely prop osed model for sperm-egg fusion suggests that fertilin alpha is the sperm c omponent that promotes membrane fusion by undergoing a conformational chang e that exposes a virus-like, hydrophobic fusion peptide. Because sperm lack ing fertilin alpha and fertilin beta can fuse with eggs at 50% the wild-typ e rate, this model is called into question. The results suggest instead tha t other gamete surface molecules act to promote membrane fusion and that fe rtilin's role in gamete fusion is in sperm-egg plasma membrane adhesion. (C ) 2000 Academic Press.