Abstract
We recently found that Xenopus dicalcin, present in the extracellular egg-coating envelope, suppresses the efficiency of fertilization in vitro through binding to envelope-constituent glycoproteins. In the present study, we explored the mouse counterpart of Xenopus dicalcin, specifically its localization in the female reproductive tract and its action on mouse fertilization. Our homology and phylogenetic analyses using known S100 proteins showed that S100A11 is most closely related to Xenopus dicalcin. S100A11 was localized in the cytosol of luteal cells, but not in the follicle, in the mouse ovary, and also in the cytosol of the oviductal epithelial cells. In addition, our quantitative analyses revealed preferential expression of S100A11 in the ampullary region of the oviduct and at the estrus stage during the mouse estrous cycle. In the cumulus cell-oocyte complex dissected from the oviduct following ovulation, S100A11 was present in the plasma membrane of cumulus cells, but not in the zona pellucida, which is comparable with Ca(2+) -dependent binding of exogenously applied S100A11 to the plasma membrane of cumulus cells. Pretreatment of the cumulus cell-oocyte complex with recombinant S100A11 substantially reduced the efficiency of in vitro fertilization, but S100A10, the next closest S100 protein to Xenopus dicalcin, had no effect. These results suggested that S100A11 is the mouse counterpart of Xenopus dicalcin, suppresses the fertilization process through its action on cumulus cells, and thereby plays a key role in fertilization success in the mouse.