Abstract
In Xenopus laevis, sperm bind to vitelline envelope (VE) that surrounds an egg, and undergo acrosome-reaction that enables sperm to penetrate the VE. Sperm-egg fusion activates an egg and resumes the cell cycle. Although there are considerable lines of evidence, molecular aspects of fertilization remain to be elucidated because of the conflicting results. In the present study, to understand molecular mechanism of sperm-egg interaction, we isolated and characterized a 26 kDa Ca2+-binding protein, Xenopus dicalcin, in Xenopus eggs. Xenopus dicalcin is localized prominently in VE and cytoplasm of the cortex of both the animal and the vegetal hemisphere of Xenopus eggs. Two VE glycoproteins bind to Xenopus dicalcin in a Ca2+-dependent manner. Since these two VE glycoproteins are considered to function as sperm-receptors in the VE, we tested the effect of dicalcin on fertilization in vitro. Preincubation of eggs with recombinant dicalcin reduced the success of fertilization to as little as 10% of control. In contrast, inhibition of intrinsic dicalcin by preincubation of anti-dicalcin antibody surprisingly increased the success of fertilization. These results indicated that dicalcin inhibits sperm-egg interaction and subsequent fertilization. Mechanism of dicalcin's action and future aims will also be discussed. [J Physiol Sci. 2007;57 Suppl:S175]
