The Binding of Concanavalin a to the Surface of Intact and Denuded Sea Urchin Eggs Affects the Fertilization Process by Altering the Structural Dynamics of Actin Filaments

In Paracentrotus lividus eggs, we assessed the binding capacity of the lectin Concanavalin A to the glycoproteins present in the vitelline layer, which is tightly bound to the membrane of the microvilli on the sea urchin egg's surface. We aimed to understand the role of carbohydrate residues in species-specific gamete recognition and binding. This interaction occurs after the sperm undergoes the jelly coat-induced acrosome reaction, which exposes the adhesive protein bindin, allowing the fusion of sperm and egg membranes, ultimately leading to fertilization. Our findings reveal that, while fertilization of both intact and denuded eggs (from which the jelly coat and vitelline layer were removed) in the presence of the lectin does not affect sperm functionality, it influences the Ca²⁺ response of the activated egg and the penetration of sperm in a dose-dependent manner. Comparative analyses of the effect of Concanavalin A on the fertilization response, assessed through light, fluorescence, confocal, and electron microscopy, revealed significant alterations in the structural dynamics of the actin filaments on the surface and cortex of the eggs, responsible for the altered Ca2+ signals. In addition to the visualization of the binding of the lectin with the vitelline layer of intact eggs and membranes of denuded eggs, the usage of a fluorescently tagged lectin has also enabled, for the first time, the visualization of the glycoprotein contents of the outer jelly coat.