Gp273, the Ligand Molecule for Sperm-Egg Interaction in the Bivalve Mollusk, Unio elongatulus, Binds to and Induces Acrosome Reaction in Human Spermatozoa Through a Protein Kinase C-Dependent Pathway1

Abstract

In a previous article, we suggested that gp273, the ligand molecule for sperm-egg interaction in the bivalve mollusk Unio elongatulus has functional carbohydrate epitopes in common with a human zona pellucida glycoprotein, probably ZP3. We demonstrated that: 1) anti-gp273-purified immunoglobulin G (IgG), which recognizes a carbohydrate gp273 epitope including a Lewisa-like structure, interacts with a zona pellucida protein; 2) human sperm specifically bind to gp273; and 3) binding is reversed by anti-gp273 IgG. In the present study, we confirm this suggestion by demonstrating that heat-solubilized zonae pellucidae reverse gp273-human sperm binding, that gp273-binding sites are restricted to the acrosomal region, and that gp273 induces the acrosome reaction in human sperm. We also demonstrated that gp273-binding sites on human sperm function as signaling receptors because exposure of spermatozoa to this glycoprotein results in significant stimulation of protein kinase C (PKC) activity. Because the PKC inhibitor, bisindolylmaleimide I, reverses both PKC activation and the acrosome reaction, this kinase is a key component of the signal transduction pathway activated by gp273 and leading to the exocytotic event.

First Steps in the Development of a Functional Assay for Human Sperm Using Pig Oocytes

“…However, Windt et al (1992) observed that human spermatozoa which had been exposed to porcine zona protein showed increased sperm‐zona binding when compared to control spermatozoa; these results would be contradictory to previous studies. Delle Monache et al (2003) demonstrated that Gp273, the ligand molecule for sperm‐egg interaction in a bivalve mollusk, binds to human sperm and induces AR. This finding, in agreement with our results, suggests that a strict species‐specificity at primary binding does not exist (Focarelli et al, 2001).…”

Aurelia aurita–Cnidarian with a prominent medusiod stage

“…Vertebrate's ZP is made up of three/four glycoproteins arranged in a delicate filamentous matrix. The glycoproteins are specific proteins with ZP‐domain (Delle Monache et al, 2003). Mouse ZP proteins are synthesized and secreted exclusively by growing oocytes in the ovary.…”

“…The sperm–egg interaction at fertilization in the mollusc bivalve Unio elongatulus is mediated by fucosylated O‐linked oligosaccharide chains of gp273, one of the two glycoproteins constituting most of the vitelline coat in this species. Gp273 has functional epitopes in common with one of the human ZP glycoproteins, probably hZP3 (Delle Monache et al, 2003). ZP‐domain proteins are found to be necessary for the cuticula formation of Caenorhabditis elegans larva (Sapio et al, 2005).…”

Glycoproteomics: Past, present and future

“…The capabilities of this tool are exemplified by our current glycoproteomic studies of a glycoprotein called gp273 which is found in the extracellular coat of the freshwater mollusc bivalve Unio elongatulus , and is believed to be the ligand for sperm–egg interaction during fertilisation [43]. This glycoprotein has attracted the attention of the mammalian developmental biology community because it has been shown to bind to human sperm and to induce the acrosomal reaction [44]. Fucosylated epitopes on the O‐glycans of gp273 appear to play a role in this binding.…”