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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

Department of Biomedical Sciences and Technology,³ University of L'Aquila, 67100 L'Aquila, Italy Department of Evolutionary Biology,⁴ University of Siena, 53100 Siena, Italy Obstetrics and Gynecology,⁵ Arcispedale S. Maria Nuova, 42100 Reggio Emilia, Italy

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 Lewis^a-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.

¹ This research was supported by a MURST (9905303471-004) grant to F.R.

² Correspondence: Dr. Riccardo Focarelli, Department of Evolutionary Biology, Via A. Moro 2, 53100 Siena, Italy. FAX: 39 0577 234476; focarelli{at}unisi.it

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