Capacitation dependent activation of tyrosine phosphorylation generates two sperm head plasma membrane proteins with high primary binding affinity for the zona pellucida

The recognition and binding of sperm cells to the zona pellucida (the extra cellular matrix of the oocyte) are essential for fertilization and are beli eved to be species specific. Freshly ejaculated sperm cells do not bind to the zona pellucida. Physiologically this interaction is initiated after spe rm activation in the female genital tract (capacitation) via a yet unknown mechanism, resulting in the binding of a receptor in the apical sperm plasm a membrane to the zona pellucida. In order to mimic this biochemically, we isolated zona pellucida fragments from gilt ovaries to prepare an affinity column with the intact zona pellucida structure and loaded this column with solubilized apical plasma membranes of boar sperm cells before and after i n vitro capacitation. With this technique we demonstrated that two plasma m embrane proteins of capacitated boar sperm cells showed high affinity for z ona pellucida fragments. Further analysis showed that these proteins were t yrosine phosphorylated. Plasma membrane proteins from freshly ejaculated sp erm cells did not exhibit any zona pellucida binding proteins, likely becau se these proteins were not tyrosine phosphorylated.