Mapping of epitopes on porcine zona pellucida-3 alpha by monoclonal antibodies inhibiting oocyte-sperm interaction.

Zona pellucida glycoprotein 3 alpha (ZP3 alpha) has been designated as the primary sperm receptor ligand in porcine gamete interaction. In this study, epitopes were mapped on porcine ZP3 alpha (pZP3 alpha) by using monoclonal antibodies (mAbs) possessing in vitro contraceptive efficacy. Using Western blots, we tested recombinant pZP3 alpha fragments expressed as fusion proteins in Escherichia coli and corresponding to pZP3 alpha precursor protein amino acid residues 18-142 (F1), 140-243 (F2), 239-363 (F3), and 359-462 (F4), for reactivities with mAbs. MAb-403 reacted with F3, and mAbs-412 and -421 with F2. MAb-420 showed weak reactivity with F1. Synthesis of overlapping 12-mer peptides further resolved the epitope for mAb-420 to amino acid residues 133-144, mAb-421 to 157-168, mAb-412 to 205-216, and mAb-403 to 301-312. MAbs-412 and -420 inhibited the binding of boar sperm to zona-encased porcine oocytes. These results, the first to define peptide epitopes of pZP3 alpha, should assist in the design of a synthetic peptide-based immunocontraceptive vaccine.