Isolation and biochemical characterization of two isoforms of a boar sperm zona pellucida-binding protein

Protein-carbohydrate complementarity has been recognized as a general mechanism of gamete recognition and adhesion in the process of fertilization throughout the whole animal kingdom. It appears that carbohydrate-binding molecules on the anterior sperm head surface mediate the binding of the male gamete to certain glycoconjugates present on the egg's extracellular coat. Subtle differences in protein and carbohydrate conformation may confer to this interaction a species-specific character. The mechanism responsible for gamete recognition is, however, poorly understood. A step in its elucidation is the characterization of the complementary molecules on the egg and sperm surfaces. With this aim we report here the isolation and partial structural characterization of two isoforms of a zona pellucida-binding protein (which we call AWN-1 and AWN-2) from boar spermatozoa, including partial sequence determination, assignment of disulphide bonds and identification of an N-terminal blocking group. AWN-1 and AWN-2 were isolated from acid extracts of washed ejaculated sperm and were present in seminal vesicle secretions, but absent in samples of epididymal fluid, suggesting a seminal vesicle origin for these sperm proteins. No analogous protein sequence could be found in the MIPS data bank, indicating that the AWN proteins may belong to a novel mammalian protein family involved in fertilization.