Sequence of a rabbit sperm zona pellucida binding protein and localization during the acrosome reaction

The interaction of the mammalian spermatozoon with the oocyte's extracellular matrix or zona pellucida is a critical first step leading to successful fertilization. In this cell-extracellular matrix interaction it is the carbohydrate of the zona pellucida which serves as the sperm receptor and the surface of the spermatozoon which provides the lectin-like adhesion molecules. To better understand sperm-zona pellucida binding we have analyzed one specific zona binding protein (ZBP). This study has determined the mRNA sequence encoding a mammalian testis and sperm specific protein of 16,891 Da, which we have designated Sp17. Analysis of Sp17 revealed that the mRNA is present in rabbit, mouse, and human testes but not in any somatic tissue tested. In the rabbit, Sp17 is the 17-kDa member of the rabbit sperm autoantigen family of sperm specific autoantigens and is encoded by two mRNAs of 0.9 and 1.1 kb. Each mRNA has a unique 5' untranslated region but both have identical coding regions. The deduced amino acid sequence of the Sp17 ZBP showed several interesting features, including a similarity to the N-terminal of human testis cAMP-dependent protein kinase. Localization of Sp17 on live spermatozoa using antibodies to recombinant Sp17 or to the Sp17 peptide, G22C, revealed that the peptide backbone of Sp17 is inaccessible until the acrosome reaction begins. However, on paraformaldehyde fixed, acrosome intact spermatozoa, the peptide backbone is accessible to the antibodies which localize Sp17 to the apical surface. In the rabbit as well as other similar species in which the corona radiata (granulosa) cells adhere tightly to the zona pellucida and synthesize zona glycoproteins, the fertilizing spermatozoon may have already begun the acrosome reaction within the cumulus oophorus. Thus, the rabbit sperm surface would be modified to expose the Sp17 polypeptide during the final phase of cumulus passage and consequently Sp17 would be available for initial zona binding. The present study has also demonstrated that recombinant Sp17 can bind zona pellucida, dextran, and dextran sulfate.