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Studies of a sperm acrosomal antigen recognized by HS-63 monoclonal antibody Liu, Ming-Sun


A sperm specific and species conserved monoclonal antibody (HS-63) was shown to inhibit in vitro fertilization of mouse oocytes and human sperm penetration to zona-free hamster ova. The sperm antigen (SA-63) which reacts with HS-63 was found to be localized on the sperm acrosome. Following sperm capacitation, this antigen becomes exposed and is shed after the acrosome reaction. SA-63 may be involved in the sperm acrosome reaction during the initial fertilization process. Sperm antigen (SA-63) from mouse (MSA-63) was purified from mouse testes with soluble and detergent extraction procedures followed by immunoaffinity chromatography. The purified MSA-63 antigen was shown to be a group of proteins with a size ranging from 25 Kd to 50 Kd and pIs of about 4.2 when analyzed by two dimensional gel electrophoresis. MSA-63 antigen may be associated with actins in its native form. A proteolytic activity was found in the solution of purified MSA-63 preparation. Purified MSA-63 was used for immunization of mice and rabbits. Following successive immunizations, antisera of high titres were raised and reacted specifically with sperm acre-some. The isoimmune sera from immunized mice exhibited significant inhibition on in vitro fertilization of mouse oocytes. Complementary deoxyribonucleic acid (cDNA) fragments encoding the MSA-63 were cloned from a mouse testis cDNA library by using an immunoscreening method with rabbit antisera against MSA-63 as the detecting probe. When a specific cDNA probe was used for Northern blot analysis, an mRNA of 1.5 Kb in size was detected only in the adult mouse testis, but not in any other somatic tissues. By Southern blot analysis, it was also demonstrated that the gene encoding for SA-63 protein is conserved among different mammalian species. The location of SA-63 antigen gene was determined to be on human chromosome 11 when analyzed with a blot of a human-hamster somatic cell hybrid panel. By DNA sequence analysis, a protein of 28 Kd in size was deduced from the MSA-63 cDNA. The amino acid sequences of trypsin-digested peptide fragments of MSA-63 were used to verify that deduced amino acid sequence from the cDNA. The recombinant fusion proteins containing MSA-63 protein fragment were produced in E. coli and used to immunize female mice. Similar to the original HS-63 monoclonal antibody, the antisera thus produced reacted only with the sperm acrosome and revealed significant inhibition of the in vitro fertilization of mouse oocytes. In the developing mouse testis, the expression of MSA-63 gene was found to be post-meiotic. Protein and mRNA of MSA-63 were not produced until day 20 after birth.

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