Human SP-10: acrosomal distribution, processing, and fate after the acrosome reaction

SP-10 is a testis-specific acrosomal protein that has been detected in several species including humans. Extracts from whole human testis and epididymal, ejaculated, and capacitated sperm were analyzed by Western blot for SP-10 polypeptides. The testis extracts contained a full-length SP-10 protein at approximately 45 kDa as well as other immunoreactive SP-10 peptides at 32, 30, 28, and 26 kDa. Extracts from epididymal, ejaculated, and capacitated sperm contained several immunoreactive SP-10 peptides that co-migrated with the 32-26-kDa SP-10 peptides in the testis extracts. Epididymal, ejaculated, and capacitated sperm extracts did not contain the 45-kDa SP-10 peptide observed in testis extracts, but did contain immunoreactive SP-10 peptides from 25 to 18 kDa that were not detected in testis extracts. These results indicate that a full-length 45-kDa SP-10 precursor protein is present in the testis and that SP-10 peptides of 32, 30, 28, and 26 kDa result from proteolytic processing of the SP-10 precursor protein in the testis and/or alternative splicing. In addition, SP-10 peptides of 25-18 kDa were first detected in extracts of caput epididymal sperm and probably resulted from the proteolytic processing of the 45- and 32-26-kDa SP-10 peptides in the initial segment or caput epididymidis. Also, no additional SP-10 bands were detected in extracts of cauda epididymal, ejaculated, or capacitated sperm, suggesting that no further processing of the 32-18-kDa SP-10 peptides occurred during epididymal transit, ejaculation, and capacitation. Electron microscopic immunocytochemical observations of epididymal, ejaculated, and capacitated sperm revealed that colloidal gold labeling of SP-10 was most abundant within the principal segment and posterior bulb of the equatorial segment of the acrosome, while the colloidal gold labeling of SP-10 was sparse in the anterior equatorial segment of the acrosome. After a follicular fluid-induced acrosome reaction, SP-10 was detected on the inner acrosomal membrane in the equatorial segment and was associated with hybrid vesicles. This localization after the acrosome reaction is consistent with the hypothesis that SP-10 may be involved in sperm-zona binding or penetration.