AN ANTIGENICALLY RELATED POLYPEPTIDE FAMILY IS A MAJOR STRUCTURAL CONSTITUENT OF A STABLE ACROSOMAL MATRIX ASSEMBLY IN BOVINE SPERMATOZOA

The apical and principal segments of the bovine acrosome contain a sta ble matrix complex that is bound to the outer acrosomal membrane and e xhibits hydrolase-binding activity. The present study was undertaken t o determine whether the outer acrosomal membrane-associated matrix com plex (OMC) is composed of a unique set of acrosomal proteins and to de fine its fate during both capacitation and the acrosome reaction. A pu rified OMC fraction was isolated from ejaculated spermatozoa, and one polypeptide of 32 kDa (OMC32) was purified to homogeneity and used for N-terminal sequence analysis and preparation of monospecific antisera . Immunofluorescence staining of sperm with anti-OMC32 demonstrated th at the polypeptide localized specifically to the apical and principal segments of the acrosome. Immunoelectron microscopy further revealed t hat OMC32 was restricted to the stable matrix assembly and was not ass ociated with the inner acrosomal membrane or the equatorial segment. I mmunoblot analyses of sperm lysates and of the purified OMC fraction r evealed that anti-OMC32 recognized an antigenically related family of polypeptides between 38 and 19 kDa. These polypeptides exhibited no si ze processing during capacitation or the acrosome reaction, and they w ere not released during the acrosome reaction but remained in the part iculate cell subfraction, associated with the hybrid membrane complex. N-terminal sequence analysis of OMC32 indicated a structural relation ship to the SP-10 polypeptide family of human and baboon spermatozoa. The potential function of the OMC complex and differences in the intra acrosomal distribution of bovine OMC32-related polypeptides from that reported for acrosomal SP-10 polypeptides in other species are discuss ed.