An Antigenically Related Polypeptide Family is a Major Structural Constituent of a Stable Acrosomal Matrix Assembly in Bovine Spermatozoa1

Abstract

The apical and principal segments of the bovine acrosome contain a stable matrix complex that is bound to the outer acrosomal membrane and exhibits hydrolase-binding activity. The present study was undertaken to determine whether the outer acrosomal membrane-associated matrix complex (OMC) is composed of a unique set of acrosomal proteins and to define its fate during both capacitation and the acrosome reaction. A purified 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 that the polypeptide localized specifically to the apical and principal segments of the acrosome. Immunoelectron microscopy further revealed that OMC32 was restricted to the stable matrix assembly and was not associated with the inner acrosomal membrane or the equatorial segment. Immunoblot analyses of sperm lysates and of the purified OMC fraction revealed that anti-OMC32 recognized an antigenically related family of polypeptides between 38 and 19 kDa. These polypeptides exhibited no size processing during capacitation or the acrosome reaction, and they were not released during the acrosome reaction but remained in the particulate cell subfraction, associated with the hybrid membrane complex. N-terminal sequence analysis of OMC32 indicated a structural relationship to the SP-10 polypeptide family of human and baboon spermatozoa. The potential function of the OMC complex and differences in the intraacrosomal distribution of bovine OMC32-related polypeptides from that reported for acrosomal SP-10 polypeptides in other species are discussed.

The acrosome of eutherian mammals

“…An extracted acrosomal matrix was recovered with the OAM of the AS of rabbit, bull and hamster spermatozoa, termed acrosomal lamina Olson et al 1997Olson et al , 1998. This superficial layer of material may be composed of glycosylated proteins (Fléchon 1979).…”

Structural modification of the hamster sperm acrosome during posttesticular development in the epididymis

“…The stable acrosomal matrix elements exhibit a specific structural association with the fusigenic domains of the outer acrosomal membrane; this attachment even persists following the membrane fusion events of the acrosome reaction and appears to maintain the structural integrity of the acrosomal cap (Barros et al, 1967;Olson et al, 1997;Yanagimachi and Phillips, 1984). However, the biochemical basis for this specific interaction between the acrosmal matrix and outer acrosomal membrane is unresolved and whether the matrix functions as a scaffold which anchors specific membrane proteins to this membrane domain remains an unsolved issue.…”

“…These hamster acrosomal polypeptides represent the orthologs of the human acrosomal protein SP-10, which is also comprised of a size-variant polypeptide family (Herr et al, 1992;Wright et al, 1990). Related acrosomal polypeptides have been identified in several species, including bovine (Coonrod et al, 1996;Olson et al, 1997), baboon and monkey (Freemerman et al, 1993), fox (Beaton et al, 1995), mouse (Reddi et al, 1995), hedgehog (Bedford et al, 2000), and mole (Bedford et al, 1999). Thus, it appears that a common protein family comprises a stable acrosomal infrastructure in various eutherian species.…”

Organization and modifications of sperm acrosomal molecules during spermatogenesis and epididymal maturation

“…Acrin3 appears to participate in zona pellucida penetration (Saxena et al, 2000). The morphological compartmentalization is also found in other species, including rabbit , hamster (NagDas et al, 1996a), bovine (NagDas et al, 1996 b;Olson et al, 1997), and hedgehog (Bedford et al, 2000).…”