Localization and role of sulfoglycolipid immobilizing protein 1 on the mouse sperm head

Sulfoglycolipid immobilizing protein 1 (SLIP 1) is an evolutionally conserved sperm head plasma membrane protein (M(r) = 68 kDa) that binds to sulfogalactosylglycerolipid (SGG), the major sulfoglycolipid present in mammalian sperm. The purpose of this study was to characterize the initial localization and the immunoaggregated relocalization of SLIP1 on the mouse sperm head. Direct immunofluorescence (DF) of live sperm using FITC-antiSLIP1 Fab fragments and FITC-antiSLIP1 IgG indicated that SLIP1 was present in the postacrosomal region of the sperm head, although the intensity of immunostaining by FITC-antiSLIP1 IgG was greatest at the border between the postacrosomal region and the acrosome. Unlike that observed with FITC-antiSLIP1 Fab, DF using FITC-antiSLIP1 IgG indicated that SLIP1 was also present in the anterior tip of the sperm head convex ridge. Results from electron microscopic studies, using antiSLIP1 IgG followed by protein A-gold on live mouse sperm, were similar to the DF findings. In contrast, indirect immunofluorescence (IIF) of live mouse sperm using antiSLIP1 IgG and FITC-secondary antibody IgG detected SLIP1 in the sperm head convex ridge only. The IIF and DF results strongly suggest that these bivalent antibodies could induce the sperm antigen relocalization on live sperm heads. SLIP1 redistribution may be dependent on availability of excess SGG, the SLIP1 binding ligand, based on the observation that purified exogenous biotinylated SLIP1 bound to live mouse sperm at both the postacrosomal and convex ridge regions of the mouse sperm head. Immunoaggregation induced by the primary antiSLIP1 IgG or antiSLIP1 Fab with secondary antibody IgG did not cause the acrosome reaction, suggesting that SLIP1 is not involved in sperm signal transduction. Furthermore, postacrosomal SLIP1 was shown to be involved in zona binding, since sperm pretreated with antiSLIP1 Fab fragments (100 micrograms/ml) bound to the egg zona pellucida in vitro at approximately 35% of control levels.