Biophysical and biochemical properties of the mammalian male germ cell specific sulfogalactosylglycerolipid (SGG): Contribution to the structure and zona pellucida affinity of pig sperm raft membranes

Abstract

Raft membranes are implicated in cell adhesion. Here, we demonstrated that rafts isolated from hypermotile capacitated pig sperm as low-density Triton X-100 insoluble membranes had the ability to bind specifically to homologous zonae pellucidae (ZP). This binding was dependent on pig ZP3alpha sulfoglycoprotein, a major player in intact sperm binding. The male germ cell specific sulfogalactosylglycerolipid (SGG) was a sperm raft component and participated in sperm raft-ZP binding, since rafts pretreated with anti-SGG IgG/Fab had decreased ability to bind to the ZP dose dependently. SGG may also partake in raft formation. Fourier transform infrared (FTIR) spectroscopic studies of mixed SGG-cholesterol liposomes revealed that the sulfoglycolipid interacted with cholesterol (a significant raft component) via intermolecular hydrogen bonding. Moreover, mixtures of SGG and cholesterol were insoluble in 1--2% Triton X-100. Since cholesterol is significant for raft formation and since sperm capacitation is associated with cholesterol efflux, we determined whether raft levels in capacitated sperm were the same as those in control sperm. Interestingly, our results revealed an increase in raft levels in the capacitated sperm, as well as an enhanced ZP affinity of these membrane domains, compared to those of control sperm. These results corroborated the implication of rafts in cell adhesion and strongly suggested that the enhanced ZP-binding ability of capacitated sperm may be attributed to increased levels of sperm rafts, with a greater affinity for the ZP.