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Cell-to-cell transfer of glycosylphosphatidylinositol-anchored membrane proteins during sperm maturation.

In spermatozoa, as in other eukaryotic cells, integral membrane proteins may be anchored by a hydrophobic protein domain, or by a glycosylphosphatidylinositol (GPI) lipid anchor. Contrary to the current understanding of sperm membrane biogenesis, recent evidence shows that some of the GPI-anchored proteins are not synthesized by the spermatozoa themselves, but by cells of the male genital tract. They are transported to the fluid secretions, possibly associated with membrane vesicles, and then incorporate into the sperm membrane. This novel mechanism, by which proteins can move from the membrane of one cell to that of another in vivo, may account for a significant proportion of the surface changes occurring during sperm maturation. The function of these GPI-anchored molecules is largely unknown. However, they are remarkably abundant, and the phenomenon of cell-to-cell transfer seems to be well conserved across mammalian species. All of the post-testicularly acquired GPI-anchored proteins identified thus far have also been found on cells of the immune system (CD62, CD55, CD59, CD73), and we speculate that they may have a role in protecting spermatozoa from immune attack in the male and female reproductive tracts.