Identification of a new sea urchin vitelline envelope sperm binding glycoprotein

Getting sperm and egg together: things conserved and things diverged

Sperm-egg interactions occur at multiple levels on the egg surface, first with the egg's extracellular matrix and then with the egg's plasma membrane. The BioPore minisymposium on "The Egg Surface" at the 1999 annual meeting of the Society for the Study of Reproduction highlighted a series of events underlying successful interactions of the sperm with the egg: 1) composition, synthesis, and assembly of the mouse egg's extracellular matrix, the zona pellucida, during oogenesis; 2) oocyte maturation and development of the sperm-binding domain of mouse eggs; and 3) characterization of functional domains in different sperm ligands (fertilin-alpha and fertilin-beta in the mouse and lysin in the abalone) that recognize cognate binding sites on the egg surface. Data that were presented are reviewed here and discussed with respect to conserved and divergent features of gamete functions.

The 350-kDa sea urchin egg receptor for sperm is localized in the vitelline layer

Previous studies have established by several methods that the 350-kDa egg receptor for sperm is localized on the plasma membrane-vitelline layer complex of the egg of the sea urchin Strongylocentrotus purpuratus. In addition, it has been found that molecules which are cross-reactive with anti-receptor antibody are present in the cortical granules located at the inner face of the plasma membrane. The objective of this study was to define more precisely the locale of the cell surface receptor. We have found that following fertilization, the immunoreactive receptor initially found on the egg surface moved to the fertilization envelope (FE) and then disappeared in parallel with the loss of sperm binding activity. A cross-linked, high-molecular-weight derivative of soybean trypsin inhibitor (hMW-SBTI) which was unable to pass through the elevating FE blocked the loss of both immunoreactivity and the sperm binding activity of the FE, but did not inhibit the vitelline delaminase activity that has been implicated in FE formation. Western blot analysis following SDS-PAGE of the proteins of the FE isolated in the presence of hMW-SBTI and benzamidine revealed the presence of the 350/320-kDa proteins which cross-reacted with anti-receptor antibody. Experiments in which molecules on the surface of unfertilized eggs were labeled with biotin and traced after FE formation revealed that a significant portion of the 350/320-kDa glycoproteins that were incorporated into the FE originated from the cell surface, rather than from the cortical granules. These findings provide strong evidence that in unfertilized eggs the egg receptor for sperm exists as part of the protein complex known as the vitelline layer which serves as a precursor of the FE. Evidence is presented indicating that some of the receptor in the vitelline layer is cryptic and a possible function for this cryptic form of the receptor is proposed.