Early Events Revealed in the Fertilization of an Egg by a Sperm
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For the first time, a detailed picture is emerging of the molecular events that occur during fertilization of an egg by a sperm. David Stout, a National Science Foundationsupported researcher at the Scripps Research Institute in La Jolla, California, and Victor Vacquier, a scientist at the University of California, San Diego, present results of their work on this process in the cover article of the September issue of the Journal of Cell Biology.
The organism they have studied is the California red abalone, Haliotis rufescens. In order for a sperm to fuse with an egg, it must first penetrate the egg's extracellular membrane. The abalone sperm uses a protein called lysin to create a hole in this membrane, through which the sperm passes.
Abalone are marine mollusks, with eight species inhabiting the west coast of North America. The species have overlapping breeding seasons and habitats, yet maintain themselves as distinct. The specific structure of lysin is responsible for the species-selective recognition. Stout and Vacquier have determined the three- dimensional molecular structure of lysin. They have shown that two molecules of lysin associate to form a dimer (a protein made up of two polypeptide chains paired together), and have also determined the structure of this dimer. Addition of egg extracellular membrane causes the lysin dimer to dissociate into two monomers. The lysin monomer then penetrates the membrane and creates a hole through which sperm passes. Specific functions can now be assigned to specific features of the lysin molecule that stabilize the dimer and allow the monomer to interact with the extracellular membrane. "These experiments," says Stout, "will allow cell biologists to visualize and understand the fundamentally important biological process of fertilization." The research is funded by NSF's division of molecular and cellular biosciences.
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