Key protein in sea urchin mating.

For sea urchin sperm, locating the right egg to fertilize appears harder than finding a needle in a haystack. Females release eggs into the sea, where they float among a myriad of other materials, including eggs from many other invertebrates. Scientists know that chemistry plays a key role in getting the right egg and sperm together, and for 15 years they have known about the sperm's egg-recognition molecule, bindin.

Now, cell biologists have identified bindin's "mate," an unusual protein that spans the egg's cell membrane. A small piece extends into the sea urchin egg and a larger portion juts out so that it can bind to the sperm's bindin, says Kathleen R. Foltz of the University of California, Santa Barbara, who discovered the protein with William J. Lennarz and Jacqueline S. Partin from the State University of New York at Stony Brook. They describe their findings in the March 5 SCIENCE.

Once bindin binds to the egg surface, it activates the egg's development. With both receptor and bindin in hand, scientists hope to learn the details of this activation, Foltz says.

The researchers also observed that sea urchin sperm binds to plastic beads coated with this receptor protein and that antibodies to the receptor can block this binding. If human fertilization depends on a similar interaction, then it may be possible to develop contraceptive antibodies that block binding in humans, they suggest.