Sperm-binding proteins on the inner layers of the extracellular matrix of vertebrate eggs are well known. However, sperm attractants in the outer jelly area have been detected in bioassays but have not been isolated. Now Olson et al. have purified a sperm chemoattractant from the egg jelly of Xenopus eggs. The protein, which they name allurin, has sequence similarity to members of the CRISP (cysteine-rich secretory protein) family. The new results add to the notion that CRISP family members escort or guide sperm on their voyage all the way from the testes to the egg. One CRISP protein, TPX-1, functions to promote adhesion of sperm to Sertoli cells in the testes. During maturation in the epididymis, sperm bind to another set of CRISP proteins, the acidic epididymal secretory glycoproteins, or AEGs. And finally, they encounter the new CRISP member, allurin, the first to be identified in the female reproductive tract. It is possible that receptors on the surface of the sperm sense a gradient in the concentration of allurin, or allurin-like molecules, which then guides the sperm to the egg.—LBR
Proc. Natl. Acad. Sci. U.S.A.98, 11205 (2001).