During mammalian fertilisation, sperm undergo capacitation before they adhere to the egg coat (zona pellucida) and enter the egg. A sperm is normally maintained in an uncapacitated state by the binding of glycoconjugates produced in the male to the sperm's cell-surface receptorβ 1,4-galactosyltransferase I (GalT I). Rodeheffer and Shur (p. 503) demonstrate that GalT I regulates capacitation by suppressing Ca2+- and HCO3–-dependent signaling cascades. On p. 491, the same authors show for the first time that sperm-egg adhesion involves at least two distinct binding events. During sperm-egg adhesion, GalT I binds to the zona pellucida glycoprotein ZP3 of ovarian eggs. Puzzlingly, however, GalT I-null sperm are able to bind to the zona pellucida of ovulated eggs. Rodeheffer and Shur have identified a novel ligand present in the zona pellucida of ovulated eggs that binds both wild-type and GalT I-null sperm, providing evidence for a ZP3- and GalT I-independent receptor-ligand interaction.
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