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First published online July 13, 2004
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Sperm must undergo a maturation process termed capacitation before they can fertilize eggs. This occurs in the female reproductive tract and involves changes in intracellular pH, reorganization of membrane lipids and increased tyrosine phosphorylation. What is phosphorylated, and where, has remained a mystery. John Aitken and co-workers have therefore examined the relationship between tyrosine phosphorylation and capacitation and identified key proteins that are phosphorylated (see p. 3645). Manipulating levels of tyrosine phosphorylation artificially, they show that tyrosine phosphorylation promotes interaction of sperm with the zona pellucida, the region of the oocyte to which the sperm binds. They then use phosphoproteome analyses to demonstrate that proteins both inside sperm and on their surface are tyrosine phosphorylated during capacitation. The surface proteins turn out to be the molecular chaperones Hsp60 and endoplasmin. The authors demonstrate that these localize to the region of the sperm head that recognizes the zona pellucida. On the basis of these findings, Aitken and co-workers put forward a novel model for sperm-egg interaction in which phosphorylation of chaperones regulates their ability to assemble a zona pellucida receptor complex on the sperm surface.
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