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First published online November 4, 2009
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Before penetrating the oocyte during fertilisation, sperm must bind to the zona pellucida (ZP), the glycoprotein membrane that surrounds the oocyte. The ZP glycoprotein ZP3 is important for sperm binding, but little has been known about which other proteins are involved. Now, Robert Lyng and Barry Shur (p. 3894) report the identification of an additional sperm-adhesion ligand. The research group showed previously that, in addition to ZP3, ovulated mouse oocytes contain a peripherally associated high-molecular-weight glycoprotein with sperm-binding activity; now, they identify this species [which they name oviduct-specific glycoprotein (OGP)] by sequence analysis and purify it under native conditions. Notably, immunodepletion of OGP from oviduct lysates inhibits sperm-oocyte adhesion, and purified OGP also competitively inhibits adhesion. The authors next show that distinct glycoforms of OGP localise to different regions of the oviduct and oocyte; in particular, a relatively scarce glycoform [which was identified on the basis that it binds to peanut agglutinin (PNA)] associates preferentially with the sperm surface and the ZP. Importantly, PNA-binding OGP can induce sperm to bind to two-cell embryos (which do not normally interact with sperm). The authors propose a specific role for PNA-binding OGP in sperm-oocyte adhesion.
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