An animal egg such as amphibian, mammalian or sea urchin egg receives only a single sperm at fertilisation. After binding of the first sperm, the egg is prevented from allowing the entry of additional sperm. In fact, polyspermy results in aborted development of the zygote. It has been generally accepted that a molecule(s) released from cortical granules participates in the block to polyspermy. As one such molecule, a cortical granule lectin has been isolated from unfertilised Xenopus eggs (Xenopus cortical granule lectin; XCGL). XCGL is released into the perivitelline space after fertilisation, and forms a complex with J1 jelly molecules to form an F layer, resulting in a block to additional sperm penetration.
A lectin molecule has also been purified from the eggs of several species of fish. The fish egg lectin is located in the cortical alveoli and is released from them after fertilisation. However, its biological function is unclear. We isolated cortical alveolar lectin from unfertilised eggs of Chinook salmon through affinity column chromatography (salmon egg lectin; SEL). The lectin activity was estimated by haemagglutination. The activity of the purified SEL was most strongly inhibited by L-rhamnose and D-galactose, but not by EDTA. Further analysis by C4 reverse-phase column chromatography using HPLC revealed that the lectin was composed of three subunit proteins: 24K, 26Ka and 26Kb proteins. In addition, we cloned cDNAs for them by RT-PCR. The deduced amino acid sequence of the 26Ka protein was homologous with that of the 26Kb protein (identity, 96.4%). Identities of the 24K with the 26Ka and the 26Kb proteins were 55.9% and 66.7%, respectively. A database search revealed that a lectin molecule similar to the SEL had been identified in Anthocidaris crassispina egg (sea urchin egg lectin; SUEL). The SUEL is composed of 105 amino acids, and is similar to both amino-terminal and carboxyl-terminal halves of the SELs. Thus, the SEL molecule is composed of two repeats of such SUEL-like domains, suggesting that the SEL gene was produced by gene duplication.