Membrane fractions have been isolated from eggs of Fucus serrratus which inhibit fertilization in a species-specific manner. This activity is destroyed by alpha-fucosidase and alpha-mannosidase. Some 6% of the protein of this membrane fraction binds to Con A-agarose, following SDS solubilization, and when eluted with alpha-methyl mannoside inhibits fertilization when preincubated with sperm, but not eggs. This inhibitory activity is species-specific and destroyed by alpha-fucosidase but not by trypsin. SDS-gel electrophoresis reveals 1 band staining strongly with Coomassie Brilliant Blue G and weakly with the PAS reagent. This major band represents a glycoprotein with an approximate molecular weight of 30 000 Daltons. Membrane fractions from sperm of Fucus serratus solubilized in KC1 yielded a protein-containing fraction, after affinity chromatography on desulphated focoidan-Sepharose. This fraction is 100-fold more effective in inhibiting fertilization after preincubation with eggs than either Con A or fucose-binding protein. It is species-specific and inhibition is reversed when pretreated eggs are washed with fucoidan. Activity is destroyed by heat and trypsin and only one diffuse band is apparent on SDS gels. This stains positively with Coomassie Brilliant Blue G but not with PAS and has a molecular weight of approximately 60 000 Daltons. Tentative calculation of the numbers of putative receptor molecules gives a figure of 2.5 X 10(9) receptors per egg and 1.8 X 10(6) receptors per sperm.

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