Structure/function analysis of the sea urchin sperm adhesive protein bindin.

The functional domain structure of bindin, an adhesive protein component of the acrosomal granule of sea urchin sperm, has been analyzed using a series of carboxyl- and amino-terminal deletion analogs of recombinant bindin expressed in Escherichia coli. Bindin, which self-associates to form macromolecular aggregates, mediates the species-specific attachment of sperm to the egg surface as well as egg-to-egg agglutination in vitro by binding to egg surface-sulfated fucose-containing glycoconjugates. Cross-species egg agglutination activity and sperm adhesion were determined for two sea urchin species, Strongylocentrotus purpuratus and S. franciscanus. We found that S. franciscanus sperm bindin agglutinates both S. franciscanus and S. purpuratus eggs whereas S. purpuratus sperm bindin displays a much more restricted specificity for homologous S. purpuratus eggs. Consistent with this result, cross-species sperm adhesion experiments demonstrate that S. franciscanus sperm also bind more efficiently to S. purpuratus eggs in comparison to S. purpuratus sperm attachment to S. franciscanus eggs. Recombinant S. purpuratus bindin demonstrates the same species-specific egg agglutination properties observed for sperm bindin. Deletion mutants of S. purpuratus bindin were tested for their ability to species specifically agglutinate eggs. We found that residues 1-74 or residues 122-236 can be deleted without a loss of the species-specific agglutination properties of bindin. Comparison of the bindin amino acid sequences from the two species reveals that all of the active deletion analogs have a central segment in common corresponding to residues 75 through 121 of S. purpuratus bindin, where the sequences are identical except for a single amino acid, Arg 77, which is an Ala residue in S. franciscanus. However, substitution of Arg 77 for Ala does not alter the species specificity of recombinant bindin, suggesting that the more divergent amino- and carboxy-terminal sequences flanking the conserved central domain determine the species specificity of bindin and either end is sufficient to impart specificity. The amino- and carboxyl-terminal thirds of S. purpuratus bindin each contain two repeats of the sequence M G G P P/V. This sequence does not occur in S. franciscanus, which instead has five repeats of the sequence M G G A/V/Q F/V/P. It is conceivable that these repeats, which vary in sequence in bindins from different species, constitute an important part of the species recognition mechanism.