The sequence of the Arbacia punctulata bindin cDNA and implications for the structural basis of species-specific sperm adhesion and fertilization

Bindin is the major protein component of the acrosome granule of sea urchin sperm which mediates the species-specific adhesion of sperm to the egg surface during fertilization. Bindin isolated from both Arbacia punctulata and Strongylocentrotus purpuratus sperm demonstrate a distinct adhesive preference for eggs of the same species although a significant amount of cross-species reactivity is observed. Here we describe the isolation and sequence of A. punctulata bindin cDNA clones and a comparison of the predicted protein sequence with the sequence previously reported for S. purpuratus bindin (Gao et al., 1986, Proc. Natl. Acad. Sci., USA 83, 8634-8638). Bindins from these genera show substantial sequence similarity in both the mature bindin domain and the probindin precursor region. The most striking identity is a region of 42 conserved amino acids in the central part of the mature bindins. This conserved domain may be responsible for conserved functions of bindin. Regions flanking this conserved element on both the amino and carboxyl side are more highly divergent, suggesting that they are responsible for the species-specific properties of bindin. The mature A. punctulata sequence contains a putative transmembrane segment between residues 431 and 451 that is absent from S. purpuratus bindin. This structural element may account for the previous observation that isolated A. punctulata bindin uniquely forms multilamellar structures reminiscent of lipid bilayers and binds significant amounts of phospholipid and detergent. The structure of this hydrophobic segment also displays a number of similarities to viral fusion peptides.