Evolution of the sea urchin sperm protein bindin

Citation

Minor, Joseph E. (1992) Evolution of the sea urchin sperm protein bindin. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/zbbb-kf36. https://resolver.caltech.edu/CaltechTHESIS:08312011-095256471

Abstract

The sperm protein bindin is responsible for the species-specific adhesion of the sperm to the egg. The role of bindin in the establishment of reproductive isolation in the species Strongylocentrotus franciscanus and S.purpuratus is considered. Evolutionary changes in the bindin molecule are described from an analysis of new cDNA sequences obtained from the species Strongylocentrotus franciscanus and Lytechinus variegatus. These sequences are compared to the previously obtained bind in sequence from S.purpuratus. The middle third of the mature bindin sequence is highly conserved in all three species, and the flanking sequences share short repeated sequences that vary in number between the species. This sequence comparison identified the regions of bindin that differ between the species, and that are therefore likely to be responsible for the species-specific properties of bindin. The regions of the bindin molecule responsible for forming the contact between the sperm and the egg were investigated by assaying the ability of bindin-derived peptides to inhibit fertilization. Twenty-four peptides were studied: seven based on the Strongylocentrotus purpuratus bindin sequence, eleven based on the S.franciscanus bindin sequence, and six control peptides. Values for IC_(50), the concentration of peptide required to inhibit 50% of the productive sperm contacts, were extracted from experimental measurements of the extent of fertilization in the presence of various concentrations of these peptides. The IC_(50) value averaged 220 µM for the control peptides. Five subregions of bindin are represented by peptides that had IC_(50) values less than 20 µM; the most potent peptide (SfO) had an IC_(50) value of 2.2 µM. Peptide SfR, derived from a region of the S.franciscanus bindin that differs from the S.purpuratus bindin, inhibited fertilization species-specifically. The peptides inhibit fertilization with a steep dose-response relationship, which probably reflects a requirement for the engagement of multiple bindin monomers in the initiation of the sperm-egg bond. These results demonstrate that a few specific regions of the bindin molecule are involved in the sperm-egg contact, and that these regions mediate the species-specificity of the interaction.

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