In the second half of the 1800s, biologists began to study the cellular basis of development. Sea urchins have become a preferred material for this endeavor because their gametes can be obtained in large numbers and fertilization and development are external to the adult body. The first research on fertilization focused on observing the interactions between sperm and ova. In most invertebrates such as sea urchins, the sperm attach to the eggs before the two very different cells merge. The article by Wessel et al. (1) focuses on Bindin, the protein in sea urchin sperm that binds sperm to the EBR1 egg receptor (2??–4). CRISPR-Cas9 methods inhibited (KO) the synthesis of Bindin as demonstrated by Western blots and immunofluorescence. The sperm from knocked out males failed to bind or fuse with the eggs. These results prove that Bindin sperm is necessary for the fertilization of sea urchins.
The initial translation product is a PreproBindin of approximately 460 to 485 amino acids. The Prepro portion is ∼244 to 254 amino acids with a Furin site at its C terminus. Cleavage of furin creates a mature Bindin of 218-284 amino acids, which is packaged in the acrosomal secretion vesicle. When the sperm come into contact with the eggs, the glycoconjugates trigger acrosomal exocytosis, exposing Bindin on the sperm membrane. The sperm binds to the EBR1 receptor in the egg cell and the two cells merge and the egg cell attracts the sperm into its cytoplasm. The central region of mature Bindin contains the B18 peptide, which does not vary between species. A second Bindin peptide, B55, is a potent membrane disruptor and is used to discharge endosomal vesicles containing dextrans, antibodies, RNase, and plasmid DNA into the cytoplasm of cultured cells (5). An unproven hypothesis is that B18 and B55 merge the membranes of sperm and ova. All B18s and most B55s are present in the Bindin Starfish with a…