In the second half of the 1800s biologists began to investigate the cellular basis of development. Sea urchins became a favorite material for this endeavor because their gametes can be obtained in vast numbers and fertilization and development are external to the adult body. The first research on fertilization centered on watching sperm interact with eggs. In most invertebrates such as sea urchins the sperm bind to the eggs before the two very different cells fuse. The paper by Wessel et al. (1) focuses on Bindin, the sea urchin sperm protein binding sperm to the egg receptor EBR1 (2⇓–4). CRISPR-Cas9 methods knocked out (KO) Bindin synthesis as demonstrated by Western blots and immunofluorescence. Sperm from KO males could not bind or fuse with eggs. These results prove that sperm Bindin is required for sea urchin fertilization.
The initial translation product is a PreproBindin of ∼460 to 485 amino acids. The Prepro portion is ∼244 to 254 amino acids with a Furin site at its C terminus. Furin cleavage creates a mature Bindin of ∼218 to 284 amino acids, which is packaged into the acrosomal secretory vesicle. When sperm contact eggs, glycoconjugates trigger acrosomal exocytosis, exposing Bindin on the sperm membrane. The sperm bind to the egg EBR1 receptor and fusion of both cells occurs and the egg draws the sperm into its cytoplasm. The central region of mature Bindin contains the B18 peptide, which does not vary among species. A second Bindin peptide, B55, is a potent membrane-perturbing agent and is used to unload endosomal vesicles containing dextrans, antibodies, RNase, and plasmid DNA into the cytoplasm of cultured cells (5). An unproven hypothesis is that B18 and B55 fuse sperm and egg membranes. All of B18 and most of B55 are present in starfish Bindin with one …
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