Fertilization: Beginning a New Organism
In Strongylocentrotus purpuratus, the acrosome reaction is initiated by a repeating polymer of fucose sulfate. When this sulfated polysaccharide binds to its receptor on the sperm, the receptor activates three sperm membrane proteins: (1) a calcium transport channel that allows Ca2+ to enter the sperm head; (2) a sodium-hydrogen exchanger that pumps sodium ions (Na+) into the sperm as it pumps hydrogen ions (H+) out; and (3) a phospholipase enzyme that makes another second messenger, the phosopholipid inositol 1,4,5-trisphosphate (IP3, of which we will hear much more later in the chapter). IP3 is able to release Ca2+ from inside the sperm, probably from within the acrosome itself (Domino and Garbers 1988; Domino et al. 1989; Hirohashi and Vacquier 2003). The elevated Ca2+ level in a relatively basic cytoplasm triggers the fusion of the acrosomal membrane with the adjacent sperm cell membrane, releasing enzymes that can lyse a path through the egg jelly to the vitelline envelope.
The second part of the acrosome reaction involves the extension of the acrosomal process by the polymerization of globular actin molecules into actin microfilaments ; Tilney et al. 1978). The influx of Ca2+ is thought to activate the protein RhoB in the acrosome and midpiece of the sperm (Castellano et al. 1997; de la Sancha et al. 2007). This GTP-binding protein helps organize the actin cytoskeleton in many types of cells and is thought to be active in polymerizing actin to make the acrosomal process.
Biochemical studies have confirmed that the bindins of closely related sea urchin species have different protein sequences. This finding implies the existence of species-specific bindin receptors on the egg vitelline envelope (FIGURE 1A). Indeed, a 350-kDa glycoprotein that displays the properties expected of a bindin receptor has been isolated from sea urchin eggs (FIGURE 1B; Kamei and Glabe 2003). These bindin receptors are thought to be aggregated into complexes on the vitelline envelope, and hundreds of such complexes may be needed to tether the sperm to the egg. The receptor for sperm bindin on the egg vitelline envelope appears to recognize the protein portion of bindin on the acrosome (FIGURE 1C) in a species-specific manner. Closely related species of sea urchins (i.e., different species in the same genus) have divergent bindin receptors, and eggs will adhere only to the bindin of their own species (FIGURE 1D). Thus, species-specific recognition of sea urchin gametes can occur at the levels of sperm attraction, sperm activation, the acrosome reaction, and sperm adhesion to the egg surface.