The Questions of Developmental Biology

Once the sea urchin sperm has penetrated the egg jelly, the acrosomal process of the
sperm contacts the surface of the egg (Figure 7.14A). A major species-specific recognition step
occurs at this point. The acrosomal protein mediating this recognition is called bindin. In 1977,
Vacquier and co-workers isolated this nonsoluble 30,500-Da protein from the acrosome of
Strongylocentrotus purpuratus and found it to be capable of binding to dejellied eggs of the same
species (Figure 7.14B; Vacquier and Moy 1977). Further, its interaction with eggs is relatively
species-specific (Glabe and Vacquier 1977; Glabe and Lennarz 1979): bindin isolated from the
acrosomes of S. purpuratus
binds to its own dejellied eggs,
but not to those of Arbacia
punctulata.
Using immunological
techniques, Moy and Vacquier
(1979) demonstrated that
bindin is located specifically
on the acrosomal process
exactly where it should be for
sperm-egg recognition (Figure
7.15).
Biochemical studies have shown that the bindins of closely related sea urchin species are indeed
different. This finding implies the existence of species-specific bindin receptors on the egg,
vitelline envelope, or plasma membrane. Such receptors were also suggested by the experiments
of Vacquier and Payne (1973), who saturated sea urchin eggs with sperm. As seen in Figure
7.16A, sperm binding does not occur over the entire egg surface. Even at saturating numbers of
sperm (approximately 1500), there appears to be room on the ovum for more sperm heads,
implying a limiting number of sperm-binding sites. The bindin receptor on the egg has recently
. been isolated (Giusti et al. 1997; Stears and Lennarz 1997).
This 350-kDa protein may have several regions that .
. interact with bindin. At least one of these sites .
…
recognizes only the bindin of the same species. The
.. other site or sites appear to recognize a . . ..
N general bindin structure and can recognize .
M
the bindin of many species. The bindin
B
receptors are thought to be aggregated into complexes on the egg
M
cell surface, and hundreds of these complexes may be needed to
M t ether the sperm to the egg (Figure 7.16B).
Thus, species-specific recognition of sea urchin gametes occurs at the levels of sperm
attraction, sperm activation, and sperm adhesion to the egg surface.