Boar sperm
acrosin is an acrosomal
protease with
trypsin-like specificity, and it functions in fertilization by assisting sperm passage through the zona pellucida by limited hydrolysis of this extracellular matrix. In addition to a proteolytic active site domain,
acrosin binds the zona pellucida at a separate binding domain that is lost during
proacrosin autolysis. In this study, we quantitate the binding of
proacrosin to the physiological substrate for
acrosin, the zona pellucida, and to a non-substrate, the polysulfated
polysaccharide fucoidan. Binding was analogous to sea urchin sperm bindin that binds egg jelly fucan and the vitelline envelope of sea urchin eggs.
Proacrosin was found to bind to
fucoidan and to the zona pellucida with binding affinities similar to bindin interaction with egg jelly fucan. These interactions were competitively inhibited by similar relative molecular mass polysulfated
polymers. Since bindin and
proacrosin have distinctly different amino acid sequences, their interaction with acidic
sulfate esters demonstrates an example of convergent evolution wherein different macromolecules localized in analogous sperm compartments have the same
biological function. From
cDNA sequence analysis of
proacrosin, this binding may be mediated through a consensus sequence for binding sulfated
glycoconjugates.
Proacrosin binding to the zona pellucida may serve as both a recognition or primary
sperm receptor, as well as maintaining the sperm on the zona pellucida once the acrosome reaction has occurred.