Effector sites in the three-dimensional structure of mammalian sperm beta-acrosin

Background: Proacrosin is a serine protease found specifically within the a crosomal vesicle of all mammalian spermatozoa. During fertilization proacro sin autoactivates to form B-acrosin, in which there is a "light" chain cros s-linked to a "heavy" chain by two disulphide bonds. p-acrosin is thought t o be multifunctional with roles in acrosomal exocytosis, as a receptor for zona pellucida proteins, and as a protease to facilitate penetration of spe rmatozoa into the egg. Results: The crystal structures of both ram and boar beta -acrosins have be en solved in complex with p-aminobenzamidine to 2.1 Angstrom and 2.9 Angstr om resolution, respectively. Both enzymes comprise a heavy chain with struc tural homology to trypsin, and a light chain covalently associated in a sim ilar manner to blood coagulation enzymes. In crystals of boar B-acrosin, th e carboxyl terminus of the heavy chain is inserted into the active site of the neighboring molecule. In both enzyme structures, there are distinctive positively charged surface "patches" close to the active site, which associ ate with carbohydrate from adjacent molecules and also bind sulfate ions. Conclusions: From the three-dimensional structure of p-acrosin, two separat e effector sites are evident. First, proteolytic activity, believed to be i mportant at various stages during fertilization, arises from the trypsin-li ke active site. Activity of this site may be autoregulated through intermol ecular associations. Second, positively charged regions on the surface adja cent to the active site may act as receptors for binding zona pellucida gly coproteins. The spatial proximity of these two effector sites suggests ther e may be synergy between them.