Role of acrosomal matrix proteases in sperm-zona pellucida interactions

The roles of acrosomal matrix proteases in sperm-zona pellucida interactions have long been one of the most puzzling questions in mammalian fertilization. Of the acrosomal proteases identified, acrosin exhibits two distinct activities: (i) an enzymatic activity as a trypsin-like serine protease; and (ii) a lectin-like carbohydrate-binding activity. Thus, acrosin has been postulated to function both in the limited proteolysis of the zona pellucida, and in the maintenance of binding of acrosome-reacted sperm to the zona. However, analysis of acrosin-deficient mouse sperm demonstrates that this protein is not essential for sperm-zona pellucida interactions, including the sperm penetration of the zona pellucida. Acrosin is presumably involved in the limited proteolysis and/or processing of other proteins in the acrosome and on the membranes during the acrosome reaction at least in the mouse, although the serine protease system in mouse sperm is very different from those in sperm of other rodent species. It is concluded that membranous proteases rather than acrosomal proteases may play an important role(s) in the sperm-zona pellucida interactions.