[Proacrosin and acrosin--retrospect of a multifunctional complex]

Proacrosin is a molecule that is located under the acrosomal cap and that acts as a secondary ligand after acrosome reaction. Proacrosin is thought to bind to ZP2 (zona pellucida receptor 2). The high affinity is based on the interaction of the zona pellucida glycoprotein's acidic sulphate groups with basic, positively charged amino acids of the proacrosin molecule. Proacrosin is a serine protease and is capable to lyse the zona pellucida locally. Our structural analysis of proacrosin shows, that the catalytic centre is surrounded by loops containing positively charged amino acids which create the binding domain. This review is intended to describe the results that lead to the elucidation of the biochemistry of proacrosin.