Protease-Induced Formation of the Sperm Acrosomal Filament

Filament extension during the sperm acrosome reaction in Sicyonia ingentis is triggered by an egg trypsin-like protease whose action can be mimicked using trypsin. Using biotinylated trypsin and either a fluorescently-labeled or colloidal gold-labeled antibody to biotin, trypsin binding was localized to the anterior granule of the sperm which is exposed upon acrosomal exocytosis. The binding was to proteinaceous material at the base of the granule juxtaposed to the inner acrosomal membrane. Other labeled proteins also bound in the same pattern but only in the presence of unlabeled trypsin; non-proteolytic proteins did not induce filament formation. Binding of all proteins tested occurred slowly over a period of about 30 min. A minimum of 30 min of trypsin exposure was required in order to trigger filament formation, and increasing trypsin concentration did not reduce this time requirement. These results indicate that the protease slowly uncovers a binding site for itself (or other proteins), and then its proteolytic activity is again required to induce filament formation. The protease kallikrein appeared to be a more potent inducer than trypsin, while thrombin and clostripain had no apparent inducing activity.