The cortical granule serine protease CGSP1 of the sea urchin, Strongylocentrotus purpuratus, is autocatalytic and contains a low-density lipoprotein receptor-like domain

Abstract

Trypsin-like activity is secreted from eggs of many species at fertilization, and this activity is believed to be critical for the block to polyspermy. Here we show that a cortical granule serine protease of sea urchins is the major and perhaps only protease family member important for fertilization. Zymography assays suggest that the cortical granules contain a single serine protease that can undergo autocatalysis and is secreted upon egg activation. We used this finding to identify a cDNA clone from a Strongylocentrotus purpuratus ovary cDNA library that encodes a 581-amino-acid-residue protein that we refer to as cortical granule serine protease 1 (CGSP1). The catalytic domain of the protein contains the essential residues of the catalytic triad characteristic of a member of the trypsin-like family of serine proteases and the N-terminus of CGSP1 resembles the ligand-binding domain of the low-density lipoprotein (LDL) receptor. Antibodies raised separately to both the protease and LDL receptor-like domains each localize to the cortical granules of unfertilized eggs. Furthermore, the full-length form of CGSP1, as well as intermediate and active forms of the protease, is detected in cortical granules by immunoblot analysis. Our evidence suggests that CGSP1 is activated at fertilization and is responsible for the protease-mediated reactions that follow cortical granule exocytosis and contribute to the block to polyspermy.