Biosynthesis and secretion of the hatching enzyme during sea urchin embryogenesis

The hatching enzyme secreted by the blastula stage sea urchin embryo proteolyzes the fertilization envelope, thereby allowing the embryo to hatch. Using an assay that measures fertilization envelope degradation, we have purified the hatching enzyme by ion-exchange and affinity chromatography from the hatching medium of Strongylocentrotus purpuratus embryos. The hatching enzyme was found to be a 33-kDa metalloprotease that exhibited a substrate preference for only a minor subset of proteins in the fertilization envelope. Secretion of the hatching enzyme by blastula stage embryos occurred during the 2-h period prior to hatching. The hatching enzyme was initially secreted as a 57-kDa protein, but during purification it was converted to the 33-kDa form. Biosynthesis of the hatching enzyme began at the late morula/early blastula stage up to 6 h before secretion. Experiments using the ionophore monensin suggest that the lag between synthesis and secretion of the hatching enzyme by the blastula stage embryos may be a result of a slow constitutive secretory process.