Purification and gene cloning of Fundulus heteroclitus hatching enzyme

Two cDNA homologues of medaka hatching enzyme - high choriolytic enzyme (HCE) and low choriolytic enzyme (LCE) - were cloned from Fundulus heteroclitus embryos. Amino acid sequences of the mature forms of Fundulus HCE (FHCE) and LCE (FLCE) were 77.9% and 63.3% identical to those of medaka HCE and LCE, respectively. In addition, phylogenetic analysis clearly showed that FHCE and FLCE belonged to the clades of HCE and LCE, respectively. Exon-intron structures of FHCE and FLCE genes were similar to those of medaka HCE (intronless) and LCE (8-exon-7-intron) genes, respectively. Northern blotting and whole-mount in situ hybridization showed that both genes were concurrently expressed in hatching gland cells. Their spatio-temporal expression pattern was basically similar to that of medaka hatching enzyme genes. We separately purified two isoforms of FHCE, FHCE1 and FHCE2, from hatching liquid through gel filtration and cation exchange column chromatography in the HPLC system. The two isoforms, slightly different in molecular weight and in MCA-peptide-cleaving activity, swelled the inner layer of chorion by their limited proteolysis, like the medaka HCE isoforms. In addition, we identified FLCE by TOF-MS. Similar to the medaka LCE, FLCE hardly digested intact chorion. FHCE and FLCE together, when incubated with chorion, rapidly and completely digested the chorion, suggesting their synergistic effect in chorion digestion. Such a cooperative digestion was confirmed by electron microscopic observation. The results suggest that a hatching enzyme system composed of HCE and LCE is conserved between two different teleosts Fundulus and medaka.