Identification of important residues in egg envelope protein for species-specific egg envelope digestion. Cleavage activity of purified FLCE (A) and MLCE (B) towards wild-type cleavage site peptides, mid-ZPd, of Fundulus and medaka egg envelope proteins (peptides 1 and 2), and towards their mutant peptides (peptides 3–11). Specific activities are expressed as nmol · 30 min-1 · μg protein-1. The sequences are listed in (C). The amino acid residues mutated to Fundulus-type and medaka-type are shown with gray and black, respectively. The cleavage site is shown with an arrowhead, and the amino acid positions from the cleavage site are indicated at the top of the sequence according to the nomenclature for the interaction of proteases with their substrates .