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Background  Serine protease inhibitors act as modulators of serine proteases, playing important roles in protecting animal toxin peptides from degradation. However, all known serine protease inhibitors discovered thus far from animal venom belong to the  Kunitz -type subfamily, and whether there are other novel types of protease inhibitors in animal venom remains unclear.    Principal Findings  Here, by screening scorpion venom gland cDNA libraries, we identified the first  Ascaris -type animal toxin family, which contains four members:  Scorpiops jendeki Ascaris -type protease inhibitor (SjAPI),  Scorpiops jendeki Ascaris -type protease inhibitor 2 (SjAPI-2),  Chaerilus tricostatus Ascaris-type protease inhibitor  (CtAPI), and  Buthus martensii Ascaris-type protease inhibitor  (BmAPI). The detailed characterization of  Ascaris -type peptide SjAPI from the venom gland of scorpion  Scorpiops jendeki  was carried out. The mature peptide of SjAPI contains 64 residues and possesses a classical  Ascaris -type cysteine framework reticulated by five disulfide bridges, different from all known protease inhibitors from venomous animals. Enzyme and inhibitor reaction kinetics experiments showed that recombinant SjAPI was a dual function peptide with α-chymotrypsin- and elastase-inhibiting properties. Recombinant SjAPI inhibited α-chymotrypsin with a Ki of 97.1 nM and elastase with a Ki of 3.7 μM, respectively. Bioinformatics analyses and chimera experiments indicated that SjAPI contained the unique short side chain functional residues “AAV” and might be a useful template to produce new serine protease inhibitors.    Conclusions/Significance  To our knowledge, SjAPI is the first functionally characterized animal toxin peptide with an  Ascaris -type fold. The structural and functional diversity of animal toxins with protease-inhibiting properties suggested that bioactive peptides from animal venom glands might be a new source of protease inhibitors, which will accelerate the development of diagnostic and therapeutic agents for human diseases that target diverse proteases.

SjAPI, the First Functionally Characterized Ascaris-Type Protease Inhibitor from Animal Venoms