Serine endopeptidase which, by cleaving prophenoloxidase PPO1 and PPO2, is required for the activation of the prophenoloxidase cascade probably following the recognition of pathogen-derived products. Protein stability and endopeptidase activity are calcium dependent (PubMed:17287215). First cleavage on prophenoloxidase PPO1 and PPO2 is not dependent on calcium; however, cleavage of PPO1 and PPO2 to their active forms is dependent on calcium and on the presence of PPAF2 and PPAF3 (PubMed:12185078). Cleavage of PPAF2 is inhibited by calcium (PubMed:11012672, PubMed:12185078). Inhibited by ethylenediaminetetraacetic acid (EDTA), p-nitrophenyl-p'-guanido-benzoate, diisopropylphosphorofluoridate (iPr2PF) and p-(Amidinophenyl)methanesulfonyl fluoride (p-APMSF) (PubMed:9652393, PubMed:17287215). In the active form, heterodimer of a light chain and a heavy chain; disulfide-linked (Probable). Secreted in the hemolymph. Expressed in larva (at protein level). The clip domain consists of 35-55 residues which are 'knitted' together usually by 3 conserved disulfide bonds forming a clip-like compact structure (By similarity). The clip domain is necessary for regulating the zymogen activation (PubMed:16362048). Cleaved following the recognition of pathogen-derived products, probably by a lysyl endopeptidase. Belongs to the peptidase S1 family. CLIP subfamily. It is not clear if the light chain is degraded after cleavage.