Binds and activates processed prophenoloxidases PPO1 and PPO2 and thus is involved in the activation of the prophenoloxidase cascade probably following the recognition of pathogen-derived products. Heterodimer of a light chain and a heavy chain; disulfide-linked (Probable). Can oligomerize into a dodecamer (PubMed:11012672, PubMed:16362048. PubMed:9652393). Interacts (via CLIP domain) with cleaved PPO1 and/or PPO2; the interaction results in PPO1 and/or PPO2 activation (PubMed:16362048). Secreted in the hemolymph. Hemocytes. Expressed in larva (at protein level). Up-regulated in response to Gram-negative bacterial infections. 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 interacts with cleaved PPO1 and/or PPO2; the interaction results in PPO1 and/or PPO2 activation (PubMed:16362048). The N-terminus is blocked (PubMed:11012672). Proteolytically cleaved by PPAF3 (PubMed:11012672, PubMed:12185078). Cleavage produces a light chain and a heavy chain, which probably remain covalently associated through an interchain disulfide bond (Probable). Stabilized by calcium. Belongs to the peptidase S1 family. CLIP subfamily. Lacks the conserved Ser residue within the catalytic triad which is replaced by a Gly residue, probably resulting in a loss of proteolytic activity. It is not clear if the light chain is degraded after cleavage.