Inactive serine protease which plays an essential role in the innate immune response against bacteria, fungi and protozoa infection by activating the melanization cascade (PubMed:16922859, PubMed:17537726, PubMed:23166497, PubMed:33045027). In the melanization cascade, acts downstream of TEP1 and SPCLIP1 to promote CLIPA28 and CLIPC9 proteolytic cleavage and CLIPC9 recruitment to microbial surfaces (PubMed:33045027). In the resistant strain L3-5, required for the melanization of killed parasite P.berghei ookinetes which results in their clearance (PubMed:16922859). In the susceptible strain G3, appears to be dispensable for ookinete elimination which occurs by lysis (PubMed:16922859, PubMed:33045027). Required for the melanization of Gram-positive and Gram-negative bacteria (PubMed:17537726). During the late stage of fungus B.bassiana-mediated infection, required for the initiation of hyphae melanization by promoting prophenoloxidase PPO activation (PubMed:23166497). Heterodimer of a light chain and a heavy chain; disulfide-linked. Secreted into the hemolymph. By Gram-positive and Gram-negative bacteria-mediated infection (PubMed:17537726). By parasite P.berghei infection (PubMed:16922859). The clip domain consists of 35-55 residues which are 'knitted' together usually by 3 conserved disulfide bonds forming a clip-like compact structure. Secreted as a full-length protein (PubMed:17537726). Proteolytically cleaved into two chains which remain covalently linked (PubMed:17537726). Cleavage is induced by Gram-positive or Gram-negative bacteria infection (PubMed:17537726). In RNAi-mediated knockdown mosquitos infected with bacteria E.coli or S.aureus, bacteria melanization, prophenoloxidase (PPO) activation, and proteolytic cleavage of CLIPA28 and CLIPC9 are impaired; however, mosquito survival is not affected (PubMed:17537726, PubMed:33045027). Melanization of wound surfaces is normal (PubMed:17537726). Increased sensitivity to fungus B.bassiana (strain 80.2) infection characterized by reduced mosquito survival and increased levels of hyphal body colonies (PubMed:23166497). Impaired melanization of hyphae, but not of germinating conidia or germ tubes, caused by a failure to recruit prophenoloxidase PPO to hyphae (PubMed:23166497). Loss of phenoloxidase (PO) activity triggered during late but not early fungal infection (PubMed:23166497). RNAi-mediated knockdown in the resistant strain L3-5 infected with P.berghei, results in impaired ookinete melanization without increasing the number of live oocysts (PubMed:16922859). RNAi-mediated knockdown in the susceptible strain G3 infected with P.berghei has no effect on the response to parasite infection (PubMed:16922859, PubMed:33045027). Simultaneous RNAi-mediated knockdown of CLIPA8 and CTL4 in the susceptible strain G3 infected with P.berghei, abolishes ookinete melanization caused by RNAi-mediated knockdown of CTL4 (PubMed:16922859, PubMed:33045027). Simultaneous RNAi-mediated knockdown of SRPN2 and CLIPA8, does not rescue the formation of abdominal melanotic tumors caused by SRPN2 RNAi-mediated knockdown (PubMed:17537726). Belongs to the peptidase S1 family. CLIP subfamily. Although it belongs to peptidase S1 family, lacks the conserved Ser residue within the catalytic triad (Asp-His-Ser) which is replaced by a Gly residue, probably resulting in a loss of proteolytic activity.