Probable serine protease which plays an essential role in the innate immune response against bacteria and protozoa infection by activating the melanization cascade (PubMed:33045027). In the susceptible strain G3, appears to be dispensable for ookinete elimination which occurs by lysis (PubMed:33045027). In the active form, heterodimer of a p12 subunit and a p30 subunit; disulfide-linked. Secreted into the hemolymph (PubMed:33045027). Recruited by CLIPA8 to microbial surfaces where it is cleaved into the two chain active form (PubMed:33045027). 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:12364791). Following bacterium E.coli infection, proteolytically cleaved into two chains, p12 and p30, which remain covalently linked (PubMed:12364791). RNAi-mediated knockdown causes a loss in phenoloxidase (PO) activity and a reduction in microbe melanization in response to E.coli infection (PubMed:33045027). RNAi-mediated knockdown in the susceptible strain G3 infected with P.berghei, does not affect the number of oocysts and ookinete melanization; however, severely reduces ookinete melanization caused by CTL4 RNAi-mediated knockdown (PubMed:33045027). Belongs to the peptidase S1 family. CLIP subfamily.