Serine endopeptidase which, by cleaving prophenoloxidase activating factor PPAF2, is required for the activation of the prophenoloxidase cascade probably following the recognition of pathogen-derived products. Cleavage of PPAF2 is Ca(2+)-independent (PubMed:12185078). Inhibited by heparin (PubMed:17287215). In the active form, heterodimer of a light chain and a heavy chain; disulfide-linked (Probable). Secreted in the hemolymph. The clip domain consists of 35-55 residues which are 'knitted' together usually by 3 conserved disulfide bonds forming a clip-like compact structure. Proteolytically cleaved. Belongs to the peptidase S1 family. CLIP subfamily.