A unique group of serine proteinases has been identified in several arthropod species. These enzymes contain a carboxyl-terminal proteinase domain typical of the chymotrypsin family and have at their amino-terminal end a "clip" domain that is 35-55 amino acid residues long. The clip domain is connected to the proteinase domain by a linker region of 23-92 residues, which is variable in sequence. Clip domains have six strictly conserved cysteine residues, which form three pairs of disulfide bond. An additional cysteine residue in the linker region forms a disulfide bond with a cysteine in the proteinase domain, linking the two domains such that after proteolytic activation of the proteinase zymogen, the two domains remain covalently attached.
Clip domain proteinases were first found as enyzmes of the hemolymph coagulation pathway in horseshoe crabs and now are also known from several insect species, including proteinases which activate prophenol oxidase.
Sequences of proteinases in the clip domain family
Medline abstracts of papers related to clip domains
Sequence alignment of clip-domain proteinases