Crystal structure of a clip‐domain serine protease and functional roles of the clip domains

Clip‐domain serine proteases (SPs) are the essential components of extracellular signaling cascades in various biological processes, especially in embryonic development and the innate immune responses of invertebrates. They consist of a chymotrypsin‐like SP domain and one or two clip domains at the N‐terminus. Prophenoloxidase‐activating factor (PPAF)‐II, which belongs to the noncatalytic clip‐domain SP family, is indispensable for the generation of the active phenoloxidase leading to melanization, a major defense mechanism of insects. Here, the crystal structure of PPAF‐II reveals that the clip domain adopts a novel fold containing a central cleft, which is distinct from the structures of defensins with a similar arrangement of cysteine residues. Ensuing studies demonstrated that PPAF‐II forms a homo‐oligomer upon cleavage by the upstream protease and that the clip domain of PPAF‐II functions as a module for binding phenoloxidase through the central cleft, while the clip domain of a catalytically active easter‐type SP plays an essential role in the rapid activation of its protease domain.