This domain is found in Prophenoloxidase-activating factor (PPAF)-II present in the beetle Holotrichia diomphalia. PPAF-II is indispensable for the generation of the active phenoloxidase leading to melanization, a major defense mechanism of insects. This domain is the clip domain and it is thought to tightly associate with regions I-III of the serine protease-like (SPL) domain. The clip domain is a protein-interaction module that plays an essential role in the binding and activation of PO76s via its central cleft [1].

1: Crystal structure of a clip-domain serine protease and functional roles of the clip domains. Piao S, Song YL, Kim JH, Park SY, Park JW, Lee BL, Oh BH, Ha NC; EMBO J. 2005;24:4404-4414. PMID:16362048