Crystal structure of the serine protease domain of prophenoloxidase activating factor-I.
A family of serine proteases (SPs) mediates the proteolytic cascades of embryonic development and immune response in invertebrates. These proteases, called easter-type SPs, consist of clip and chymotrypsin-like SP domains. The SP domain of easter-type proteases differs from those of typical SPs in its primary structure. Herein, we report the first crystal structure of the SP domain of easter-type proteases, presented as that of prophenoloxidase activating factor (PPAF)-I in zymogen form. This structure reveals several important structural features including a bound calcium ion, an additional loop with a unique disulfide linkage, a canyon-like deep active site, and an exposed activation loop. We subsequently show the role of the bound calcium and the proteolytic susceptibility of the activation loop, which occurs in a clip domain-independent manner. Based on biochemical study in the presence of heparin, we suggest that PPAF-III, highly homologous to PPAF-I, contains a surface patch that is responsible for enhancing the catalytic activity through interaction with a nonsubstrate region of a target protein. These results provide insights into an activation mechanism of easter-type proteases in proteolytic cascades, in comparison with the well studied blood coagulation enzymes in mammals.
Piao
Shunfu
S
College of Pharmacy and Research Institute for Drug Development, Pusan National University, Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea.
Kim
Seulki
S
Kim
Jung Hyun
JH
Park
Ji Won
JW
Lee
Bok Leul
BL
Ha
Nam-Chul
NC
D000818:Animals; D001665:Binding Sites / Q000502:physiology; D002118:Calcium / Q000737:chemistry / Q000276:immunology / Q000378:metabolism; D004156:Catechol Oxidase / Q000737:chemistry / Q000276:immunology / Q000378:metabolism; D001517:Coleoptera / Q000196:embryology / Q000201:enzymology* / Q000276:immunology; D004789:Enzyme Activation / Q000502:physiology; D004792:Enzyme Precursors / Q000737:chemistry / Q000276:immunology / Q000378:metabolism; D019476:Insect Proteins / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D012697:Serine Endopeptidases / Q000737:chemistry* / Q000276:immunology / Q000378:metabolism; D040681:Structural Homology, Protein
D004792:Enzyme Precursors; D019476:Insect Proteins; C038451:pro-phenoloxidase; D004156:Catechol Oxidase; D012697:Serine Endopeptidases; D002118:Calcium