title: Nonproteolytic serine proteinase homologs are involved in prophenoloxidase activation in the tobacco hornworm, Manduca sexta.
abstract: In insects, the prophenoloxidase activation system is a defense mechanism against parasites and pathogens. Recognition of parasites or pathogens by pattern recognition receptors triggers activation of a serine proteinase cascade, leading to activation of prophenoloxidase-activating proteinase (PAP). PAP converts inactive prophenoloxidase (proPO) to active phenoloxidase (PO), which then catalyzes oxidation of phenolic compounds that can polymerize to form melanin. Because quinone intermediates and melanin are toxic to both hosts and pathogens, activation of proPO must be tightly regulated and localized. We report here purification and cDNA cloning of serine proteinase homologs (SPHs) from the tobacco hornworm, Manduca sexta, which interact with PAP-1 in proPO activation. Two SPHs were co-purified from plasma of M. sexta larvae with immulectin-2, a C-type lectin that binds to bacterial lipopolysaccharide. They contain an amino-terminal clip domain connected to a carboxyl-terminal serine proteinase-like domain. PAP-1 alone cannot efficiently activate proPO, but a mixture of SPHs and PAP-1 was much more effective for proPO activation. Immulectin-2, proPO and PAP-1 in hemolymph bound to the immobilized recombinant proteinase-like domain of SPH-1, indicating that a complex containing these proteins may exist in hemolymph. Since immulectin-2 is a pattern recognition receptor that binds to surface carbohydrates on pathogens, such a protein complex may localize activation of proPO on the surface of pathogens. SPH, which binds to immulectin-2, may function as a mediator to recruit proPO and PAP to the site of infection.
authors.affiliation: Department of Cell Biology and Biophysics, School of Biological Sciences, University of Missouri--Kansas City, Kansas City, MO 64110, USA.
authors.forename: Xiao-Qiang
authors.initials: XQ
authors.lastname: Yu
authors.forename: Haobo
authors.initials: H
authors.lastname: Jiang
authors.forename: Yang
authors.initials: Y
authors.lastname: Wang
authors.forename: Michael R
authors.initials: MR
authors.lastname: Kanost
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D004156:Catechol Oxidase / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation; D004792:Enzyme Precursors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D007814:Larva; D018402:Manduca / Q000201:enzymology* / Q000254:growth & development / Q000472:pathogenicity; D008543:Melanins / Q000096:biosynthesis; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D000073718:Pancreatitis-Associated Proteins; D010636:Phenols / Q000378:metabolism; D016133:Polymerase Chain Reaction; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D012697:Serine Endopeptidases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*
chemical_list: D004792:Enzyme Precursors; D008543:Melanins; D000073718:Pancreatitis-Associated Proteins; D010636:Phenols; C000612141:REG3A protein, human; C038451:pro-phenoloxidase; D004156:Catechol Oxidase; D012697:Serine Endopeptidases