title: Factor C acts as a lipopolysaccharide-responsive C3 convertase in horseshoe crab complement activation.
abstract: The complement system in vertebrates plays an important role in host defense against and clearance of invading microbes, in which complement component C3 plays an essential role in the opsonization of pathogens, whereas the molecular mechanism underlying C3 activation in invertebrates remains unknown. In an effort to understand the molecular activation mechanism of invertebrate C3, we isolated and characterized an ortholog of C3 (designated TtC3) from the horseshoe crab Tachypleus tridentatus. Flow cytometric analysis using an Ab against TtC3 revealed that the horseshoe crab complement system opsonizes both Gram-negative and Gram-positive bacteria. Evaluation of the ability of various pathogen-associated molecular patterns to promote the proteolytic conversion of TtC3 to TtC3b in hemocyanin-depleted plasma indicated that LPS, but not zymosan, peptidoglycan, or laminarin, strongly induces this conversion, highlighting the selective response of the complement system to LPS stimulation. Although originally characterized as an LPS-sensitive initiator of hemolymph coagulation stored within hemocytes, we identified factor C in hemolymph plasma. An anti-factor C Ab inhibited various LPS-induced phenomena, including plasma amidase activity, the proteolytic activation of TtC3, and the deposition of TtC3b on the surface of Gram-negative bacteria. Moreover, activated factor C present on the surface of Gram-negative bacteria directly catalyzed the proteolytic conversion of the purified TtC3, thereby promoting TtC3b deposition. We conclude that factor C acts as an LPS-responsive C3 convertase on the surface of invading Gram-negative bacteria in the initial phase of horseshoe crab complement activation.
authors.affiliation: Department of Biology, Faculty of Sciences, Kyushu University, Fukuoka, Japan.
authors.forename: Shigeru
authors.initials: S
authors.lastname: Ariki
authors.forename: Shusaku
authors.initials: S
authors.lastname: Takahara
authors.forename: Toshio
authors.initials: T
authors.lastname: Shibata
authors.forename: Takaaki
authors.initials: T
authors.lastname: Fukuoka
authors.forename: Aya
authors.initials: A
authors.lastname: Ozaki
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Endo
authors.forename: Teizo
authors.initials: T
authors.lastname: Fujita
authors.forename: Takumi
authors.initials: T
authors.lastname: Koshiba
authors.forename: Shun-ichiro
authors.initials: S
authors.lastname: Kawabata
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D060829:Arthropod Proteins; D003167:Complement Activation / Q000187:drug effects* / Q000235:genetics / Q000276:immunology; D050577:Complement C3-C5 Convertases / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D004792:Enzyme Precursors / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D006090:Gram-Negative Bacteria / Q000276:immunology; D006094:Gram-Positive Bacteria / Q000276:immunology; D006434:Hemocytes / Q000201:enzymology / Q000276:immunology; D006458:Hemolymph / Q000201:enzymology / Q000276:immunology; D006737:Horseshoe Crabs / Q000201:enzymology / Q000235:genetics / Q000276:immunology*; D008070:Lipopolysaccharides / Q000494:pharmacology*; D008969:Molecular Sequence Data; D012697:Serine Endopeptidases / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D013379:Substrate Specificity / Q000235:genetics / Q000276:immunology
chemical_list: D060829:Arthropod Proteins; D004792:Enzyme Precursors; D008070:Lipopolysaccharides; D050577:Complement C3-C5 Convertases; D012697:Serine Endopeptidases; C057151:Limulus clotting factor C