title: Mass spectrometry analysis of the oligomeric C1q protein reveals the B chain as the target of trypsin cleavage and interaction with fucoidan.
abstract: C1q is a subunit of the C1 complex that triggers activation of the complement classical pathway through recognition and binding of immune complexes. C1q also binds to nonimmune ligands such as the sulfated polysaccharide fucoidan, a potent anticomplementary agent. C1q was submitted for the first time to mass spectrometry analysis, yielding insights into its assembly and its interaction with fucoidan. The MALDI-TOF mass spectrometry technique on membrane allowed partial preservation of noncovalent interactions, allowing precise analysis of its substructure and estimation of the C1q molecular weight at 459520-461883, with an average mass of 460793 g x mol(-1). The disulfide-linked A-B and C-C dimers as well as the noncovalent structural unit (A-B:C)-(C:B-A) were detected, providing experimental support to the C1q model based on covalent and noncovalent associations of six heterotrimers. Trypsin treatment of native C1q led to proteolysis of the B chain only, at a single cleavage site (Arg(109)) located in the globular region. Unlike DNA, fucoidan protected C1q from trypsin cleavage, indicating that this polysaccharide binds to the B moiety of the globular head. Given the involvement of the C1q globular heads in the recognition of IgG, this interaction may account for the observed anticomplementary activity of fucoidan.
authors.affiliation: Laboratoire de Thérapie Cellulaire, CHU Morvan, 5 Avenue Foch, 29200 Brest, France.
authors.forename: Bérangère
authors.initials: B
authors.lastname: Tissot
authors.forename: Florence
authors.initials: F
authors.lastname: Gonnet
authors.forename: Alban
authors.initials: A
authors.lastname: Iborra
authors.forename: Christian
authors.initials: C
authors.lastname: Berthou
authors.forename: Nicole
authors.initials: N
authors.lastname: Thielens
authors.forename: Gérard J
authors.initials: GJ
authors.lastname: Arlaud
authors.forename: Régis
authors.initials: R
authors.lastname: Daniel
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002240:Carbohydrate Sequence; D002417:Cattle; D015922:Complement C1q / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D006868:Hydrolysis; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000378:metabolism; D011134:Polysaccharides / Q000378:metabolism*; D021122:Protein Subunits / Q000037:antagonists & inhibitors / Q000302:isolation & purification / Q000378:metabolism*; D015842:Serine Proteinase Inhibitors / Q000737:chemistry; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D014357:Trypsin / Q000378:metabolism*
chemical_list: D010446:Peptide Fragments; D011134:Polysaccharides; D021122:Protein Subunits; D015842:Serine Proteinase Inhibitors; D015922:Complement C1q; C007789:fucoidan; D014357:Trypsin