title: Novel subunit in C4b-binding protein required for protein S binding.
abstract: C4b-binding protein (C4BP) is a multimeric protein with regulatory functions in the complement system. It also interacts with vitamin K-dependent protein S, which is involved in the regulation of the coagulation system. It has been demonstrated that C4BP consists of seven disulfide-linked, identical 70-kDa subunits, which are arranged to give the molecule a spider-like structure. We now have evidence for the presence of a new subunit in C4BP. On sodium dodecyl sulfate-poly-acrylamide gel electrophoresis it appears as a weakly stainable band with a molecular weight of approximately 45,000. The subunit was isolated by gel filtration in 6 M guanidine hydrochloride of reduced and carboxymethylated C4BP. Its amino-terminal sequence is distinct from previously known protein sequences. The stoichiometry of 45- to 70-kDa subunits was estimated to be 1:9, indicating the presence of one 45-kDa subunit per C4BP molecule. The new subunit was demonstrated to be a disulfide-linked component of the central core of C4BP. It was sensitive to proteolysis by chymotrypsin, and when cleaved the protein S binding ability of C4BP was lost. With protein S bound to C4BP, the 45-kDa subunit was protected from degradation by chymotrypsin, and the protein S binding site remained intact. These data suggest that the new subunit is directly involved in protein S binding.
authors.affiliation: Department of Clinical Chemistry, University of Lund, Malmö General Hospital, Sweden.
authors.forename: A
authors.initials: A
authors.lastname: Hillarp
authors.forename: B
authors.initials: B
authors.lastname: Dahlbäck
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D002352:Carrier Proteins / Q000302:isolation & purification / Q000378:metabolism*; D002850:Chromatography, Gel; D002918:Chymotrypsin / Q000378:metabolism; D003181:Complement C4; D003169:Complement Inactivator Proteins; D004220:Disulfides / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D006023:Glycoproteins / Q000378:metabolism*; D019791:Guanidine; D006146:Guanidines; D006801:Humans; D007118:Immunoassay; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010446:Peptide Fragments / Q000302:isolation & purification; D017293:Protein S; D011951:Receptors, Complement
chemical_list: D002352:Carrier Proteins; D003181:Complement C4; D003169:Complement Inactivator Proteins; D004220:Disulfides; D006023:Glycoproteins; D006146:Guanidines; D046911:Macromolecular Substances; D010446:Peptide Fragments; D017293:Protein S; D011951:Receptors, Complement; D002918:Chymotrypsin; D019791:Guanidine