title: C4b-binding protein protects coagulation factor Va from inactivation by activated protein C.
abstract: We investigated the effect of C4BP on APC-mediated inactivation of factor Va (FVa) in the absence and presence of protein S. FVa inactivation was biphasic (k(506) = 4.4 x 10(8) M(-)(1) s(-)(1), k(306) = 2.7 x 10(7) M(-)(1) s(-)(1)), and protein S accelerated Arg(306) cleavage approximately 10-fold. Preincubation of protein S with C4BP resulted in a total abrogation of protein S cofactor activity. C4BP also protected FVa from inactivation by APC in the absence of protein S. Control experiments with CLB-PS13, a monoclonal anti-protein S antibody, indicated that inhibition of FVa inactivation by C4BP was not mediated through contaminating traces of protein S in our reaction systems. Protection of FVa was prevented by a monoclonal antibody directed against the C4BP alpha-chain. Recombinant rC4BPalpha comprised of only alpha-chains also protected FVa, but in the presence of protein S, the level of protection was decreased, since rC4BPalpha lacks the beta-chain responsible for C4BP binding to protein S. A truncated C4BP beta-chain (SCR-1+2) inhibited protein S cofactor activity, but had no effect on FVa inactivation by APC in the absence of protein S. In conclusion, C4BP protects FVa from APC-catalyzed cleavage in a protein S-independent way through direct interactions of the alpha-chaims of C4BP with FVa and/or APC.
authors.affiliation: Thrombosis and Haemostasis Laboratory, Department of Haematology, University Medical Center, and Institute of Biomembranes, Utrecht University, Utrecht, The Netherlands.
authors.forename: R H
authors.initials: RH
authors.lastname: van de Poel
authors.forename: J C
authors.initials: JC
authors.lastname: Meijers
authors.forename: J
authors.initials: J
authors.lastname: Rosing
authors.forename: G
authors.initials: G
authors.lastname: Tans
authors.forename: B N
authors.initials: BN
authors.lastname: Bouma
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal / Q000378:metabolism; D001120:Arginine / Q000235:genetics; D001666:Binding Sites, Antibody / Q000235:genetics; D001667:Binding, Competitive / Q000235:genetics / Q000276:immunology; D002460:Cell Line; D015935:Complement C4b / Q000378:metabolism*; D003169:Complement Inactivator Proteins; D006224:Cricetinae; D004789:Enzyme Activation; D015943:Factor Va / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000378:metabolism*; D005973:Glutamine / Q000235:genetics; D006023:Glycoproteins; D006801:Humans; D007668:Kidney; D011485:Protein Binding / Q000235:genetics; D011486:Protein C / Q000037:antagonists & inhibitors / Q000378:metabolism / Q000502:physiology*; D017293:Protein S / Q000502:physiology; D011951:Receptors, Complement / Q000235:genetics / Q000276:immunology / Q000378:metabolism / Q000502:physiology*; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000276:immunology / Q000302:isolation & purification / Q000378:metabolism; D017384:Sequence Deletion
chemical_list: D000911:Antibodies, Monoclonal; D003169:Complement Inactivator Proteins; D006023:Glycoproteins; D011486:Protein C; D017293:Protein S; D011951:Receptors, Complement; D011994:Recombinant Proteins; D005973:Glutamine; D015943:Factor Va; D015935:Complement C4b; D001120:Arginine