The region Ser333-Arg356 of the alpha-chain of human C4b-binding protein is involved in the binding of complement C4b

Stringent regulation of complement lectin pathway C3/C5 convertase by C4b-binding protein (C4BP)

The complement lectin pathway, an essential component of the innate immune system, is geared for rapid recognition of infections as each C4b deposited via this pathway is capable of forming a C3/C5 convertase. In the present study, role of C4b-binding protein (C4BP) in regulating the lectin pathway C3/C5 convertase assembled on zymosan and sheep erythrocytes coated with mannan (E(Man)) was examined. While the C4BP concentration for inhibiting 50% (IC(50)) formation of surface-bound C3 convertase on the two surfaces was similar to that obtained for the soluble C3 convertase (1.05nM), approximately 3- and 41-fold more was required to inhibit assembly of the C5 convertase on zymosan (2.81nM) and E(Man) (42.66nM). No difference in binding interactions between C4BP and surface-bound C4b alone or in complex with C3b was observed. Increasing the C4b density on zymosan (14,000-431,000 C4b/Zym) increased the number of C4b bound per C4BP from 2.87 to 8.23 indicating that at high C4b density all seven alpha-chains of C4BP are engaged in C4b-binding. In contrast, the number of C4b bound per C4BP remained constant (3.79+/-0.60) when the C4b density on E(Man) was increased. The data also show that C4BP regulates assembly and decay of the lectin pathway C3/C5 convertase more stringently than the classical pathway C3/C5 convertase because of a approximately 7- to 13-fold greater affinity for C4b deposited via the lectin pathway than the classical pathway. C4BP thus regulates efficiently the four times greater potential of the lectin pathway than the classical pathway in generating the C3/C5 convertase and hence production of pro-inflammatory products, which are required to fight infections but occasionally cause pathological inflammatory reactions.

Structural Requirements for the Complement Regulatory Activities of C4BP

C4b-binding protein (C4BP) is a regulator of the classical complement pathway C3 convertase (C4bC2a complex). It is a disulfide-linked polymer of seven alpha-chains and a unique beta-chain; the alpha- and beta-chains are composed of eight and three complement control protein (CCP) domains, respectively. To elucidate the importance of the polymeric nature of C4BP and the structural requirements for the interaction between C4b and the alpha-chain, 19 recombinant C4BP variants were created. Six truncated monomeric variants, nine polymeric variants in which individual CCPs were deleted, and finally, four variants in which double alanine residues were introduced between CCPs were functionally characterized. The smallest truncated C4BP variant still active in regulating fluid phase C4b comprised CCP1-3. The monomeric variants were less efficient than polymeric C4BP in degrading C4b on cell surfaces. All three N-terminal CCP domains contributed to the binding of C4b and were important for full functional activity; CCP2 and CCP3 were the most important. The spatial arrangements of the first CCPs were found to be important, as introduction of alanine residues between CCPs 1 and 2, CCPs 2 and 3, and CCPs 3 and 4 resulted in functional impairment. The results presented here elucidate the structural requirements of individual CCPs of C4BP, as well as their spatial arrangements within and between subunits for expression of full functional activity.

A cluster of positively charged amino acids in the C4BP alpha-chain is crucial for C4b binding and factor I cofactor function

C4b-binding protein (C4BP) is a regulator of the classical complement pathway, acting as a cofactor to factor I in the degradation of C4b. Computer modeling and structural analysis predicted a cluster of positively charged amino acids at the interface between complement control protein modules 1 and 2 of the C4BP alpha-chain to be involved in C4b binding. Three C4BP mutants, R39Q, R64Q/R66Q, and R39Q/R64Q/R66Q, were expressed and assayed for their ability to bind C4b and to function as factor I cofactors. The apparent affinities of R39Q, R64Q/R66Q, and R39Q/R64Q/R66Q for immobilized C4b were 15-, 50-, and 140-fold lower, respectively, than that of recombinant wild type C4BP. The C4b binding site demonstrated herein was also found to be a specific heparin binding site. In C4b degradation, the mutants demonstrated decreased ability to serve as factor I cofactors. In particular, the R39Q/R64Q/R66Q mutant was inefficient as cofactor for cleavage of the Arg937-Thr938 peptide bond in C4b. In contrast, the factor I mediated cleavage of Arg1317-Asn1318 bond was less affected by the C4BP mutations. In conclusion, we identify a cluster of amino acids that is part of a C4b binding site involved in the regulation of the complement system.

The complement regulator C4b-binding protein analyzed by molecular modeling, bioinformatics and computer-aided experimental design

Molecular modeling and bioinformatics have gained recognition as scientific disciplines of importance in the field of biomedical research. Molecular modeling not only allows to predict the three-dimensional structure of a protein but also helps to define its function. Careful incorporation of the experimental findings in the structural/theoretical data provides means to understand molecular mechanisms for highly complex biological systems. C4b-binding protein (C4BP) is composed of one beta-chain and seven alpha-chains essentially built from three- and eight-complement control protein (CCP) modules, respectively, followed by a non-repeat carboxy-terminal region involved in polymerization of the chains. C4BP is involved in the regulation of the complement system and interacts with many molecules such as C4b, Arp, protein S and heparin. Here, we report experimental and computer data obtained for C4BP. Protein modeling together with site directed mutagenesis indicate that R39, R64 and R66 from the C4BP alpha-chain form a key binding site for heparin, suggesting that this region could be of major importance for interaction with C4b. We also propose that the first CCP of the C4BP beta-chain displays a key hydrophobic surface of major importance for the interaction with the coagulation cofactor protein S.

Molecular remodeling of complement regulatory proteins for xenotransplantation

In pig-to-human discordant xenotransplantation, human complement is a major barrier against long survival of xenografts. Human complement regulatory proteins expressed on xenografts have been adapted as safeguards against host-induced hyperacute rejection of xenografts. For successful xenotransplantation, there have been many attempts to generate molecules with potent human complement regulatory activity but without activities related to harmful functions such as infection, immunosuppression and signal transduction devastating cellular homeostasis. Here, we summarize the strategy by which molecules for xenotransplantation should be designed and propose a GPI-anchored form of monomeric human C4bp as a candidate for efficient protection of swine xenografts from human complement attack.

Structural investigation of C4b-binding protein by molecular modeling: localization of putative binding sites

C4b-binding protein (C4BP) contributes to the regulation of the classical pathway of the complement system and plays an important role in blood coagulation. The main human C4BP isoform is composed of one beta-chain and seven alpha-chains essentially built from three and eight complement control protein (CCP) modules, respectively, followed by a nonrepeat carboxy-terminal region involved in polymerization of the chains. C4BP is known to interact with heparin, C4b, complement factor I, serum amyloid P component, streptococcal Arp and Sir proteins, and factor VIII/VIIIa via its alpha-chains and with protein S through its beta-chain. The principal aim of the present study was to localize regions of C4BP involved in the interaction with C4b, Arp, and heparin. For this purpose, a computer model of the 8 CCP modules of C4BP alpha-chain was constructed, taking into account data from previous electron microscopy (EM) studies. This structure was investigated in the context of known and/or new experimental data. Analysis of the alpha-chain model, together with monoclonal antibody studies and heparin binding experiments, suggests that a patch of positively charged residues, at the interface between the first and second CCP modules, plays an important role in the interaction between C4BP and C4b/Arp/Sir/heparin. Putative binding sites, secondary-structure prediction for the central core, and an overall reevaluation of the size of the C4BP molecule are also presented. An understanding of these intermolecular interactions should contribute to the rational design of potential therapeutic agents aiming at interfering specifically some of these protein-protein interactions.

A monomeric human C4b-binding protein (C4bp) more efficiently inactivates C3b than natural C4bp: participation of C-terminal domains in factor I-cofactor activity

We designed a cDNA construct encoding an artificial membrane molecule consisting of all 8 short consensus repeats (SCRs) of human monomeric C4b-binding protein (C4bp) followed by DAF's GPI anchor, named mC4bp, and expressed the protein on swine endothelial cells (SEC). At the same level of expression, mC4bp protected host cells as effectively as DAF, the most potent complement (C) regulator on the membrane. This result was unexpected from the reported functional properties of natural multimeric C4bp. Here, we investigated the mechanism whereby mC4bp has potent cell-protective activity. Our results were as follows: (1) mC4bp serves more efficiently as a methylamine-treated C3 (C3ma)-inactivating factor I-cofactor than natural C4bp and as efficiently as MCP as a methylamine-treated (C4ma)-inactivating cofactor by fluid-phase cofactor assay: (2) the potency of C3ma inactivation by mC4bp and factor I is quite high compared to those of other cofactors: (3)blocking studies using mAbs against C4bp suggested that both the 48 kDa N-terminal fragment and the C-terminal domain near the portion responsible for bundle formation participate in the high C3ma-inactivating capacity of mC4bp. Thus, acquiring high C3ma-inactivating capacity secondary to monomeric alteration leads to high C regulatory activity of mC4bp. These results infer that mC4bp differs from C4bp in its potent factor I-cofactor activity and is a good candidate as a safeguard against hyperacute rejection of xenografts.

Regulation of complement-mediated swine endothelial cell lysis by a surface-bound form of human C4b binding protein

BACKGROUND

Human C4b-binding protein (C4bp) functions as a cofactor for factor I in the degradation of C4b and C3b and, in addition, accelerates the rate of decay of the C4b2a complex.

METHODS

In this study, we constructed a surface-bound form of human C4b-binding protein (C4bp-PI) consisting of a short consensus repeat 1-8 of the alpha-chain of C4bp and a glycosyl phosphatidylinositol (GPI) of the decay-accelerating factor (CD55) and established stable swine endothelial cell (SEC) lines expressing C4bp-PI by transfection of cDNA. Amelioration of complement-mediated lysis by the transfectant molecules was tested as an in vitro hyperacute rejection model of swine to human discordant xenograft, using the lactate dehydrogenase assay.

RESULTS

Flow cytometric profiles of the stable SEC lines with C4bp-PI showed a high level of expression of this molecule. The cell lysate of the SEC line with C4bp-PI showed strong cofactor activity in not only C4b but also C3b, whereas the activity of plasma C4bp to bind to C3 was very weak. Approximately 150 x 10(4) molecules of C4bp-PI per SEC blocked human complement-mediated cell lysis by approximately 75%.

CONCLUSIONS

The results suggest that the surface-bound form of C4bp will be very useful in clinical xenotransplantation.

The amino-terminal module of the C4b-binding protein alpha-chain is crucial for C4b binding and factor I-cofactor function

C4b-binding protein (C4BP) regulates the classical pathway C3-convertase of the complement system. Human C4BP is composed of seven identical subunits (alpha-chains) and one unique one (beta-chain). Both types of chains contain homologous repeats called complement control proteins (CCPs); the alpha-chain contains eight CCPs and the beta-chain three. Each alpha-chain contains a binding site for C4b although the detailed localization of this binding site is not known. We have used three different chimeric proteins, originally designed to localize the protein S-binding site on C4BP, to demonstrate the importance of the amino-terminal part of the alpha-chain for the complement-regulatory functions of C4BP. These recombinant proteins were composed of C4BP alpha-chains with one, two or three of the amino-terminal CCPs replaced by corresponding CCPs from the C4BP beta-chain. Furthermore, seven different monoclonal antibodies were raised against C4BP and characterized using the recombinant chimeric proteins. Whereas all three recombinant chimeras bind protein S with the same affinity as plasma-purified C4BP, none of them bound to C4b. Three of the antibodies, which were found to bind to alpha-chain CCP 1 and CCP 2, completely inhibited the binding of plasma-purified C4BP to immobilized C4b. In addition, two of these antibodies totally blocked the factor I-cofactor activity of C4BP in a C4b-degradation assay. The binding site for one of the monoclonal antibodies was also studied using electron microscopy where it was confirmed that this antibody bound to the amino-terminal tip of the alpha-chain. These results show that the amino-terminal CCP of the C4BP alpha-chain (CCP 1) is crucial for the C4b binding and factor I-cofactor activity.

Serum amyloid P component binding to C4b-binding protein

Human C4b-binding protein (C4BP), which is a regulator of the classical complement pathway C3 convertase, forms high affinity complexes with anticoagulant protein S and with the pentraxin serum amyloid P component (SAP). SAP is a plasma protein present in all amyloid deposits. Recently, SAP was shown to inhibit the complement regulatory functions of C4BP. In this investigation, we have studied the structural requirements for the C4BP-SAP interaction. C4BP was subjected to chymotrypsin digestion, which yielded two major fragments corresponding to the central core (160 kDa) and to the cleaved-off tentacles (48 kDa). SAP-Sepharose specifically bound the 160-kDa fragment, suggesting that the central core of C4BP contains the binding site for SAP. In a quantitative affinity chromatography assay, the dissociation constants for binding of intact C4BP and of the 160-kDa central core fragment to SAP were found to be 30 and 70 nM, respectively. Recombinant C4BP composed of only alpha-chains bound SAP with similar affinity (Kd = 22 nM), whereas nonglycosylated recombinant alpha-chain C4BP (synthesized in the presence of tunicamycin) bound SAP with lower affinity (Kd = 126 nM). This suggests that the carbohydrate moiety of the central core of C4BP is important for binding of C4BP to SAP in contrast to the C4BP beta-chain, which is not required. EDTA, heparin, and phosphorylethanolamine as well as a peptide comprising amino acids 27-39 of SAP were found to completely displace C4BP from the SAP matrix. Moreover, the immobilized SAP peptide bound C4BP in a reaction that, in contrast to the C4BP-SAP interaction, was not dependent on calcium.

Expression and characterization of a recombinant C4b-binding protein lacking the beta-chain

C4b-binding protein (C4BP) is a high-molecular-mass glycoprotein which contains binding sites for complement component C4b, anti-coagulant vitamin K-dependent protein S and serum amyloid P component (SAP). The major form of C4BP in plasma is composed of seven identical alpha-chains and a single beta-chain. We have expressed full-length cDNA for the alpha-chain in a eukaryotic expression system and characterized functional properties of non-beta-chain-containing C4BP. During synthesis, recombinant alpha-chains polymerized into two different high-molecular-mass C4BP forms which were composed of seven or eight alpha-chains. Recombinant C4BP bound C4(H2O) (used instead of C4b) equally as well as native C4BP, functioned equally as well as factor I cofactor in the degradation of C4(H2O) and bound to SAP. In contrast, the recombinant C4BP did not bind protein S and therefore did not inhibit the ability of protein S to function as a cofactor to activated protein C. Tunicamycin treatment of the transfected cells prevented N-linked glycosylation, but did not affect polymerization of the alpha-chains into a high-molecular-mass C4BP. The non-glycosylated C4BP had comparable properties to glycosylated C4BP in several functional assays. These results demonstrate polymerization of C4BP alpha-chains to be independent both of the beta-chain and of the N-linked carbohydrates. Moreover, N-linked carbohydrates and the beta-chain were neither required for the ability of C4BP to bind C4b and to function as factor I cofactor nor for the interaction with SAP.

cDNA structure of rabbit C4b-binding protein alpha-chain. Preserved sequence motive in complement regulatory protein modules which bind C4b

A full length cDNA clone for the alpha-chain of the rabbit complement regulatory protein C4b-binding protein (C4BP) was isolated from a liver cDNA library. The clone encoded an open reading frame of 597 amino acids, which included a signal peptide, eight short consensus repeats (SCR) and a carboxy terminal non-repeat region. Gel filtration of rabbit plasma and testing of fractions for factor I cofactor activity (C4BP-like) revealed two peaks of activity, the one with highest molecular weight corresponding in size to that of human C4b-binding protein. Comparison of the rabbit C4BP alpha-chain sequence with other SCR containing C3b/C4b binding proteins revealed highest sequence similarities between the second SCRs in C4BP from rabbit, human and murine species and SCRs at corresponding position in complement receptor 1 (CR1) whereas in decay accelerating factor (DAF), the third SCR was most similar. A conserved sequence motive was identified in these C4b-binding SCRs.