Evidence for histidine residues in the immunoglobulin-binding site of human Clq

Thermodynamic, conformational and functional properties of the human C1q globular heads in the intact C1q molecule in solution

Thermodynamic. conformational and functional properties of the human C1q globular heads (hgC1q) were studied with the experimental approaches, which allow investigating these properties in the intact hC1q molecule in solution. Surprisingly, the scanning calorimetry data reveal a low level of cooperativity of interactions between the hgC1q A, B and C domains even at a neutral pH area. Ionization of His residues due to acidification of the medium at the pH range from 6 to 5 or the chemical modification of His residues completely abolishes the cooperative interactions between the domains without significant effect on their conformation. The thermodynamic data provide evidence that the hgC1q module is composed of three structurally independent A, B and C globular domains characterized by the practically identical thermal stability and very similar enthalpy of melting. The spectroscopic studies and modification with 2-oxy-5-nitrobenzylbromide (ONBB) indicate that Trp residues in the hgC1q A and C domains are accessible to the solvent that has been confirmed by the hgC1q crystal structure solved and refined to 1.9 A. The modification of Trp residues significantly affects the complement-dependent cytotoxicity without noticeable effect on the hC1q conformation. These data provide evidence that Trp residues are the components of immunoglobulin-binding sites both in the hgC1q A and C domains.

Identification of Residues in the First Domain of Human Fcα Receptor Essential for Interaction with IgA

The FcR family contains multiple receptors for Igs, of which the most distantly related (∼20%) is the IgA receptor (human FcαR), being more homologous (∼35%) to another family of killer-inhibitory receptor-related immunoreceptors with a 19q13.4 chromosomal location in humans. This study of the FcαR demonstrated that, like several IgG receptors, FcαR is a low affinity receptor for Ab (Ka ∼ 106 M−1). Rapid dissociation of the rsFcαR:IgA complex (t1/2 ∼ 25 s) suggests that monomer IgA would bind transiently to cellular FcαRs, while IgA immune complexes could bind avidly. Mutagenesis of histidyl 85 and arginyl 82, in the FG loop of domain 1, demonstrated that these residues were essential for the IgA-binding activity of FcαR, while arginyl 87 makes a minor contribution to the binding activity of the receptor. This site is unusual among the Fc receptors (FcγRII, FcγRIII, and FcεRI), in which the ligand binding site is in domain 2 rather than domain 1, but like FcαR, the FG loop comprises part of the ligand binding site. The putative F and G strands flanking the FcαR ligand binding site are highly homologous in the other killer-inhibitory receptor-related immunoreceptors, suggesting they comprise a conserved structural element on which divergent FG loops are presented and participate in the specific ligand interactions of each of these receptors.

Dissection of C1q capability of interacting with IgG. Time-dependent formation of a tight and only partly reversible association

C1q-bearing immune complexes have been observed in diseases such as rheumatoid arthritis and human immunodeficiency virus infection-associated neuropathy. For the purpose of understanding better the phenomenon of C1q-bearing immune complexes, we investigated the constancy of the C1q-IgG interaction. An enzyme-linked immunosorbent assay was developed in which wells were coated with IgG to mimic antigen-complexed IgG. Serial dilutions of C1q were applied for distinct time intervals, and bound C1q was detected either directly or after exposure to one of several elution buffers. Our results show that a part of C1q attached to IgG forms a tight association that is not reversible under treatment with buffers containing usually protein-protein interaction-dissociating reagents such as 3 M NaCl, 5 M urea, sodium dodecyl sulfate, or beta-mercaptoethanol. The formation of the highly stable C1q-IgG complex was found to be time-, temperature-, and pH-dependent and to proceed with bound C1q even in the absence of free C1q in the supernatant. In ligand blotting experiments we demonstrate for the first time directly that all three chains of C1q can individually bind IgG. Altogether, our results provide a suitable explanation for the formation and persistence of C1q-bearing immune complexes.

Beta-sheet secondary structure of the trimeric globular domain of C1q of complement and collagen types VIII and X by Fourier-transform infrared spectroscopy and averaged structure predictions

C1q plays a key role in the recognition of immune complexes, thereby initiating the classical pathway of complement activation. Although the triple-helix conformation of its N-terminal segment is well established, the secondary structure of the trimeric globular C-terminal domain is as yet unknown. The secondary structures of human C1q and C1q stalks and pepsin-extracted human collagen types I, III and IV (with no significant non-collagen-like structure) were studied by Fourier-transform i.r. spectroscopy in 2H2O buffers. After second-derivative calculation to resolve the fine structure of the broad amide I band, the Fourier-transform i.r. spectrum of C1q showed two major bands, one at 1637 cm-1, which is a characteristic frequency for beta-sheets, and one at 1661 cm-1. Both major bands were also detected for Clq in H2O buffers. Only the second major band was observed at 1655 cm-1 in pepsin-digested C1q which contains primarily the N-terminal triple-helix region. The Fourier-transform i.r. spectra of collagen in 2H2O also showed a major band at 1659 cm-1 (and minor bands at 1632 cm-1 and 1682 cm-1). It is concluded that the C1q globular heads contain primarily beta-sheet structure. The C-terminal domains of C1q show approximately 25% sequence identity with the non-collagen-like C-terminal regions of the short-chain collagen types VIII and X. To complement the Fourier-transform-i.r. spectroscopic data, averaged Robson and Chou-Fasman structure predictions on 15 similar sequences for the globular domains of C1q and collagen types VIII and X were performed. These showed a clear pattern of ten beta-strands interspersed by beta-turns and /or loops. Residues thought to be important for C1q-immune complex interactions with IgG and IgM were predicted to be at a surface-exposed loop. Sequence insertions and deletions, glycosylation sites, the free cysteine residue and RGD recognition sequences were also predicted to be at surface-exposed positions.

Carbodiimide crosslinking of human C1q and rabbit IgG

The water soluble carbodiimide 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide was used to covalently link carboxyl groups on rabbit IgG to lysyl groups on complement protein C1q. The interaction between C1q and IgG was disrupted by varying the pH, modifying essential residues in the IgG binding site of C1q and by reducing the interchain disulfides of IgG. Under each of these conditions the correlation found between binding and crosslinking indicated a strong requirement for the proteins to bind normally in order for crosslinking to occur. SDS-PAGE analysis of the crosslinked material showed a 210 kDa band consistent with one IgG crosslinked to two disulfide linked C1q chains. Blotting and autoradiography showed the crosslinking involved the A and/or B and C chains of C1q. The lysines flanking the intrachain half cystines are proposed as the likely candidates for crosslinking to IgG, thus delineating the immunoglobulin binding site of C1q.

Interaction between complement subcomponent C1q and bacterial lipopolysaccharides

The heptose-less mutant of Escherichia coli, D31m4, bound complement subcomponent C1q and its collagen-like fragments (C1qCLF) with Ka values of 1.4 x 10(8) and 2.0 x 10(8) M-1 respectively. This binding was suppressed by chemical modification of C1q and C1qCLF using diethyl pyrocarbonate (DEPC). To investigate the role of lipopolysaccharides (LPS) in this binding, biosynthetically labelled [14C]LPS were purified from E. coli D31m4 and incorporated into liposomes prepared from phosphatidylcholine (PC) and phosphatidylethanolamine (PE) [PC/PE/LPS, 2:2:1, by wt.]. Binding of C1q or its collagen-like fragments to the liposomes was estimated via a flotation test. These liposomes bound C1q and C1qCLF with Ka values of 8.0 x 10(7) and 2.0 x 10(7) M-1; this binding was totally inhibited after chemical modification of C1q and C1qCLF by DEPC. Liposomes containing LPS purified from the wild-strain E. coli K-12 S also bound C1q and C1qCLF, whereas direct binding of C1q or C1qCLF to the bacteria was negligible. Diamines at concentrations which dissociate C1 into C1q and (C1r, C1s)2, strongly inhibited the interaction of C1q or C1qCLF with LPS. Removal of 3-deoxy-D-manno-octulosonic acid (2-keto-3-deoxyoctonic acid; KDO) from E. coli D31m4 LPS decreases the binding of C1qCLF to the bacteria by 65%. When this purified and modified LPS was incorporated into liposomes, the C1qCLF binding was completely abolished. These results show: (i) the essential role of the collagen-like moiety and probably its histidine residues in the interaction between C1q and the mutant D31m4; (ii) the contribution of LPS, particularly the anionic charges of KDO, to this interaction.

Enhancement of the binding of C1q to immune complexes by polyethylene glycol

The binding of 125I-labelled human C1q to insoluble rabbit IgG:ovalbumin immune complexes was enhanced by polyethylene glycol (PEG, Mr 8 x 10(3)) in the concn range 0-2.5% (w/v). C1q with native immunoglobulin bindings sites rendered inactive by diethylpyrocarbonate treatment did not bind to immune complexes in the presence of PEG. The ionic strength dependence of the binding was independent of the presence of PEG. There was a linear relationship between the logarithm of the apparent affinity constant of the C1q:immune complex interaction and PEG concn.

The presence of histidine residues at or near the C1q binding site of rabbit immunoglobulin G

Treatment of covalently crosslinked rabbit IgG oligomers with diethylpyrocarbonate resulted in the loss of their C1q binding activity. The inactivation was a first-order process with respect to time in the range 0-8 min, and modifier concentration from 0 to 2.39 mM. Hydroxylamine treatment of diethylpyrocarbonate-treated IgG oligomers led to 80% recovery of their C1q binding activity. Diethylpyrocarbonate treatment of IgG oligomers had little effect on their absorbance at 278 nm, but led to an increase in their absorbance at 242 nm. The apparent pKa of the modified residues was 6.91 +/- 0.12. These data are consistent with diethylpyrocarbonate modification of histidine residues leading to loss of C1q binding activity in rabbit IgG oligomers. Modification of four histidine residues per IgG molecule was associated with the loss of C1q binding activity. Thus, there may be two histidine residues at or near the C1q binding sites in the CH2 domains of rabbit IgG.

Activation of the classical complement pathway by BioRex-70

The cation exchange resin BioRex-70 was able to activate the classical complement pathway in human serum at 37 degrees C over the resin concentration range 0-5% (v/v). Using zymosan-treated human serum, it was found that the activation proceeded as far as complement protein C3.

Conformational changes in C1q upon binding to IgG oligomers

The interaction between C1q and immune complexes is inhibited by 1-anilino-8-naphthalenesulfonate (ANS) in the concentration range of 2-4 mM. ANS binds to Clq with a 20-fold higher affinity than to IgG [(1986) Mol. Immunol. 23, 39-44] and therefore it is possible to label only C1q with ANS in the presence of IgG. Under such conditions no inhibition is observed. Addition of monomer IgG to a solution of C1q-bound ANS did not significantly alter the fluorescence of the ANS. However when oligomeric IgG was added there was a 2-fold increase in fluorescence over the same IgG concentration range. When C1q was pretreated with diethylpyrocarbonate there was little change in the fluorescence when IgG oligomers were added to C1q:ANS solutions. These results suggest that C1q undergoes conformational changes upon binding to IgG oligomers.

Binding of complement component C1q by spectrin

125I-labelled human C1q was found to bind to human spectrin. Scatchard plots for the binding process were non-linear, indicating the possible presence of multiple classes of binding sites for C1q on spectrin. The binding was ionic-strength-dependent; the extent of binding decreased with increasing ionic strength. Chemical modification of arginine and histidine residues on C1q as well as pretreatment of C1q at pH 4.45 or at 56 degrees C reduced its spectrin binding activity. The amount of 125I-labelled C1q bound to immune complexes was reduced by the presence of spectrin. Spectrin was also able to deplete the complement haemolytic activity of human serum in a dose-dependent manner.

Evidence for arginine residues in the immunoglobulin-binding sites of human Clq

The immune complex binding activity of human Clq was lost following treatment of the protein with the arginine-selective reagents cyclohexane 1,2-dione and phenylglyoxal. Both inactivations followed pseudo-first-order kinetics. The affinity of Clq for immune complexes was reduced 7-fold following cyclohexane-1,2-dione treatment, and could be substantially restored by treatment of the modified protein with hydroxylamine. Heat-aggregated IgG protected Clq against inactivation by both reagents. Incorporation of 25 molecules of [7-14C]phenylglyoxal per Clq molecule completely inactivated the protein. These data are consistent with the presence of arginyl residues in the immunoglobulin recognition sites of human Clq.

Binding of complement component C1q by rat adipocyte membranes

Human C1q was found to bind to rat adipocyte membranes with an affinity comparable to that for aggregated immunoglobulin. The binding was ionic strength dependent, and modification of arginyl and histidyl residues in C1q abrogated its binding activity. Treatment of the adipocyte membranes with either high ionic strength buffers, EDTA or trypsin had little effect on their C1q-binding activity.

Basic proteins bind immunoglobulin G: a mechanism for demyelinating disease?

Heat-aggregated immunoglobulin G ( HAGG ) bound avidly to solid-phase basic proteins, including myelin basic protein. In contrast, monomeric immunoglobulin bound weakly. Bound HAGG could activate complement. Normal human serum strongly inhibited the binding of HAGG , even when decomplemented or greatly diluted. Cerebrospinal fluid was also inhibitory, but the effect was weaker. Apart from inhibition by decomplemented serum, the biochemical characteristics of the interaction were similar to those of other Fc ligands with IgG, particularly C1q. In multiple sclerosis this interaction could occur between IgG and central-nervous-system myelin basic protein, leading to demyelination by activation of immune mechanisms of tissue damage. Bound IgG is present in multiple sclerosis plaques and IgG from multiple sclerosis patients can produce demyelination in experimental models. However, there is little evidence of any specific immunity to central-nervous-system antigens in multiple sclerosis, and this non-specific interaction might be an important link in the pathogenesis of the disorder.