Studies of the association of the eighth and ninth components of human complement within the membrane-bound cytolytic complex

The association of the eighth (C8) and ninth (C9) components of human complement within membrane-bound C5b-9 was investigated using the photosensitive cross-linking reagent N-succinimidyl-6-(4'-azido-2'-nitrophenylamino)hexanoate. Reaction of this reagent with either the purified alpha-gamma or beta subunit of C8 resulted in the introduction of 6-8 mol/mol of photosensitive 6-(4'-azido-2'-nitrophenylamino)hexanoate (ANH) as an intrinsic ligand on each protein. The resulting ANH-(alpha-gamma) or ANH-(beta) was capable of recombining with equimolar amounts of beta or alpha-gamma, respectively, to yield ANH-C8. Parallel modifications of purified C9 resulted in incorporation of 3-4 mol/mol of ANH-ligand. Both ANH-C8 and ANH-C9 retained their ability to incorporate into C5b-9. Two approaches were used to determine the proximity of C8 subunits to C9 within C5b-9. In one, the complex was assembled on erythrocytes by incubating EAC1-7 cells separately with each form of ANH-C8 and subsequently saturating with 125I-C9. After lysis, membranes were irradiated, solubilized, and analyzed by gel electrophoresis. Cross-linking was assessed by a shift in electrophoretic mobility of 125I-C9 to a higher molecular weight. Results using either form of ANH-C8 in C5b-9 showed that, although at least 30% was involved in cross-linking, none was cross-linked to C9. Similar results were obtained using a second approach in which cross-linker and radiolabel were transposed between C8 and C9. Here, EAC1-7 cells were incubated first with 125I-C8 containing either 125I-(alpha-gamma) or 125I-(beta) and subsequently with ANH-C9. Although at least 48% of ANH-C9 in C5b-9 was involved in cross-linking in these experiments, no cross-linking to either subunit of C8 was detected. These results suggest that C8 is not in close physical association with C9 within membrane-bound C5b-9.     

Binding of human complement C8 to C9: role of the N-terminal modules in the C8 alpha subunit

Human C8 is one of five components of the membrane attack complex of complement (MAC). It is composed of a disulfide-linked C8alpha-gamma heterodimer and a noncovalently associated C8beta chain. The C8alpha and C8beta subunits contain a pair of N-terminal modules [thrombospondin type 1 (TSP1) + low-density lipoprotein receptor class A (LDLRA)] and a pair of C-terminal modules [epidermal growth factor (EGF) + TSP1]. The middle segment of each protein is referred to as the membrane attack complex/perforin domain (MACPF). During MAC formation, C8alpha mediates binding and self-polymerization of C9 to form a pore-like structure on the membrane of target cells. In this study, the portion of C8alpha involved in binding C9 was identified using recombinant C8alpha constructs in which the N- and/or C-terminal modules were either exchanged with those from C8beta or deleted. Those constructs containing the C8alpha N-terminal TSP1 or LDLRA module together with the C8alpha MACPF domain retained the ability to bind C9 and express C8 hemolytic activity. By contrast, those containing the C8alpha MACPF domain alone or the C8alpha MACPF domain and C8alpha C-terminal modules lost this ability. These results indicate that both N-terminal modules in C8alpha have a role in forming the principal binding site for C9 and that binding may be dependent on a cooperative interaction between these modules and the C8alpha MACPF domain.     

Functional domains of the alpha subunit of the eighth component of human complement: identification and characterization of a distinct binding site for the gamma chain

The purified gamma subunit of the eighth component of human complement (C8) was used to characterize its site of interaction within C8 and to probe the ultrastructure of membrane-bound C5b-8 and C5b-9 complexes. Purification of gamma was accomplished by separating the disulfide-linked alpha-gamma subunit from the noncovalently associated beta chain and subjecting the former to limited reduction, alkylation, and ion-exchange chromatography. Upon mixing, purified alpha and gamma exhibited a high affinity for each other, as evidenced by their ability to form a noncovalent, equimolar complex at dilute concentrations and in the presence of excess serum albumin. Purified gamma also exhibited an affinity for C8', a previously described derivative that is functionally similar to C8 although it is composed of only alpha and beta. These results indicate that alpha possesses a specific site for interaction with gamma and that this site is preserved in the isolated subunit. Furthermore, this site remains accessible when alpha is associated with beta. In related experiments, gamma was found to specifically associate with membrane-bound C5b-8' and C5b-(8')9 complexes. These results indicate that the site for gamma interaction remains accessible on alpha in C5b-8' and is not shielded by C9 within C5b-(8')9. It is concluded that the gamma subunit of C8 is located on the surface of membrane-bound C5b-8 and C5b-9.     

Complementary DNA and derived amino acid sequence of the alpha subunit of human complement protein C8: evidence for the existence of a separate alpha subunit messenger RNA

The entire amino acid sequence of the alpha subunit (Mr 64,000) of the eighth component of complement (C8) was determined by characterizing cDNA clones isolated from a human liver cDNA library. Two clones with overlapping inserts of net length 2.44 kilobases (kb) were isolated and found to contain the entire alpha coding region [1659 base pairs (bp)]. The 5' end consists of an untranslated region and a leader sequence of 30 amino acids. This sequence contains an apparent initiation Met, signal peptide, and propeptide which ends with an arginine-rich sequence that is characteristic of proteolytic processing sites found in the pro form of protein precursors. The 3' untranslated region contains two polyadenylation signals and a poly(A) sequence. RNA blot analysis of total cellular RNA from the human hepatoma cell line HepG2 revealed a message size of approximately 2.5 kb. Features of the 5' and 3' sequences and the message size suggest that a separate mRNA codes for alpha and argues against the occurrence of a single-chain precursor form of the disulfide-linked alpha-gamma subunit found in mature C8. Analysis of the derived amino acid sequence revealed several membrane surface seeking domains and a possible transmembrane domain. These occur in a cysteine-free region of the subunit and may constitute the structural basis for alpha interaction with target membranes. Analysis of the carbohydrate composition indicates 1 or 2 asparagine-linked but no O-linked oligosaccharide chains, a result consistent with predictions from the amino acid sequence.(ABSTRACT TRUNCATED AT 250 WORDS)     

An indel within the C8 alpha subunit of human complement C8 mediates intracellular binding of C8 gamma and formation of C8 alpha-gamma

Human C8 is one of five complement components (C5b, C6, C7, C8, and C9) that interact to form the cytolytic membrane attack complex, or MAC. It is an oligomeric protein composed of three subunits (C8alpha, C8beta, C8gamma) that are products of different genes. In C8 from serum, these are arranged as a disulfide-linked C8alpha-gamma dimer that is noncovalently associated with C8beta. In this study, the site on C8alpha that mediates intracellular binding of C8gamma to form C8alpha-gamma was identified. From a comparative analysis of indels (insertions/deletions) in C8alpha and its structural homologues C8beta, C6, C7, and C9, it was determined that C8alpha contains a unique insertion (residues 159-175), which includes Cys(164) that forms the disulfide bond to C8gamma. Incorporation of this sequence into C8beta and coexpression of the resulting construct (iC8beta) with C8gamma produced iC8beta-gamma, an atypical disulfide-linked dimer. In related experiments, C8gamma was shown to bind noncovalently to mutant forms of C8alpha and iC8beta in which Cys(164)-->Gly(164) substitutions were made. In addition, C8gamma bound specifically to an immobilized synthetic peptide containing the mutant indel sequence. Together, these results indicate (a) intracellular binding of C8gamma to C8alpha is mediated principally by residues contained within the C8alpha indel, (b) binding is not strictly dependent on Cys(164), and (c) C8gamma must contain a complementary binding site for the C8alpha indel.     

Complementary DNA and derived amino acid sequence of the beta subunit of human complement protein C8: identification of a close structural and ancestral relationship to the alpha subunit and C9

A cDNA clone encoding the beta subunit (Mr 64,000) of the eighth component of complement (C8) has been isolated from a human liver cDNA library. This clone has a cDNA insert of 1.95 kilobases (kb) and contains the entire beta sequence [1608 base pairs (bp)]. Analysis of total cellular RNA isolated from the hepatoma cell line HepG2 revealed the mRNA for beta to be approximately 2.5 kb. This is similar to the message size for the alpha subunit of C8 and confirms the existence of different mRNAs for alpha and beta. This finding supports genetic evidence that alpha and beta are encoded at different loci. Analysis of the derived amino acid sequence revealed several membrane surface seeking segments that may facilitate beta interaction with target membranes during complement-mediated cytolysis. Determination of the carbohydrate composition indicated 1 or 2 asparagine-linked but no O-linked oligosaccharide chains. Comparison of the beta sequence to that reported for alpha in the preceding paper [Rao, A. G., Howard, O. M. Z., Ng, S. C., Whitehead, A. S., Colten, H. R. & Sodetz, J. M. (1987) Biochemistry (preceding paper in this issue)] and to that of human C9 revealed a striking homology between all three proteins. For beta and alpha, the overall homology is 33% on the basis of identity and 53% when conserved substitutions are allowed. For beta and C9, the values are 26% and 47%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Crystal structure of the MACPF domain of human complement protein C8 alpha in complex with the C8 gamma subunit

Human C8 is one of five complement components (C5b, C6, C7, C8, and C9) that assemble on bacterial membranes to form a porelike structure referred to as the “membrane attack complex” (MAC). C8 contains three genetically distinct subunits (C8α, C8β, C8γ) arranged as a disulfide-linked C8α-γ dimer that is noncovalently associated with C8β. C6, C7 C8α, C8β, and C9 are homologous. All contain N- and C-terminal modules and an intervening 40-kDa segment referred to as the membrane attack complex/perforin (MACPF) domain. The C8γ subunit is unrelated and belongs to the lipocalin family of proteins that display a β-barrel fold and generally bind small, hydrophobic ligands. Several hundred proteins with MACPF domains have been identified based on sequence similarity; however, the structure and function of most are unknown. Crystal structures of the secreted bacterial protein Plu-MACPF and the human C8α MACPF domain were recently reported and both display a fold similar to those of the bacterial pore-forming cholesterol-dependent cytolysins (CDCs). In the present study, we determined the crystal structure of the human C8α MACPF domain disulfide-linked to C8γ (αMACPF-γ) at 2.15 Å resolution. The αMACPF portion has the predicted CDC-like fold and shows two regions of interaction with C8γ. One is in a previously characterized 19-residue insertion (indel) in C8α and fills the entrance to the putative C8γ ligand-binding site. The second is a hydrophobic pocket that makes contact with residues on the side of the C8γ β-barrel. The latter interaction induces conformational changes in αMACPF that are likely important for C8 function. Also observed is structural conservation of the MACPF signature motif Y/W-G-T/S-H-F/Y-X₆-G-G in αMACPF and Plu-MACPF, and conservation of several key glycine residues known to be important for refolding and pore formation by CDCs.     

The eighth component of human complement: molecular basis of C8A (C81) polymorphism

Using an exon-specific polymerase chain reaction (PCR) followed by direct DNA sequence analysis we have analyzed the polymorphism of the -chain of the eighth component of human complement (C8) at the DNA level. We found that two common alleles, C8A^*A and C8A^*B, are characterized by the substitution of a single amino acid (Gln to Lys), which is caused by a point mutation of a single nucleotide (C to A) in exon 3 at position 187 of the mature C8 cDNA sequence. Based on this mutation, an allele-specific PCR was designed detecting the two alleles of C8A. We applied this method to type the C8A polymorphism using DNA samples from a Chinese Han population. The comparison with the data of protein typing of the same samples proved that the described method is efficient and reliable for the identification of C8A genotypes and may be valuable for further application in population studies and forensic science.     

Inhibition of the formation of the complement membrane-attack complex by a monoclonal antibody to the complement component C8 alpha subunit.

The effect of nine monoclonal antibodies to complement component C8 on the interaction of C9 with preformed cell-surface C5b-8 complexes and on the functional insertion of C8 into the membrane-attack complex (MAC) was investigated. None of the antibodies prevented C9 insertion into a preformed C5b-8 complex. One antibody (F1) directed to the C8 alpha subunit clearly inhibited formation of a functional MAC. It is proposed that this antibody prevents the C8 alpha subunit unfolding and distorting the bilayer to allow C9 insertion.     

Terminal complement components play a role in the expression of C5a

This study examined the expression of C5a detected antigenically (RIA) and functionally (PMN-myeloperoxidase release) consequent to classical or alternative pathway convertase cleavage. Maximal C5a expression occurred when C5 was cleaved in the presence of the later-acting complement components, C6, C7, and C8. This effect was detected by using both purified components and normal human serum immunochemically depleted of C7 or C8 and reconstituted with the purified component. C6 alone was not sufficient to augment C5a expression. Subsequent incubation of C6 and C7 with C5 cleaved in the absence of the terminal components was not sufficient for C5a release. Repeated freezing and thawing of C5 cleaved in the absence of C6 and C7 produced C5a equivalent to that detected when convertase cleavage occurred in the presence of the terminal components. Mild detergent treatment of convertase-cleaved C5 was not sufficient for C5a release. We believe that these data indicate a role for the terminal complement components in the expression of both C5a antigen and function. The mechanism for this effect is not known, but it may involve conformational changes in the C5 molecule that occur during membrane attack complex formation.     

Chromosomal assignment of genes encoding the alpha, beta, and gamma subunits of human complement protein C8: identification of a close physical linkage between the alpha and the beta loci

The eighth component of human complement (C8) is a serum protein containing three nonidentical subunits (alpha, beta, gamma) that are arranged as a disulfide-linked alpha-gamma dimer and a noncovalently associated beta chain. In earlier genetic studies, electrophoretic analysis of C8 protein polymorphisms revealed several allelic variants of alpha-gamma and beta. These were governed by separate loci designated C8A and C8B for alpha-gamma and beta, respectively. Genetic linkage analyses indicated that these loci were linked to each other and to chromosome 1 marker loci PGM1 and Rh, but it was unclear at the time if C8A was a single locus coding for a single-chain precursor form of alpha-gamma or if separate loci existed for alpha and gamma. Since evidence now indicates that alpha, beta, and gamma are encoded by separate genes, cDNA probes corresponding to each subunit were used to make direct assignments of the individual loci. Analysis of somatic cell hybrids revealed that only the alpha and beta loci are located on chromosome 1. Parallel analysis of genomic DNA digests using 5' and 3'-specific cDNA probes showed they are physically linked (less than 2.5 kb) and oriented 5' alpha-beta 3'. Further probing of the hybrid panel revealed that gamma is located on chromosome 9q. Thus, the observed genetic linkage of alpha-gamma to beta must be determined solely by alpha. In accordance with these findings, the C8 loci should now be designated C8A, C8B, and C8G for alpha, beta and gamma, respectively.     

Functional studies of the MACPF domain of human complement protein C8alpha reveal sites for simultaneous binding of C8beta, C8gamma, and C9

Human C8 is one of five components of the membrane attack complex of complement (MAC). It contains three subunits (C8alpha, C8beta, C8gamma) arranged as a disulfide-linked C8alpha-gamma dimer that is noncovalently associated with C8beta. C8alpha, C8beta, and complement components C6, C7, and C9 form the MAC family of proteins. All contain N- and C-terminal modules and an intervening 40-kDa segment referred to as the membrane attack complex/perforin (MACPF) domain. During MAC formation, C8alpha binds and mediates the self-polymerization of C9 to form a pore-like structure on target cells. The C9 binding site was previously shown to reside within a 52-kDa segment composed of the C8alpha N-terminal modules and MACPF domain (alphaMACPF). In the present study, we examined the role of the MACPF domain in binding C9. Recombinant alphaMACPF and a disulfide-linked alphaMACPF-gamma dimer were successfully produced in Escherichia coli and purified. alphaMACPF was shown to simultaneously bind C8beta, C8gamma, and C9 and form a noncovalent alphaMACPF.C8beta.C8gamma.C9 complex. Similar results were obtained for the recombinant alphaMACPF-gamma dimer. This dimer bound C8beta and C9 to form a hemolytically active (alphaMACPF-gamma).C8beta.C9 complex. These results indicate that the principal binding site for C9 lies within the MACPF domain of C8alpha. They also suggest this site and the binding sites for C8beta and C8gamma are distinct. alphaMACPF is the first human MACPF domain to be produced recombinantly and in a functional form. Such a result suggests that this segment of C8alpha and corresponding segments of the other MAC family members are independently folded domains.     

Interaction between alphaB-crystallin and the human 20S proteasomal subunit C8/alpha7

alphaB-Crystallin, a member of the small heat shock protein (sHsp) family, can bind unfolding proteins, but is unable to refold them. To fulfil its protective function in vivo it is therefore likely to interact with other cellular proteins. Here we report that alphaB-crystallin binds very specifically both in vitro and in vivo to C8/alpha7, one of the 14 subunits of the 20S proteasome. The C8/alpha7 protein forms heterogeneous complexes with alphaB-crystallin of about 540 kDa. However, no strong interaction between alphaB-crystallin and 20S proteasomes was observed. Since both proteins are localized in the cytoplasm, the interaction between alphaB-crystallin and C8/alpha7 subunit might affect the assembly of the proteasome complex or facilitate the degradation of unfolded proteins bound to alphaB-crystallin.     

Functional role of the noncatalytic subunit of complement C5 convertase

The C5 convertase is a serine protease that consists of two subunits: a catalytic subunit which is bound in a Mg2+-dependent complex to a noncatalytic subunit. To understand the functional role of the noncatalytic subunit, we have determined the C5-cleaving properties of the cobra venom factor-dependent C5 convertase (CVF, Bb) made with CVF purified from the venom of Naja naja (CVFn) and Naja haje (CVFh) and compared them to those for two C3b-dependent C5 convertases (ZymC3b,Bb and C3b,Bb). A comparison of the kinetic parameters indicated that although the four C5 convertases (CVFn,Bb, ZymC3b,Bb, CVFh,Bb, and C3b,Bb) had similar catalytic rate constants (kcat = 0.004-0.012 s-1) they differed 700-fold in their affinity for the substrate as indicated by the Km values (CVFn,Bb = 0.036 microM, ZymC3b,Bb = 1.24 microM, CVFh,Bb = 14.0 microM, and C3b,Bb = 24 microM). Analysis of binding interactions between C5 and the noncatalytic subunits (CVFh or C3b, or CVFn) using the BIAcore, revealed dissociation binding constants (Kd) that were similar to the Km values of the respective enzymes. The kinetic and binding data demonstrate that the binding site for C5 resides in the noncatalytic subunit of the enzyme, the affinity for the substrate is solely determined by the noncatalytic subunit and the catalytic efficiency of the enzyme appears not to be influenced by the nature of this subunit.     

The ninth component of human complement: purification and physicochemical characterization

A procedure for the isolation of the human complement (C) protein C9 is described. The procedure allowin. The purified protein has the electrophoretic mobility of an alpha-globulin, and is a single polypeptide chain with a m.w. of 71,000. No impurities were detected either on gel electrophoretic or immunochemical examination. C9 is a glycoprotein containing 7.8% carbohydrate, and in terms of residues per mole, 3.0 glucosamine, 17.6 neutral hexose, and 7.4 sialic acid. Its amino acid composition is typical of a globular serum protein. Upon automated Edman degradation of reduced and alkylated C9, no amino acid residues were released, suggesting a blocked N-terminus. The concentration of C9 in normal human serum is 58 +/- 8 microgram/ml. A high titer rabbit antiserum was produced and employed to immunochemically deplete serum of C9. The CH50 of the C9-depleted serum was identical to that of whole human serum; however, membrane fragments of erythrocytes lysed by C9-depleted serum lacked the typical ultrastructural C lesions, which constitute the dimeric membrane attack complex.     

The eighth FIII domain of human fibronectin promotes integrin alpha5beta1 binding via stabilization of the ninth FIII domain

Binding of the extracellular matrix molecule fibronectin to the integrin receptor alpha(5)beta(1) elicits downstream signaling pathways that modulate cell function. Fibronectin-alpha(5)beta(1) interaction occurs via the conserved RGD sequence in the tenth FIII (FIII10) domain of fibronectin. A synergistic site containing the sequence PHSRN in the adjacent FIII9 domain has also been identified. Here we investigate the function of the eighth FIII domain in integrin-mediated cell adhesion using a wide range of methods, including biochemical, biological, and biophysical assays of integrin binding, cell adhesion, and protein denaturation. Mutation of the FIII9 synergistic site (PHSRN to PHAAA) in FIII9-10 reduced the binding activity for integrin alpha(5)beta(1) to levels observed for FIII10 alone, but the corresponding mutant in FIII8-9-10 showed no loss of binding activity. Cell adhesion assays also demonstrated enhanced functional activity of constructs containing FIII8. Equilibrium chemical denaturation studies indicated that FIII8 confers conformational stability upon FIII9, but only if the exposed loops, PHSRN and VKNEED on FIII9 and FIII8, respectively, are intact. These results demonstrate that the loss of integrin binding activity, observed upon alteration of the PHSRN synergistic site of FIII9-10, results partly from a loss of conformational stability of FIII9. Our data suggest a mechanism for integrin alpha(5)beta(1)-fibronectin interaction, which in addition to the primary RGD binding event, involves a conformation-sensitive scanning by the integrin for accessible sites on the ligand, whereupon full activation of downstream signaling occurs.     

Chimeric and truncated forms of human complement protein C8 alpha reveal binding sites for C8 beta and C8 gamma within the membrane attack complex/perforin region.

Human C8 is one of five components of the membrane attack complex of complement. It is an oligomeric protein composed of three subunits (C8 alpha, C8 beta, and C8 gamma) that are derived from different genes. C8 alpha and C8 beta are homologous and both contain a pair of tandemly arranged N-terminal modules [thrombospondin type 1 (TSP1) + low-density lipoprotein receptor class A (LDLRA)], an extended middle segment referred to as the membrane attack complex/perforin region (MACPF), and a pair of C-terminal modules [epidermal growth factor (EGF) + TSP1]. During biosynthetic processing, C8 alpha and C8 gamma associate to form a disulfide-linked dimer (C8 alpha-gamma) that binds to C8 beta through a site located on C8 alpha. In this study, the location of binding sites for C8 beta and C8 gamma and the importance of the modules in these interactions were investigated by use of chimeric and truncated forms of C8 alpha in which module pairs were either exchanged for those in C8 beta or deleted. Results show that exchange or deletion of one or both pairs of modules does not abrogate the ability of C8 alpha to form a disulfide-linked dimer when coexpressed with C8 gamma in COS cells. Furthermore, each chimeric and truncated form of C8 alpha-gamma retains the ability to bind C8 beta; however, only those containing the TSP1 + LDLRA modules from C8 alpha are hemolytically active. These results indicate that binding sites for C8 beta and C8 gamma reside within the MACPF region of C8 alpha and that interaction with either subunit is not dependent on the modules. They also suggest that the N-terminal modules in C8 alpha are important for C9 binding and/or expression of C8 activity.