Binding of the lipocalin C8gamma to human complement protein C8alpha is mediated by loops located at the entrance to the C8gamma ligand binding site

Human C8 is one of five complement components (C5b, C6, C7, C8 and C9) that interact to form the membrane attack complex (MAC). C8 is composed of a disulfide-linked C8alpha-gamma heterodimer and a noncovalently associated C8beta chain. C8alpha and C8beta are homologous to C6, C7 and C9, whereas C8gamma is the only lipocalin in the complement system. Lipocalins have a core beta-barrel structure forming a calyx with a binding site for a small molecule. In C8gamma, the calyx opening is surrounded by four loops that connect beta-strands. Loop 1 is the largest and contains Cys40 that links to Cys164 in C8alpha. To determine if these loops mediate binding of C8alpha prior to interchain disulfide bond formation in C8alpha-gamma, the loops were substituted separately and in combination for the corresponding loops in siderocalin (NGAL, Lcn2), a lipocalin that is structurally similar to C8gamma. The siderocalin-C8gamma chimeric constructs were expressed in E. coli, purified, and assayed for their ability to bind C8alpha. Results indicate at least three of the four loops surrounding the entrance to the C8gamma calyx are involved in binding C8alpha. Binding near the calyx entrance suggests C8alpha may restrict and possibly regulate access to the C8gamma ligand binding site.     

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.     

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.     

Human complement protein C8 gamma

Human C8 gamma is a 22 kDa subunit of complement component C8, which is one of five components (C5b, C6, C7, C8, C9) that interact to form the cytolytic membrane attack complex (MAC) of complement. C8 contains three nonidentical subunits (alpha, beta, gamma) that are products of different genes. These subunits are arranged asymmetrically to form a disulfide-linked C8 alpha-gamma dimer that is noncovalently associated with C8 beta. C8 alpha and C8 beta are homologous to C6, C7 and C9 and together these proteins comprise what is referred to as the 'MAC protein family'. By comparison, C8 gamma is distinct in that it belongs to the lipocalin family of small, secreted proteins which have the common ability to bind small hydrophobic ligands. While specific roles have been identified for C8 alpha and C8 beta in the formation and function of the MAC, a function for C8 gamma and the identity of its ligand are unknown. This review summarizes the current status of C8 gamma structure and function and the progress made from efforts to determine its role in the complement system.     

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.     

Interaction between the C8 alpha-gamma and C8 beta subunits of human complement C8: role of the C8 beta N-terminal thrombospondin type 1 module and membrane attack complex/perforin domain

Human C8 is one of five complement components (C5b, C6, C7, C8, and C9) that interact to form the cytolytic membrane attack complex (MAC). It is an oligomeric protein composed of a disulfide-linked C8alpha-gamma heterodimer and a noncovalently associated C8beta chain. C8alpha and C8beta are homologous; both contain an N-terminal thrombospondin type 1 (TSP1) module, a low-density lipoprotein receptor class A (LDLRA) module, an extended central segment referred to as the membrane attack/perforin (MACPF) domain, an epidermal growth factor (EGF) module, and a second TSP1 module at the C-terminus. In this study, the segment of C8beta that confers binding specificity toward C8alpha-gamma was identified using recombinant C8beta constructs in which the N- and/or C-terminal modules were deleted or exchanged with those from C8alpha. Constructs were tested for their ability to bind C8alpha-gamma in solution and express C8 hemolytic activity. Binding to C8alpha-gamma was found to be dependent on the TSP1 + LDLRA + MACPF segment of C8beta. Within this segment, the TSP1 module and MACPF domain are principally involved and act cooperatively to mediate binding. Results from activity assays suggest that residues within this segment also mediate binding and incorporation of C8 into the MAC.     

The eighth component of human complement: evidence that it is an oligomeric serum protein assembled from products of three different genes

The eighth component of human complement (C8) consists of three nonidentical subunits arranged asymmetrically as a disulfide-linked alpha-gamma dimer and a noncovalently associated beta chain. Genetic studies of C8 polymorphisms established that alpha-gamma and beta are encoded at different loci. Implicit in this finding was the existence of two different genes and the likelihood that alpha-gamma would be synthesized in single-chain precursor form. However, recent characterization of cDNA clones revealed separate mRNAs for human alpha and beta but no evidence of a single-chain precursor for alpha-gamma. A cDNA clone containing the entire coding region for human gamma has now been characterized, and its sequence supports the existence of a separate gamma mRNA. Included are a consensus translation initiation sequence, an apparent initiation methionine, and a signal peptide. By use of cDNA probes specific for human alpha, beta, or gamma, analysis of poly(A) RNA from normal baboon liver revealed separate mRNAs of 2.5, 2.6, and 1.0 kilobases (kb), respectively. Parallel analysis of poly(A) RNA from rat liver identified mRNAs of 3.4, 2.3, and 0.9 kb. These results argue against the possibility that C8 is assembled from products of two different genes (alpha-gamma and beta) and suggest it is comprised of three different gene products (alpha, beta, and gamma) that undergo both covalent and noncovalent association to yield the mature protein.     

Crystal structure of human complement protein C8gamma at 1.2 A resolution reveals a lipocalin fold and a distinct ligand binding site

C8gamma is a 22-kDa subunit of human C8, which is one of five components of the cytolytic membrane attack complex of complement (MAC). C8gamma is disulfide-linked to a C8alpha subunit that is noncovalently associated with a C8beta chain. In the present study, the three-dimensional structure of recombinant C8gamma was determined by X-ray diffraction to 1.2 A resolution. The structure displays a typical lipocalin fold forming a calyx with a distinct binding pocket that is indicative of a ligand-binding function for C8gamma. When compared to other lipocalins, the overall structure is most similar to neutrophil gelatinase associated lipocalin (NGAL), a protein released from granules of activated neutrophils. Notable differences include a much deeper binding pocket in C8gamma as well as variation in the identity and position of residues lining the pocket. In C8gamma, these residues allow ligand access to a large hydrophobic cavity at the base of the calyx, whereas corresponding residues in NGAL restrict access. This suggests the natural ligands for C8gamma and NGAL are significantly different in size. Cys40 in C8gamma, which forms the disulfide bond to C8alpha, is located in a partially disordered loop (loop 1, residues 38-52) near the opening of the calyx. Access to the calyx may be regulated by movement of this loop in response to conformational changes in C8alpha during MAC formation.     

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.     

Structural features of the ligand binding site on human complement protein C8gamma: a member of the lipocalin family

Human C8 is one of five components of the cytolytic membrane attack complex of complement. It contains three subunits (C8alpha, C8beta, C8gamma) arranged as a disulfide-linked C8alpha-gamma heterodimer that is noncovalently associated with C8beta. C8gamma has the distinction of being the only lipocalin in the complement system. Lipocalins have a core beta-barrel structure forming a calyx with a binding site for a small hydrophobic ligand. A natural ligand for C8gamma has not been identified; however previous structural studies indicate C8gamma has a typical lipocalin fold that is suggestive of a ligand-binding capability. A distinctive feature of C8gamma is the division of its putative ligand binding pocket into a hydrophilic upper portion and a large hydrophobic lower cavity. Access to the latter is restricted by the close proximity of two tyrosine side chains (Y83 and Y131). In the present study, binding experiments were performed using lauric acid as a pseudoligand to investigate the potential accessibility of the lower cavity. The crystal structure of a C8gamma.laurate complex revealed that Y83 and Y131 can move to allow penetration of the hydrocarbon chain of laurate into the lower cavity. Introducing a Y83W mutation blocked access but had no effect on the ability of C8gamma to enhance C8 cytolytic activity. Together, these results indicate that the lower cavity in C8gamma could accommodate a ligand if such a ligand has a narrow hydrophobic moiety at one end. Entry of that moiety into the lower cavity would require movement of Y83 and Y131, which act as a gate at the cavity entrance.     

Analysis of the specific association of the eighth and ninth components of human complement: identification of a direct role for the alpha subunit of C8

The basis for the physical association between C8 and C9 in solution was examined by isolating the noncovalently associated alpha-gamma and beta subunits of C8 and determining their respective affinities for C9. Results indicate that only alpha-gamma associates with C9 and this association, though reversible, is complete at near equimolar ratios of each component. Further experiments using purified alpha or gamma revealed that only alpha was capable of forming a stable complex with C9. Although the strength of this interaction was dependent on salt concentration, association was observed in buffer of physiological ionic strength and in human serum. These results establish that the domain on C8 responsible for interaction with C9 is located entirely within alpha. In related experiments, addition of beta to performed dimers of either (alpha-gamma + C9) or (alpha + C9) resulted in complete association of this subunit. These particular results indicate that there are two physically distinct sites on alpha that separately mediate association of alpha with beta and with C9. Furthermore, occupation of one site does not impair interaction at the other.     

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.     

Analysis of human C8 with monoclonal antibodies. Characterization of a monoclonal antibody that recognizes free C8 alpha-gamma subunit

The eighth component of human C is essential for the formation of the membranolytic C attack complex. C8 has a unique structure in that two covalently linked chains, C8 alpha and C8 gamma, are associated non-covalently with the third chain, C8 beta. In order to study the structure and assembly of the C8 molecule, a panel of mAb has been produced against the C component C8. Eight of these mAb had reactivity to the C8 alpha-gamma subunit, whereas four reacted with C8 beta. One of the C8 alpha-gamma mAb, C8A2, had specificity for an epitope on the C8 alpha-chain and exhibited no cross-reactivity to any of the other terminal C components, including C8 beta. C8A2 inhibited the hemolytic activity of the C8 alpha-gamma subunit but had no effect on the activity of fluid phase whole C8 or C8 within membrane-bound C5b-8. Functional experiments suggest that C8A2 inhibits C8 alpha-gamma activity by interfering with its interaction with the C8 beta-chain. In an enzyme immunoassay using the C8A2 mAb, free C8 alpha-gamma subunit could be detected in both homozygous and heterozygous C8 beta-deficient serum. However, only low level binding was observed when homozygous C5- and C7-deficient sera were tested. Thus the mAb, C8A2, recognizes an epitope expressed on the C8 alpha-gamma subunit but not on intact C8 and can detect free C8 alpha-gamma in the presence of native C8.     

Biosynthesis of C8 by hepatocytes. Differential expression and intracellular association of the alpha-gamma- and beta-subunits

The subunit structure of C8 is unusual in that it consists of a disulfide-linked alpha-gamma-dimer and a non-covalently associated beta-chain. To gain insight into the biosynthesis and assembly of C8, we examined its production by short term cultures of rat hepatocytes. Synthesis was detected by functional assays as well as immunologically with antisera against C8, alpha-gamma or beta. Control experiments showed that all three antisera were capable of precipitating C8 from rat serum. When antiserum against C8 was used to analyze hepatocyte cell lysates, precursor as well as mature forms of alpha-gamma and beta were detected. Precursor-product relationships were confirmed by pulse-chase experiments, which also revealed a significantly faster rate of synthesis for alpha-gamma. Importantly, no single chain form of alpha-gamma was detected in the intracellular pool. Evidence that C8 is assembled before secretion was provided by analyzing cell lysates with antiserum specific for alpha-gamma or beta. Precipitation of both subunits with either antiserum confirmed they were associated. Parallel analyses of the extracellular pool revealed that alpha-gamma and beta remain associated in the media but that excess alpha-gamma is secreted as a consequence of a faster synthesis rate. These results provide evidence for differential expression of alpha-gamma and beta and are consistent with the existence of different loci for these subunits. Furthermore, they indicate that association of alpha-gamma with beta is a presecretory event in normal C8 synthesis.     

The human complement regulatory protein CD59 binds to the alpha-chain of C8 and to the "b"domain of C9.

The erythrocyte membrane inhibitor of the human terminal complement proteins, surface antigen CD59, has previously been shown to enter into a detergent-resistant complex with either the membrane-bound complex of C5b-8 or C5b-9 (Meri, S., Morgan, B. P., Davies, A., Daniels, R. H., Olavesen, M. G., Waldmann, H. and Lachmann, P. J. (1990) Immunology 71, 1-9; Rollins, S. A., Zhao, J., Ninomiya, H., and Sims, P. J. (1991) J. Immunol, 146, 2345-2351). In order to further define the interactions that underlie the complement-inhibitory function of CD59, we have examined the binding interactions between 125I-CD59 and the isolated components of human complement membrane attack complex, C5b6, C7, C8, and C9. By density gradient analysis, we were unable to detect interaction of 125I-CD59 with any of these isolated complement components in solution. Specific binding of 125I-CD59 to C8 and C9 was detected when these human complement proteins were adsorbed to either plastic or to nitrocellulose, suggesting that a conformational change that accompanies surface adsorption exposes a CD59-binding site that is normally buried in these serum proteins. The binding of 125I-CD59 to plastic-adsorbed C8 and C9 was saturable and competed by excess unlabeled CD59, with half-maximal binding observed at 125I-CD59 concentrations of 80 and 36 nM, respectively. No specific binding of 125I-CD59 was detected for surface-adsorbed human C5b6 or C7 nor was such binding observed for C8 or C9 isolated from rabbit serum. Binding of CD59 to human C8 and C9 was not mediated by the phospholipid moiety of CD59, implying association by protein-protein interaction. In order to further define the binding sites for CD59, ligand blotting with 125I-CD59 was performed after separation of C8 into its noncovalently associated subunits (C8 alpha-gamma and C8 beta) and after alpha-thrombin digestion of C9. These experiments revealed specific and saturable binding of 125I-CD59 to C8 alpha-gamma subunit (half-maximal binding at 75 nM), but not to C8 beta, and specific and saturable binding to the 37-kDa fragment (C9b) of thrombin-cleaved C9 (half-maximal binding at 35 nM), but not to the 25-kDa C9a fragment. Partial reduction of C8 alpha-gamma revealed that only C8 alpha polypeptide exhibited affinity for CD59, and no specific binding to the C8 gamma chain was detected.(ABSTRACT TRUNCATED AT 400 WORDS)     

Substructural analysis of the insulin receptor by microsequence analyses of limited tryptic fragments isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the absence or presence of dithiothreitol

Human placental insulin receptor contains 47 Cys per an alpha beta dimer. Most of the 94 Cys in an intact alpha 2 beta 2 receptor are expected to form interchain or intrachain disulfide bonds, since there appears to be only one free cysteine residue in each beta subunit. In order to gain more insight into the three-dimensional organization of the insulin receptor, we have used limited trypsin digestion, SDS-PAGE, and protein microsequencing. The present study revealed the following; major tryptic cleavages occurred at alpha 164, alpha 270, alpha 582, and beta 1115, generating Mr 175,000, 130,000, 100,000, 70,000, and 55,000 disulfide-linked complexes. Under reducing conditions, tryptic fragments of Mr values = 30,000, 70,000, 20,000, 55,000, and 20,000 were identified to be alpha(1-164), alpha(165-582), alpha(165-270), alpha(271-582), and alpha(583-C-terminal), respectively. The major beta subunit tryptic fragment of Mr = 55,000 was assumed to have beta(724-1115) or beta(N-terminal-392). The Mr 175,000 complex appeared to contain two alpha(1-164) and two alpha(165-582), whereas the Mr 70,000 complex contained alpha(583-C-terminal) and beta(724-1115). Tryptic cleavage at alpha 582 apparently produced one Mr 175,000 and two Mr 70,000 complexes, suggesting that the alpha(583-C-terminal) domain interacts with the extracellular domain of the beta subunit by disulfide bonds. Tryptic cleavage at alpha 270 resulting in a formation of one Mr 100,000 complex consisting of two alpha(1-270) and two Mr 130,000 complexes consisting of alpha(271-C-terminal) and beta(724-1115) suggests that Cys residues involved with disulfide bonds between the two alpha subunits are located in the alpha(1-270) domain. The identification of the Mr 55,000 complex consisting of small tryptic fragments between alpha(122-270) indicates that 40 Cys residues in the two alpha(122-270) domains are inter- and intramolecularly associated by disulfide bonds. The alpha(1-121) domain does not appear to be linked to any other domains by disulfide bonds. These results are consistent with the structural model that the N-terminal domains of alpha subunits (122-270) are disulfide-linked together while the C-terminal domain (583-C-terminal) of the alpha subunit is linked to the N-terminal domain of the beta subunit by disulfide bonds.     

Surface expression of mutated subunits of the high affinity mast cell receptor for IgE

The mast cell receptor with high affinity for IgE consists of four transmembrane polypeptides which are held together by detergent-sensitive interactions: an IgE-binding alpha chain, a single beta chain, and a disulfide-linked dimer of gamma chains. Now that the cDNAs that code for each of the subunits have been isolated, it should be possible to probe by site-specific mutations, which portions of the receptor are critical for transmembrane signaling. One prerequisite for such studies is that the mutant receptors be expressible on the cell surface. We have explored this issue by transiently transfecting COS 7 cells with mutant subunits and assessing surface expression by IgE binding. Removal of any single cytoplasmic domain of the receptor's subunits had little influence on surface expression, and even receptors missing all five cytoplasmic domains were expressed, albeit less efficiently. Minor changes within the transmembrane domains (TMs) sometimes produced major effects and more drastic changes in the TMs ablated surface expression entirely. These data suggest that the TMs are critical loci for receptor display. Cys7 (residue 2 in the gamma TM) was shown to form the inter-gamma disulfide bond and to be nonessential for surface expression. By localizing this bond, residues in the TM of gamma that are buried in the interface between the gamma subunits could be predicted. Consistent with observations on other membrane proteins (Rees, D. C., DeAntonio, L., and Eisenberg, D. (1989) Science 245, 510-513), maximal interspecies conservation was observed for those residues in the gamma TM predicted to be buried. This was also true for those residues in the alpha and beta TMs predicted to be buried by analysis of the TM hydrophobic moments.     

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.     

Differential functional expression of the C8 subunits. Primary role of C8 beta in assembly of intact C8

The eighth component of C (C8) is composed of two subunits C8 beta and C8 alpha-gamma, which are non-covalently bound in a 1/1 ratio in the intact molecule. The genes encoding the polypeptide chains composing the subunits demonstrate close genetic linkage. To assess the functional expression of these genes at the protein level, normal human serum and C8-deficient sera were electrophoresed in native polyacrylamide gels following which C8, C8 beta, and C8 alpha-gamma were detected using hemolytic overlays. These experiments demonstrated that normal sera contained free C8 alpha-gamma in addition to intact C8. Free C8 alpha-gamma was not observed when C8 was reconstituted by mixing C8 beta-deficient serum with C8 alpha-gamma-deficient serum in a ratio optimized for C8 activity, suggesting that the free C8 alpha-gamma observed in normal serum was not due to dissociation of intact C8. Inasmuch as this technique did not adequately separate C8 and C8 beta, sera were also examined by anion exchange chromatography. C8 alpha-gamma-deficient serum contained C8 beta in a single peak in the 1.4 ms/cm fall through. C8 beta-deficient serum contained a major peak of C8 alpha-gamma at 7.1 ms/cm and a lesser peak coeluting with C9 at 9.5 ms/cm. Normal serum contained both intact C8 eluting between 2.4 to 5.5 ms/cm and C8 alpha-gamma eluting at 7.1 ms/cm. Free C8 beta was not detectable in normal serum indicating that free C8 alpha-gamma was not due to C8 dissociation. Mixing aliquots from the chromatographic peak of C8 beta activity with the peaks of C8 alpha-gamma activity in C8 beta-deficient serum or in normal serum generated intact C8 hemolytic activity. Non-reducing SDS-PAGE and Western blotting with anti-C8 confirmed the presence of antigenic material of appropriate m.w. in each peak. These findings demonstrate that serum contains excess C8 alpha-gamma relative to C8 beta, despite the equimolar presence of the subunits in intact C8. Thus the availability of C8 beta determines the quantity of C8 produced. Further, these data suggest the possibility that the C8 structural genes may be differentially expressed despite their close genetic linkage.     

Assembly and secretion of fibrinogen. Degradation of individual chains.

Hep G2 cells produce surplus A alpha and gamma fibrinogen chains. These excess chains, which are not secreted, exist primarily as free gamma chains and as an A alpha-gamma complex. We have determined the intracellular location and the degradative fate of these polypeptides by treatment with endoglycosidase-H and by inhibiting lysosomal enzyme activity, using NH4Cl, chloroquine, and leupeptin. Free gamma chain and the gamma component of A alpha-gamma are both cleaved by endoglycosidase-H, indicating that the gamma chains accumulate in a pre-Golgi compartment. Lysosomal enzyme inhibitors did not affect the disappearance of free gamma chains but inhibited A alpha-gamma by 50%, suggesting that A alpha-gamma is degraded in lysosomes. The degradative fate of individual chains was determined in transfected COS cells which express but do not secrete single chains. Leupeptin did not affect B beta chain degradation, had very little affect on gamma chain, but markedly inhibited A alpha chain degradation. Antibody to immunoglobulin heavy chain-binding protein (GRP 78) co-immunoprecipitated B beta but not A alpha or gamma chains. Preferential binding of heavy chain-binding protein to B beta was also noted in Hep G2 cells and in chicken hepatocytes. Taken together these studies indicate that B beta and gamma chains are degraded in the endoplasmic reticulum, but only B beta is bound to BiP. By contrast A alpha chains and the A alpha-gamma complex undergo lysosomal degradation.