The Molecular Architecture of Human Complement Component C6

Comprehensive Update and Revision of Nomenclature on Complement C6 and C7 Variants

Complement genes encompass a wide array of variants, giving rise to numerous protein isoforms that have often been shown to exhibit clinical significance. Given that these variants have been discovered over a span of 50 y, one challenging consequence is the inconsistency in the terminology used to classify them. This issue is prominently evident in the nomenclature used for complement C6 and C7 variants, for which we observed a great discrepancy between previously published works and variants described in current genome browsers. This report discusses the causes for the discrepancies in C6 and C7 nomenclature and seeks to establish a classification system that would unify existing and future variants. The inconsistency in the methods used to annotate amino acids and the modifications pinpointed in the C6 and C7 primers are some of the factors that contribute greatly to the discrepancy in the nomenclature. Several variants that were classified incorrectly are highlighted in this report, and we showcase first-hand how a unified classification system is important to match previous with current genetic information. Ultimately, we hope that the proposed classification system of nomenclature becomes an incentive for studies on complement variants and their physiological and/or pathological effects.

Selective cross-linking of coinciding protein assemblies by in-gel cross-linking mass spectrometry

Cross-linking mass spectrometry has developed into an important method to study protein structures and interactions. The in-solution cross-linking workflows involve time and sample consuming steps and do not provide sensible solutions for differentiating cross-links obtained from co-occurring protein oligomers, complexes, or conformers. Here we developed a cross-linking workflow combining blue native PAGE with in-gel cross-linking mass spectrometry (IGX-MS). This workflow circumvents steps, such as buffer exchange and cross-linker concentration optimization. Additionally, IGX-MS enables the parallel analysis of co-occurring protein complexes using only small amounts of sample. Another benefit of IGX-MS, demonstrated by experiments on GroEL and purified bovine heart mitochondria, is the substantial reduction of undesired over-length cross-links compared to in-solution cross-linking. We next used IGX-MS to investigate the complement components C5, C6, and their hetero-dimeric C5b6 complex. The obtained cross-links were used to generate a refined structural model of the complement component C6, resembling C6 in its inactivated state. This finding shows that IGX-MS can provide new insights into the initial stages of the terminal complement pathway.

Identification of the Sixth Complement Component as Potential Key Genes in Hepatocellular Carcinoma via Bioinformatics Analysis

The present study is designed to determine potential target genes involved in hepatocellular carcinoma (HCC) and provide possible underlying mechanisms of action. Several studies (GSE112790, GSE87630, and GSE56140) from the GEO database looking at molecular characteristics in HCC were screened and analyzed by GEO2R, which led to the identification of a total of 93 differentially expressed genes (DEGs). From the protein–protein interaction (PPI) network, we selected 13 key genes with high degree of variability in expression in HCC. Expression of three key genes (NQO1, CYP2C9, and C6) presented with poor overall survival (OS) in HCC patients by UALCAN. C6, which is a complement component, was found by ONCOMINE and TIMER to have low expression in many solid cancers including HCC. Besides, Kaplan-Meier plotter and UALCAN database analysis to access diseases prognosis suggested that low expression of C6 is significantly related to worse OS in LIHC patients, especially in advanced HCC patients. Finally, the TIMER analysis suggested that the C6 expression showed significant negative correlation with infiltrating levels of six immune cells. The somatic copy number alterations (SCNAs) of C6 were associated with CD4+ T cell infiltration in HCC. Taken together, these results together identified C6 as a potential key gene in the diagnosis and prognosis of HCC.

Recurrent meningococcal meningitis with complement 6 (C6) deficiency: A case report

RATIONALE

Late complement deficiency increases susceptibility to meningococcal disease and recurrent infections. In Korea, 5 case reports have described meningococcal disease with complement deficiency. However, C6 deficiency has not been described previously.

PATIENT CONCERNS

A 21-year-old police trainee presented with recurrent meningococcal meningitis. He was housed in communal living quarters until 20 days before the initial symptom onset.

DIAGNOSIS

He was diagnosed with meningococcal meningitis with C6 deficiency.

INTERVENTIONS

He was treated with intravenous ceftriaxone. An additional dose of quadrivalent meningococcal conjugate vaccine was administered after discharge.

OUTCOMES

He was discharged without complications.

LESSONS

Screening for complement deficiency is necessary in patients with a history of recurrent meningococcal infections to provide appropriate care and prevent recurrent infections.

Gene Variations of Sixth Complement Component Affecting Tacrolimus Metabolism in Patients with Liver Transplantation for Hepatocellular Carcinoma

BACKGROUND

Orthotopic liver transplantation (OLT) improves the prognosis of patients with hepatocellular carcinoma (HCC). Moreover, the complement system is a powerful immune effector that can affect liver function and process of liver cirrhosis. However, studies correlating the complement system with tacrolimus metabolism after OLT are scarce. In this study, the role of single nucleotide polymorphisms (SNPs) associated with the sixth complement component (C6) in tacrolimus metabolism was investigated during the early stages of liver transplantation.

METHODS

The study enrolled 135 adult patients treated with OLT for HCC between August 2011 and October 2013. Ten SNPs in C6 gene and rs776746 in cytochrome P450 3A5 (CYP3A5) gene were investigated. The tacrolimus levels were monitored daily during 4 weeks after transplantation.

RESULTS

Both donor and recipient CYP3A5 rs776746 allele A were correlated with decreased concentration/dose (C/D) ratios. Recipient C6 rs9200 allele G and donor C6 rs10052999 homozygotes were correlated with lower C/D ratios. Recipient CYP3A5 rs776746 allele A (yielded median tacrolimus C/D ratios of 225.90 at week 1 and 123.61 at week 2), C6 rs9200 allele G (exhibited median tacrolimus C/D ratios of 211.31 at week 1, 110.23 at week 2, and 99.88 at week 3), and donor CYP3A5 rs776746 allele A (exhibited median C/D ratios of 210.82 at week 1, 111.06 at week 2, 77.49 at week 3, and 85.60 at week 4) and C6 rs10052999 homozygote (exhibited median C/D ratios of 167.59 at week 2, 157.99 at week 3, and 155.36 at week 4) were associated with rapid tacrolimus metabolism. With increasing number of these alleles, patients were found to have lower tacrolimus C/D ratios at various time points during the 4 weeks after transplantation. In multiple linear regression analysis, recipient C6 rs9200 group (AA vs. GG/GA) was found to be related to tacrolimus metabolism at weeks 1, 2, and 3 (P = 0.005, P = 0.045, and P = 0.033, respectively), whereas donor C6 rs10052999 group (CC/TT vs. TC) was demonstrated to be correlated with tacrolimus metabolism only at week 4 (P = 0.001).

CONCLUSIONS

Recipient C6 gene rs9200 polymorphism and donor C6 gene rs10052999 polymorphism are new genetic loci that affect tacrolimus metabolism in patients with HCC after OLT.

Recipient C6 rs9200 genotype is associated with hepatocellular carcinoma recurrence after orthotopic liver transplantation in a Han Chinese population

Hepatocellular carcinoma (HCC) recurrence is one of the leading causes of death after orthotopic liver transplantation (OLT). The sixth complement component (C6) is a late-acting complement protein that participates in the assembly of the membrane attack complex, which has an indispensable role in innate and acquired immune responses, as well as cancer immune surveillance. However, studies assessing the association between C6 and HCC recurrence after OLT are scarce. This study aimed to evaluate the association of donor and recipient C6 single-nucleotide polymorphisms with the risk for HCC recurrence after OLT. A total of 71 adult patients who underwent primary LT for HCC were enrolled. HCC recurrence was observed in 26 (36.6%) patients. Ten single-nucleotide polymorphisms were genotyped and analyzed in both donor and recipient groups. Patients with the rs9200 heterozygous GA variant presented significantly higher HCC recurrence rates (54.17 vs 27.66%, P=0.028), and lower cumulative tumor-free survival and overall survival (P=0.006 and P=0.013, respectively) compared with those harboring the GG/AA genotype, in multivariate logistic regression and Cox regression analyses. The rs9200 heterozygous GA variant in C6 persisted as a statistically independent prognostic factor (P<0.05) for predicting HCC recurrence after OLT. In conclusion, recipient C6 rs9200 polymorphism is associated with HCC recurrence after OLT, and improves the predictive value of clinical models.

Structural biology of the membrane attack complex

The complement system is an intricate network of serum proteins that mediates humoral innate immunity through an amplification cascade that ultimately leads to recruitment of inflammatory cells or opsonisation or killing of pathogens. One effector arm of this network is the terminal pathway of complement, which leads to the formation of the membrane attack complex (MAC) composed of complement components C5b, C6, C7, C8 and C9. Upon formation of C5 convertases via the classical or alternative pathways of complement activation, C5b is generated from C5 by proteolytic cleavage, nucleating a series of association and polymerisation reactions of the MAC-constituting complement components that culminate in pore formation of pathogenic membranes. Recent structures of MAC components and homologous proteins significantly increased our understanding of oligomerisation, membrane association and integration, shedding light onto the molecular mechanism of this important branch of the innate immune system.

An unexpected loss of domains in the conservative evolution ninth complement component in a higher teleost, Miichthys miiuy

The complement systems of fish are well developed and play an important role in the innate immune response. C9 is the ninth member of complement components, involved in creating the membrane attack complex (MAC). In the present study, a truncated C9 cDNA sequence encoding 461 amino acids was cloned and characterized in the miiuy croaker (Miichthys miiuy). Typical fish C9 molecules have five characteristic modular domains, i.e. TSP1, LDLRA, MACPF, EGF, and a second TSP domain which is absent in mammalian counterparts. While in miiuy croaker, this truncated C9 presents only TSP1, LDLRA and MACPF domains. It is the first time of finding a truncated C9 in teleost components. RT-PCR analysis detected these C9 transcripts among all tissues examined, demonstrating its constitutive expression pattern in healthy fish. The highest levels of transcripts were detected in liver of both healthy and pathogen-infected miiuy croaker. Its constitutive and inducible expression pattern of this truncated C9 in liver is similar to most complement components which belong to the type of acute-phase proteins and are in general of hepatic origin. We cannot exclude the possibility that miiuy croaker presents the typical C9 although it has not yet been found. The molecular evolutionary analysis showed that this truncated C9 of miiuy croaker had a significantly higher omega value comparing with other fish and the positive selection pressure had happened on it after its divergence with other fish.

Structure of complement C6 suggests a mechanism for initiation and unidirectional, sequential assembly of membrane attack complex (MAC)

The complement membrane attack complex (MAC) is formed by the sequential assembly of C5b with four homologous proteins as follows: one copy each of C6, C7, and C8 and 12-14 copies of C9. Together these form a lytic pore in bacterial membranes. C6 through C9 comprise a MAC-perforin domain flanked by 4-9 "auxiliary" domains. Here, we report the crystal structure of C6, the first and longest of the pore proteins to be recruited by C5b. Comparisons with the structures of the C8αβγ heterodimer and perforin show that the central domain of C6 adopts a "closed" (perforin-like) state that is distinct from the "open" conformations in C8. We further show that C6, C8α, and C8β contain three homologous subdomains ("upper," "lower," and "regulatory") related by rotations about two hinge points. In C6, the regulatory segment includes four auxiliary domains that stabilize the closed conformation, inhibiting release of membrane-inserting elements. In C8β, rotation of the regulatory segment is linked to an opening of the central β-sheet of its clockwise partner, C8α. Based on these observations, we propose a model for initiation and unidirectional propagation of the MAC in which the auxiliary domains play key roles: in the assembly of the C5b-8 initiation complex; in driving and regulating the opening of the β-sheet of the MAC-performin domain of each new recruit as it adds to the growing pore; and in stabilizing the final pore. Our model of the assembled pore resembles those of the cholesterol-dependent cytolysins but is distinct from that recently proposed for perforin.

Protein ultrastructure and the nanoscience of complement activation

The complement system constitutes an important barrier to infection of the human body. Over more than four decades structural properties of the proteins of the complement system have been investigated with X-ray crystallography, electron microscopy, small-angle scattering, and atomic force microscopy. Here, we review the accumulated evidence that the nm-scaled dimensions and conformational changes of these proteins support functions of the complement system with regard to tissue distribution, molecular crowding effects, avidity binding, and conformational regulation of complement activation. In the targeting of complement activation to the surfaces of nanoparticulate material, such as engineered nanoparticles or fragments of the microbial cell wall, these processes play intimately together. This way the complement system is an excellent example where nanoscience may serve to unravel the molecular biology of the immune response.

Cloning of the sixth complement component and, spatial and temporal expression profile of MAC structural and regulatory genes in chicken

Humoral cytotoxicity results from the assembly of terminal components of complement, called membrane attack complex (MAC), which lead to the formation of pores on pathogen membranes. The complement components involved in MAC formation are C5b, C6, C7, C8alpha, C8beta, C8gamma and C9. Among them, C6 protein interacts with C5b through a metastable binding site to form a soluble C5b-6 dimer in the vicinity of the activating cell. Formation of the MAC is controlled by complement regulatory molecules, such as CD59, vitronectin and clusterin. Here, we report the molecular characterization of the C6 complement component, as well as the spatial and temporal expression profile of MAC structural (C6, C7, C8alpha, C8beta, C8gamma) and regulatory (CD59, vitronectin and clusterin) genes in chicken (Gallus gallus). The deduced polypeptide sequence of chicken C6 consists of 935 amino acid residues and exhibits 81%, 58%, 56% and 44% identity with zebra finch, human, frog and trout orthologs, respectively. The 'domain' architecture of chicken C6 resembles that of mammalian counterparts and the cysteine backbone is also conserved. MAC structural and regulatory genes are expressed in a wide range of adult chicken tissues, with the liver being the major source of their produced transcripts. The developmental expression profile of chicken MAC structural genes shows that their transcripts initially appear in the 12th embryonic day in the liver, exhibiting a pick in the 17th, while no expression was detected in the early whole embryo (day 4 and 6), as well as in the 2-day old neonate chicken liver. On the other hand, MAC regulatory genes are expressed in all the developmental stages investigated.

Solution structure of factor I-like modules from complement C7 reveals a pair of follistatin domains in compact pseudosymmetric arrangement

Factor I-like modules (FIMs) of complement proteins C6, C7, and factor I participate in protein-protein interactions critical to the progress of a complement-mediated immune response to infections and other trauma. For instance, the carboxyl-terminal FIM pair of C7 (C7-FIMs) binds to the C345C domain of C5 and its activated product, C5b, during self-assembly of the cytolytic membrane-attack complex. FIMs share sequence similarity with follistatin domains (FDs) of known three-dimensional structure, suggesting that FIM structures could be reliably modeled. However, conflicting disulfide maps, inconsistent orientations of subdomains within FDs, and the presence of binding partners in all FD structures led us to determine the three-dimensional structure of C7-FIMs by NMR spectroscopy. The solution structure reveals that each FIM within C7 contains a small amino-terminal FOLN subdomain connected to a larger carboxyl-terminal KAZAL domain. The open arrangement of the subdomains within FIMs resembles that of first FDs within structures of tandem FDs but differs from the more compact subdomain arrangement of second or third FDs. Unexpectedly, the two C7-FIMs pack closely together with an approximate 2-fold rotational symmetry that is rarely seen in module pairs and has not been observed in FD-containing proteins. Interfaces between subdomains and between modules include numerous hydrophobic and electrostatic contributions, suggesting that this is a physiologically relevant conformation that persists in the context of the parent protein. Similar interfaces were predicted in a homology-based model of the C6-FIM pair. The C7-FIM structures also facilitated construction of a model of the single FIM of factor I.

Prevalence of mutations leading to complete C6 deficiency (C6Q0) in the Western Cape, South Africa and detection of novel mutations leading to C6Q0 in an Irish family

Complement component C6 is one of five terminal complement components incorporated into the membrane attack complex. Complete deficiency of C6 (C6Q0) leads to an increased susceptibility to Neisseria meningitidis infections, and affected individuals typically present with recurrent meningococcal disease. There is a relatively high prevalence of C6Q0 in the Western Cape, South Africa and three frameshift mutations have previously been described to be responsible for C6Q0 in this area-879delG, 1195delC, and 1936delG (current nomenclature). We have now genotyped a further nine genetically independent individuals with C6Q0, confirming previous reports that the most common defect in the Western Cape is 879delG. Moreover, we report the first identification of the 878delA mutation within the Western Cape, which has previously only been reported in individuals of African descent living in the United States or Europe. We also investigated the genotype of an Irish C6Q0 individual and her sibling, and report two previously undescribed mutations. One mutation alters a tyrosine codon to a stop codon within exon 10. The second mutation is within the 5' donor splice site of intron 3, and would, in all probability, disrupt splicing. These two mutations were shown to segregate independently. We also discuss the nomenclature for reporting C6 and C7 gene mutations, as the current nomenclature does not follow the recognised guidelines.

Complement Components C5 and C7: Recombinant Factor I Modules of C7 Bind to the C345C Domain of C5

Studies reported over 30 years ago revealed that latent, nonactivated C5 binds specifically and reversibly to C6 and C7. These reversible reactions are distinct from the essentially nonreversible associations with activated C5b that occur during assembly of the membrane attack complex, but they likely involve some, perhaps many, of the same molecular contacts. We recently reported that these reversible reactions are mediated by the C345C (NTR) domain at the C terminus of the C5 alpha-chain. Earlier work by others localized the complementary binding sites to a tryptic fragment of C6 composed entirely of two adjacent factor I modules (FIMs), and to a larger fragment of C7 composed of its homologous FIMs as well as two adjoining short consensus repeat modules. In this work, we expressed the tandem FIMs from C7 in bacteria. The mobility on SDS-polyacrylamide gels, lack of free sulfhydryl groups, and atypical circular dichroism spectrum of the recombinant product rC7-FIMs were all consistent with a native structure. Using surface plasmon resonance, we found that rC7-FIMs binds specifically to both C5 and the rC5-C345C domain with K(D) approximately 50 nM, and competes with C7 for binding to C5, as expected for an active domain. These results indicate that, like C6, the FIMs alone in C7 mediate reversible binding to C5. Based on available evidence, we suggest a model for an irreversible membrane attack complex assembly in which the C7 FIMs, but not those in C6, are bound to the C345C domain of C5 within the fully assembled complex.

Two cDNAs from the purple sea urchin, Strongylocentrotus purpuratus , encoding mosaic proteins with domains found in factor H, factor I, and complement components C6 and C7

The vertebrate complement system is composed of about 30 serum and cell surface proteins that make up three activation pathways, a lytic pathway, and a set of proteins that regulate complement. Regulatory proteins are required for host protection against autologous complement attack and to control the amplification feedback loop of the alternative pathway. Purple sea urchin, Strongylocentrotus purpuratus, homologues of complement C3 (SpC3) and factor B (SpBf) have been identified, suggesting the presence of an alternative complement pathway. This implies that echinoderms require a complement regulatory system for the same reasons that it is required in higher vertebrates. Two cDNAs, Sp5 and Sp5013, have been characterized from coelomocytes and the deduced structures of the encoded mosaic proteins, SpCRL ( S. p urpuratus complement related protein, long form) and SpCRS ( short form), have domains that are also found in regulatory proteins such as factor H and factor I and the terminal pathway components C6 and C7. These domains include multiple short consensus repeats, a fucolectin domain, Ser/Thr/Pro-rich regions, a Cys-rich region, and a factor I-membrane attack complex domain. The genes are constitutively expressed in all tissues of the sea urchin and are not induced in response to immune challenge. Multiple bands of varying intensity on both genome blots and RNA blots suggest that Sp5 and Sp5013 are members of a small gene family and that they might undergo alternative splicing. Based on the domains present in SpCRL and SpCRS, they might be either examples of complement regulatory proteins or members of the terminal pathway of complement.

Detection of mRNA for the terminal complement components C5, C6, C8 and C9 in human umbilical vein endothelial cells in vitro

Human umbilical vein endothelial cells (HUVEC) have previously been shown to synthesize the functional terminal pathway of complement based on the detection by radioimmunoassay of the terminal complement complex (TCC) on coincubated agarose beads. In addition, C7 secretion by these cells in amounts comparable to C3, as well as C7 mRNA, has recently been demonstrated. However, it has not been possible to detect C5-6 and C8 in the fluid phase, and only trace amounts of soluble C9. Against this background we examined whether mRNA for the remaining terminal complement factors was present in HUVEC. By the use of reverse transcription (RT)-polymerase chain reaction (PCR) and Northern blot the presence of mRNA for complement factors C5, C6, C8 and C9 was demonstrated.

High prevalence of complement component C6 deficiency among African-Americans in the south-eastern USA

Complement component C6 is a part of the membrane attack complex that forms a pore-like structure in cell membranes following complement activation. Deficiency of terminal complement components including C6 predisposes individuals to infection with Neisseriae. Using polymerase chain reaction/single-strand conformation polymorphism analysis followed by DNA sequencing, we screened genomic DNA from 200 randomly chosen blacks and an equal number from whites for three loss-of-function C6 mutations. Ten blacks and two whites were found to be heterozygous for one of the mutations. Two of the mutations, 1195delC and 1936delG, were found exclusively in black individuals. A third previously undescribed mutation, 878delA, was found at equal frequency among the two groups. The difference between the two groups was significant (P = 0.027), indicating that C6 deficiency due to these three mutations is more common among blacks than whites in the local area, principally Jefferson County, Alabama. In addition, three previously undescribed point mutations, two of which result in amino acid substitutions, were identified within exon 6. A review of the county health department records over the past 6 years revealed a higher incidence of meningococcal meningitis in blacks due to serogroups Y and W-135 which paralleled the difference in the estimated prevalence of C6 deficiency. Among black residents of the county (n = 235 598) there were 15 cases of meningitis due to these two serogroups, compared with two cases in the white population (n = 422 604) (P = 0.002). We conclude that C6 deficiency is more common among blacks than whites in the south-eastern United States, with a frequency approaching 1 in 1600 black individuals.

The four terminal components of the complement system are C-mannosylated on multiple tryptophan residues

C-Mannosylation is a unique form of protein glycosylation, involving the C-glycosidic attachment of a mannosyl residue to the indole moiety of Trp. In the two examples found so far, human RNase 2 and interleukin-12, only the first Trp in the recognition motif WXXW is specifically C-mannosylated. To establish the generality of protein C-mannosylation, and to learn more about its mechanism, the terminal components of the human complement system (C6, C7, C8,and C9), which contain multiple and complex recognition motifs, were examined. Together with C5b they form the cytolytic agent, the membrane attack complex. These are the first proteins that are C-mannosylated on more than one Trp residue as follows: six in C6, four in C7, C8alpha, and C8beta, and two in C9. Thus, from the 113 Trp residues in the complete membrane attack complex, 50 were found to undergo C-mannosylation. The other important finding is that in C6, C7, C8, and C9 Trp residues without a second Trp (or another aromatic residue) at the +3 position can be C-mannosylated. This shows that they must contain an additional C-mannosylation signal. Whether this is encoded in the primary or tertiary structure is presently unknown. Finally, all modified Trp residues are part of the highly conserved core of the thrombospondin type 1 repeats present in these proteins. Since this module has been found in a large number of other proteins, the results suggest further candidates for C-mannosylation.

Function of the factor I modules (FIMS) of human complement component C6

In order to elucidate the function of complement component C6, truncated C6 molecules were expressed recombinantly. These were either deleted of the factor I modules (FIMs) (C6des-748-913) or both complement control protein (CCP) modules and FIMs (C6des-611-913). C6des-748-913 exhibited approximately 60-70% of the hemolytic activity of full-length C6 when assayed for Alternative Pathway activity, but when measured for the Classical Pathway, C6des-748-914 was only 4-6% as effective as C6. The activity difference between C6 and C6des-748-913 for the two complement pathways can be explained by a greater stability of newly formed metastable C5b* when produced by the Alternative Pathway compared with that made by the Classical Pathway. The half-lives of metastable C5b* and the decay of (125)I-C5b measured from cells used to activate the Alternative Pathway were found to be about 5-12-fold longer than those same parameters derived from cells that had activated the Classical Pathway. (125)I-C5 binds reversibly to C6 in an ionic strength-dependent fashion, but (125)I-C5 binds only weakly to C6des-FIMs and not at all to C6des-CCP/FIMs. Therefore, although the FIMs are not required absolutely for C6 activity, these modules promote interaction of C6 with C5 enabling a more efficient bimolecular coupling ultimately leading to the formation of the C5b-6 complex.

Rat complement factor I: molecular cloning, sequencing and expression in tissues and isolated cells

Factor I (FI) is a regulatory serine protease of the complement system which cleaves three peptide bonds in the alpha-chain of C3b and two bonds in the alpha-chain of C4b thereby inactivating these proteins. The human protein and the recently characterized mouse factor I are heterodimers of about 88,000 MW which consist of a non-catalytic heavy chain of 50,000 MW which is linked to a catalytic light chain of 38,000 MW by a disulphide bond. For the screening of a rat liver cDNA library we used a hybridization probe produced by polymerase chain reaction (PCR) using degenerated primers which corresponded to conserved parts of the human and the murine factor I nucleotide sequences. One of the identified sequences, which had a length of 2243 base pairs (bp), contained the complete coding region and the whole 3' untranslated region. The length of the coding region in rat consisted of 1812 bp followed by a 3' untranslated region of 207 bp including the polyadenylation signal and the beginning of the poly A tail. Comparison of the rat cDNA-derived coding sequence revealed identities of 87% to the mouse and of 78% to the human FI nucleotide sequence. The translation product of rat FI mRNA was 604 amino acid residues (aa) in length with an identity of 85% to the mouse (603 aa) and 69% to the human protein (583 aa). The comparison of the molecular mass predicted by the primary structure and derived from rat FI isolated from rat serum as detected in immunoblot analyses suggested a glycosylation of more than 20% of the total mass of the FI protein. Expression studies using reverse transcription (RT)-PCR assays indicated that FI-specific mRNA could neither be identified in B cells, nor in T cells, monocytes or granulocytes from rat and human peripheral blood nor in rat peritoneal macrophages. These data were in agreement with the results of RT-PCR obtained with several human lymphoma cell lines (Jurkat, MOLT-4, HUT102, Wil 2-NS, Ramos, Raji, U937) all of which were devoid of FI-specific mRNA. In accord with our data from two rat hepatoma cell lines (FAO and H4IIE) and one from man (HepG2) only isolated rat hepatocytes (HC) but neither Kupffer cells (KC), hepatic stellate cells (HSC; Ito cells) nor sinusoidal endothelial cells (SEC) expressed FI-specific mRNA. FI mRNA was also detected in human umbilical vein endothelial cells (HUVEC) and in the uterus and small intestine of the rat. Spleen and lymph nodes did not contain any detectable FI-specific mRNA.

Molecular defects leading to human complement component C6 deficiency in an African-American family

Complement component C6 deficiency (C6D) was diagnosed in a 16-year-old African-American male with meningococcal meningitis. The patient's father and two brothers also had C6D, but gave no history of meningitis or other neisserial infection. By using exon-specific polymerase chain reaction (PCR)/single-strand conformation polymorphism as a screening step and nucleotide sequencing of target exons, we determined that the proband was a compound heterozygote for two C6 gene mutations. The first, 1195delC located in exon 7, is a novel mutation, while the second, 1936delG in exon 12, has been described before to cause C6D in an unrelated African-American individual. Both mutations result in premature termination codons and C6 null alleles. Allele-specific PCR indicated that the proband's two brothers also inherited the 1195delC mutation from their heterozygous mother and the 1936delG mutation from their homozygous father.

Elucidation of the disulfide-bonding pattern in the factor I modules of the sixth component (C6) of human complement

Complement component C6 is known to contain two factor I modules in tandem at its C-terminus. To localize the disulfide bridges in those domains, native C6 was cleaved with trypsin, followed by subtilisin. The resulting digests were separated by reversed-phase HPLC, and all of the potential cystine-containing fragments were detected by a fluorescence assay and amino acid composition analyses. Final identification of the disulfide bonds was achieved by Edman degradation of the corresponding peptides. From the data gained a 1-3, 2-9, 4-7, 5-10, 6-8 pattern was determined (Cys752-Cys802, Cys763-Cys780, Cys765-Cys816, Cys772-Cys795, Cys841-Cys852, Cys846-Cys898, Cys859-Cys876, Cys861-Cys911, Cys867-Cys891). These findings are compared with the strongly related complement components C7 and factor I.

Neuronal expression of mRNAs for complement proteins of the classical pathway in Alzheimer brain

To determine possible sources of complement proteins in the brain, we investigated by in situ hybridization expression of the mRNAs of C1q, C2, C3, C4, C5, C6, C7, C8 and C9 in postmortem Alzheimer disease (AD) and control brain tissue. We found detectable hybridization for all these components in the temporal cortex and hippocampus, with significantly higher levels being found in AD tissue. Hybridization signals were strongest over pyramidal neurons. Low or absent hybridization was seen in the visual cortex or cerebellum. These results suggest that the activated complement components found in association with AD lesions may be, in part, derived from neurons.

Terminal components of carp complement constituting a membrane attack complex

The membrane attack complex (MAC) of carp complement was extracted with deoxycholate from rabbit erythrocytes lysed by carp serum and purified by a two-step chromatographic procedure. On two-dimensional SDS-PAGE of carp MAC, eight bands were detected. The band of M(r) 91,000 was identified as carp C9 by western blotting using anti-carp C9, and two bands of M(r) 62,000 and one band of M(r) 22,000 were confirmed as those of carp C8 alpha, C8 beta and C8 gamma, respectively, by their N-terminal amino acid sequences. The bands of M(r) 102,000 and 73,000, which generated from a 180,000 band under reducing conditions, were those corresponding to human C5b alpha and C5b beta, respectively. The remaining bands of M(r) 115,000 and 106,000 were identified as those corresponding to human C6 and C7, as determined by their molecular size, single-chain structures and similarities in N-terminal amino acid sequences to their mammalian counterparts. Densitometric scan of the gels showed the molar ratio of C5b, C6, C7, C8 and C9 in carp MAC to be 1:1:1:1:4. Based on these results, it appears that, as with mammals, the cytolytic pathway of bony fish complement is composed of five terminal components from C5 to C9.

Identification of disulfide bonds in the ninth component (C9) of human complement

C9 is the most abundant protein of the membrane attack complex of complement. By means of limited proteolysis, different chromatographic techniques, a thiol-specific fluorescence assay, amino acid analysis, and Edman degradation 9 out of 12 disulfide bridges are definitely assigned (Cys22-Cys57, Cys33-Cys36, Cys67-Cys73, Cys121-Cys160, Cys233- Cys234, Cys359-Cys384, Cys489-Cys505, Cys492-Cys507, Cys509-Cys518). Weaker evidence permits to reduce the number of possible configurations for the remaining 3 cystines (Cys80-Cys91, Cys86-Cys104, Cys98-Cys113, or Cys80-Cys91, Cys86-Cys113, Cys98-Cys104). These findings are discussed in comparison with the strongly related components C6, C7, C8alpha, and C8beta.

cDNA cloning, sequencing and chromosomal assignment of the gene for mouse complement factor I (C3b/C4b inactivator): identification of a species specific divergent segment in factor I

Factor I is an essential regulatory serine proteinase of the complement cascade. It cleaves and inactivates the C3b and C4b constituents of the C3 and C5 convertases and thereby regulates many complement-mediated activities. The human protein is a heterodimer composed of a 50 kDa non-catalytic subunit (which contains several domains, i.e. FIM, CD5, LDLr type A) disulfide linked to a 38 kDa catalytic subunit. Recent characterization of Xenopus factor I cDNA revealed a 29 residue negatively charged region in its heavy chain which is absent in the human protein (Kunnath-Muglia et al., Molec. Immun. 30, 1249-1256, 1993). We report the complete cDNA sequence of mouse factor I as well as a partial chicken factor I cDNA sequence. Alignment of these two sequences with the published sequences for human and Xenopus proteins (a) demonstrates an overall conservation of primary structure and domain organization of mouse factor I, and (b) defines a divergent segment (D segment) in each species. In Xenopus protein, the D segment includes the 29 residue negatively charged region. In each of the four species examined, the D segment differed in length, sequence, organization, and number of repeated subregions. These differences reflect a considerable evolution of D segment. The significance of the diversity of the D segment is at present unclear. We also report the chromosomal localization of the mouse factor I gene (Cfi) to distal chromosome 3 near Egf.

Processing of human factor I in COS-1 cells co-transfected with factor I and paired basic amino acid cleaving enzyme (PACE) cDNA

Factor I is an active serine proteinase in plasma that regulates both the classical and alternative complement pathways by cleaving C3b and C4b thereby preventing the assembly of C3 and C5 convertase enzymes. In this study, a full-length human factor I cDNA was cloned into the pMT2 expression vector and the pMT2-fI construct was expressed transiently in COS-1 cells and stably in CHO-K1 cells. The transfected COS-1 cells secreted large amounts of recombinant pro-factor I (85 kD). Co-transfection of COS-1 cells with pMT2-fI and the cDNA expression plasmid for PACE (paired basic amino acid cleaving enzyme), resulted predominantly in the secretion of a proteolytically processed form of recombinant factor I (heavy chain, 47 kD; light chain, 35 kD). Following co-transfection of pMT2-fI and pSVNeo.1 into CHO-K1 cells and selection in medium containing G418, a stably transfected clone was isolated that secreted pro-factor I (85 kd) and proteolytically processed factor I (heavy chain, 48 kD; light chain, 37 kD) in approximately equal amounts. The molecular sizes of the subunit chains of the expressed factor I were generally slightly smaller than those of human plasma factor I. The activity of recombinant factor I present in the culture supernatants of transfected COS-1 and CHO-K1 cells was assayed by its ability to cleave 125I-C3b in the presence of factor H and was found to be low when compared with factor I purified from human plasma. However, since the functional activity of purified factor I was reduced approximately 50% in the presence of conditioned medium from non-transfected cells, it is suggested that the cold C3b present in the factor I-deficient serum used to supplement the culture medium probably competed with the 125I-C3b tracer, thereby decreasing the sensitivity of the assay for the recombinant factor I proteins.

DNA polymorphisms of the complement C6 and C7 genes

The linked C6 and C7 loci are rich in genetic markers, both at the protein and DNA levels. There are now seven common DNA polymorphisms distributed over about 300 kbp of chromosome 5p12-14. We report a new TaqI RFLP for C7 and a method for typing a C7 variant (T368S) hitherto known only from cDNA clones. We have re-investigated the published RFLPs to provide information on their frequency in North European Caucasian (predominantly British and Irish) subjects and have revised some of the published parameters, especially the sizes of polymorphic restriction fragments. Their precise locations within the genes are also reported: the three markers for C6 are in exon 3, intron 3 and adjacent to exon 17 and the four markers for C7 are in introns 15 and 13 and in exons 13 and 9. The gene frequencies of the second commonest allele of all seven markers lie in the range 0.2 to 0.37, except C6 A/B in the Japanese, where the frequencies of both common alleles are about 0.45. We have estimated the gene frequencies for the DNA polymorphisms which correlate with C7 M/N phenotype and for the C6 A/B phenotype and find them to be the same as the phenotypic estimates in Caucasians and in the Japanese respectively. The markers provide the possibility of defining 128 haplotypes, many (28) of which have been observed. Allelic associations in these genes are generally surprisingly weak.

Genetic detection of the silent allele (*Q0) in hereditary deficiencies of the human complement C6, C7, and C9 components

DNA polymorphisms (RFLPs) of the human complement component C6, C7, and C9 genes were studied in three C7-deficient (C7D) families, one C6-deficient (C6D) family, and one C9-deficient (C9D) family. The 3 loci are closely linked on human chromosome 5. The haplotypes carrying the "silent" allele (C7*Q0, C6*Q0, and C9*Q0) were defined in each family, allowing for the detection of carriers among asymptomatic relatives. This paper describes familial studies on a type of hereditary trait, characterized by recurrent Neisseria infections in individuals homozygous for "silent" alleles at the C6, C7, or C9 loci.

Physiology and pathophysiology of complement: progress and trends

The complement system comprises a family of at least 20 plasma and membrane proteins that interact in a tightly regulated cascade system to destroy invading bacteria and prevent the deposition of immune complexes in the tissues. This brief review addresses the basic mechanisms of complement activation and control and describes the active fragments produced during complement activation. The biological importance of the complement system is amply illustrated in patients with complement deficiencies, who are susceptible to bacterial infections and immune complex diseases. The involvement of complement in other immunological diseases is an expanding area of clinical research, supported by the development of new assays for the identification of complement activation. This area is discussed here with particular reference to neurological diseases. A promising new prospect involves the use of complement inhibitory molecules in therapy of complement-mediated disease and this exciting area is also discussed. Novel physiological roles of complement also are being revealed and new evidence that complement and complement receptors play an important role in reproduction is summarized. It is hoped that this brief overview will convey some of the enthusiasm currently pervading research in this underappreciated area of immunology.

C6 haplotypes: associations of a Dde I site polymorphism to complement deficiency genes and the Msp I restriction fragment length polymorphism (RFLP)

Complement C6 has a common charge polymorphism designated A and B with gene frequencies of 0.65 and 0.35. The probable molecular basis for this is a Glu (C6A) for Ala (C6B) substitution at amino acid position 98, and is detected by digestion with the restriction enzyme Dde I of a polymerase chain reaction (PCR)-amplified fragment of genomic DNA. C6A was found to be Dde I-positive and C6B corresponds to Dde I-negative. We have applied our Dde I A/B polymorphism genotyping method to the investigation of C6-deficient individuals with complete (C6Q0) and sub-total deficiency (C6SD) protein phenotypes, including members of four families. We have also investigated the RFLP detected by digestion of genomic DNA with the enzyme Msp I, which is due to a polymorphic site located in the 5' section of the gene, the variable sequence of which has yet to be determined. Sixteen out of seventeen unrelated C6Q0 subjects were found to be genotypically Dde I B/Msp I-negative; the remaining subject was heterozygous at both the loci under investigation. The C6SD phenotype was found to be associated with the Dde I A/Msp I-positive genotype in two families with combined C6/C7 subtotal deficiency and two with C6SD. It can be concluded that the two forms of C6 deficiency, C6Q0 and C6SD, arose independently on two different C6 allelic backgrounds. These associations have allowed the genotyping of the rare families that contain both types of deficiency. We have also defined a number of normal C6 Dde I/Msp I haplotypes in Caucasians and Cape Coloured populations.

Structure of complement poly-C9 determined in projection by cryo-electron microscopy and single particle analysis

The ring-like complement 'lesions' found on membranes of complement lysed cells comprise a complex of components C5b through C9 that coalesce to form hollow cylinders which penetrate the membrane bilayer and create lytic pores. Walls of these C5b-9 membrane attack complex cylinders may consist primarily of the C9 component, since samples of purified, isolated C9 can polymerize into cylindrical structures which appear identical with the fully assembled C5b-9 complex. The structure of these poly-C9 molecules has been investigated using the techniques of cryo-electron microscopy and single particle analysis. Sets of single poly-C9 particles viewed as rings were selected from cryo-EM images, then particles were aligned and treated by correspondence analysis to identify the principle interparticle similarities and variations. The highest ranking variation found was the presence or absence of a dense inner ring of protein density. Other important variations were interpreted as different types of particle tilt. These results were used in selecting a subgroup of untilted particles for averaging and symmetry analysis. The rotational power spectrum of the initial average suggested 13-fold symmetry. The 13-fold symmetry was used to select and group particles for further analysis. Individual particles were 13-fold rotational averaged and those with enhanced peripheral features were placed into either a right-handed subgroup or into a left-handed subgroup based on orientation of the peripheral features. Particles within each group were aligned and averaged, and a poly-C9 structure was produced which shows important structural details and from which the C9 monomer structure can be deduced. The poly-C9 structure contains a dense inner ring of diameter between 113-181 A and which is modulated into 13 discrete peaks with peak-to-peak separation of approx. 35 A. The dense inner ring is surrounded by a less dense, concentric outer rim extending to 254 A diameter. The outer rim contains projections that are contiguous with the inner peaks but are skewed relative to the ring radius to produce the appearance of a pin-wheel. These projections correspond with the peripheral features picked up in the rotationally averaged individual particles; the left- or right-handed orientation of projections may result from the up/down orientation of individual particles in ice. The C9 monomer structure within the cylinder is suggested by the density distribution. The monomer would be a rod with diameter of 35 A, oriented parallel to the cylinder axis and would be roughly perpendicular to a membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of Xenopus laevis complement factor I structure--conservation of modular structure except for an unusual insert not present in human factor I

Factor I (C3b/C4b inactivator) is a regulatory protein of the classical and alternative complement pathways. In this paper, we report the sequence of Xenopus factor I cDNA and the deduced protein structure. The basic structure of human preprofactor I, NH2-heavy chain-cleavage peptide-light chain-COOH, is conserved in the frog. However, the frog heavy chain contains a highly charged segment of 29 amino acids, encoded by a poly dA-rich mRNA insert, which is not found in human factor I. The modular structure of the frog heavy chain was analyzed, and found to differ vis-à-vis previously published analyses of human factor I. We also evaluate the timing of factor I transcription during frog embryogenesis.

Linkage between complement components 6 and 7 and glutamic pyruvate transaminase in the marsupial Monodelphis domestica

The sixth and seventh components of complement were found to be polymorphic and tightly linked in the laboratory opossum (Monodelphis domestica), as they are in eutherian mammals. In addition, strong evidence for linkage of the C6-C7 haplotype to the gene for glutamic pyruvate transaminase (GPT) was obtained for females but not for males. This result, combined with previous observations, established as a generality that recombination is severely reduced in females of this species by comparison with males. It also establishes synteny of C6-C7 and GPT in a marsupial species, as exists in mice. Because these loci are not syntenic in humans, the results imply that this synteny is ancestral to the separation of marsupials and eutherians and that it was broken relatively recently in the mammalian lineage leading to human beings. The newly described C6 and C7 polymorphisms provide additional power for developing a linkage map for M. domestica and for localizing genes that confer susceptibility to diseases for which this species is used as a model.

C6 epitope expression by an unrelated antisense cDNA clone: an inadvertent surface-simulation mimotope

A cDNA clone which directs the expression of a fusion protein reacting with anti-C6 antibodies has been isolated and sequenced. A synthetic peptide corresponding to the 14 C-terminal residues of the expressed protein elicited the production of antibodies which are specific for native C6, confirming the presence of a C6 epitope on the expressed protein. However, analysis of the intron-exon boundaries of a corresponding genomic clone revealed that the expression clone is in antisense orientation, and is therefore not C6 cDNA. Comparison of the sequences of the expression clone and expressed protein with those for C6 have not demonstrated any significant sequence homology. It is therefore apparent that what has been cloned is a mimotope for C6 which includes in its continuous sequence an epitope that is conformational in C6 and not represented as a continuous sequence in the C6 molecule. Although this was not the purpose of the investigation, these results confirm that screening random expression libraries with antibodies may be an alternative to the synthetic peptide approach to obtain mimotopes reacting with particular antibodies.

Expression of the components and regulatory proteins of the alternative complement pathway and the membrane attack complex in normal and diseased synovium

We have studied synthesis of the complement components and regulatory proteins of the alternative pathway and the membrane attack complex in synovial membrane. RNA was extracted from synovial tissue of patients with rheumatoid arthritis (RA) or osteoarthritis (OA) as well as from normal synovial membrane. Dot blot analysis showed the presence of mRNAs for all the complement components and regulatory proteins (C3, factor B, factor D, C5, C6, C7, C9, factor H, factor I, S-protein, SP-40, 40, DAF, MCP, CR1, CD59), except for properdin, C8 alpha, C8 beta and C8 gamma in all three types of synovial membrane studied. In an attempt to determine which components were synthesised by each cell type, monocytes (mononuclear phagocytes), human umbilical vein endothelial cells (HUVEC), synovial membrane fibroblasts (from normal, OA and RA synovial membrane) and peripheral blood lymphocytes were cultured in vitro and secretion rates of individual components were measured and total cellular RNA analysed by northern blotting. Monocytes secreted properdin, C3, and factor H but not factor B, factor I, C5, C6, C7, C8 or C9. Fibroblasts and endothelial cells secreted factor B, factor H and factor I, but not properdin, C5, C6, C7, C8 or C9. Lymphocytes did not secrete any of these components. mRNAs encoding C3, factor B, factor H, S-protein, SP-40, 40, MCP and DAF were detected in all three other cell types (monocytes, fibroblasts and HU-VEC), but factor I and CD59 mRNAs were not detected in monocytes. C5, C6, C7, C8 alpha, C8 beta, CD8 gamma and C9 mRNAs were not detected in any of the cell types studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Complete determination of disulfide bonds localized within the short consensus repeat units of decay accelerating factor (CD55 antigen)

Decay accelerating factor (DAF) has 4 SCR (short consensus repeat) units. Each SCR unit consists of approx. 60 amino acids characterized by having four conserved cysteine residues and several other highly conserved residues which include proline, tryptophan, tyrosine/phenylalanine and glycine. To determine the disulfide-bonding pattern, we used the urine form of DAF. After thermolysin and trypsin digestion, we isolated seven disulfide-linked peptides by HPLC purification. Because all of the cysteine residues are disulfide-bonded, DAF should contain eight disulfide bonds. After subtilisin and trypsin digestion, we isolated the eighth disulfide-bonded peptides by HPLC purification. From sequence analyses of these peptides, we could identify all disulfide bonds in the 4 SCR units of DAF as being between the first and the third and between the second and the fourth half-cystines within each SCR unit.

SP-40,40, a protein involved in the control of the complement pathway, possesses a unique array of disulphide bridges

SP-40,40 is a two-chain serum protein which acts in vitro as a potent inhibitor of the assembly of the membrane attack complex of human complement. It contains 10 cysteine residues, the numbers and locations of which are conserved in several mammalian species. Evidence is presented that all the cysteine residues are involved in interchain (alpha-beta) disulphide bonds. There are no free cysteine residues. The disulphide bond motif established in this study for SP-40,40 is unique and bears no obvious homology to those complement components whose disulphide bonds have been assigned, nor is there any homology apparent between SP-40,40 and other multi-chain proteins containing disulphide bonds.

Evolution of the complement system

The ancestral form of the alternative pathway of complement activation probably originated as a primitive independent immune system. Subsequent evolution of an adaptive immune response drove the specialization of the classical pathway to connect antibody-mediated nonself recognition to the complement-dependent effector mechanisms. In this article Timothy Farries and John Atkinson consider how the contemporary complexity arose by a succession of credible alterations at the genetic level, and the selective advantages provided at each step.

Neutron and X-ray scattering studies on the human complement protein properdin provide an analysis of the thrombospondin repeat

Properdin is a regulatory glycoprotein of the alternative pathway of the complement system of immune defense. It is responsible for the stabilization of the C3 convertase complex formed between C3b and the Bb fragment of factor B. Neutron and X-ray solution scattering experiments were performed on the dimeric and trimeric forms of properdin. These have RG values of 9.1 and 10.7 nm, respectively. The scattering curves were compared with Debye sphere modeling simulations for properdin. Good agreements were obtained for models similar to published electron micrographs showing that the properdin trimer has a triangular structure with sides of 26 nm. Such a structure also accounted for sedimentation coefficient data on properdin. Primary structure analyses for mouse and human properdin have shown that this contains six homologous motifs known as the thrombospondin repeat (TSR), which is the second most abundant domain type found in the complement proteins. Sequences for these 12 TSRs were aligned with 19 others found in thrombospondin and the late complement components. Three distinct groups of TSRs were identified, namely, the TSRs found in thrombospondin and properdin, the TSRs mostly found at the N-terminus of the late complement components, and the TSRs found at the C-terminus of the late components. Averaged secondary structure predictions suggested that all three groups contain similar backbone structures with two amphipathic turn regions and one hydrophilic beta-strand region. The mean dimensions of the TSRs of properdin in solution were determined to be approximately 4 nm X 1.7 nm X 1.7 nm, showing that these are elongated in structure.

Cell death mechanisms and the immune system

The immune system provides good models for cell death, a phenomenon now recognized to be of fundamental importance in many fields of biology. Cell death is strikingly polymorphic: it can proceed via necrosis (as in complement-mediated cell death) or apoptosis, but the latter displays different patterns (in the receptor-mediated death of some thymocytes, in cell death mediated by TNF alpha or by cytotoxic T cells), perhaps reflecting different pathways of control of a common core mechanism. Even though there are differences in the morphological and metabolic changes associated with the different patterns of apoptosis, some recurrent sequences of events are observed in almost all dying cells. The metabolic state of a cell often seems to play a major role in determining if and how this cell will die in given external circumstances. The nature of molecules causally involved in the dying cell can now be approached in some systems.

Molecular cloning of the cDNA coding for properdin, a positive regulator of the alternative pathway of human complement

Northern blot analysis indicated that the mRNA for human properdin is approximately 1.5 kb long and that its level in U-937 cells is increased by pretreating the cells with phorbol 12-myristate 13-acetate (PMA). Using a human genomic probe clones coding for human properdin were isolated from a lambda gt10 cDNA library derived from PMA-treated U-937 cells. The sequence of the 1474-bp cDNA insert of the longest clone revealed an open reading fram of 1326 bp coding for the entire 442 amino acids of the mature form of human properdin and 67 bp coding for 22 amino acids of typical, but incomplete leader sequence. Polymerase chain reaction "RACE" experiments identified the start site ATG and revealed the complete, 27-amino acid-long, leader peptide sequence. Within the 81-bp 3' non-translated extension a polyadenylation signal was identified 41 bp downstream from the stop codon, TAA, and 12 bp upstream of a 19 nucleotide long poly(A) tail. The amino acid sequence of human properdin is clearly divided into three distinct regions: a 49 residue-long N-terminal region, a 32 residue-long C-terminal region and a middle region, covering residues 50 to 411, composed of six tandemly repeated thrombospondin repeat (TSR) motifs of the type first described in the adhesive glycoprotein thrombospondin and also known to be present in the C6, C7, C8 alpha, C8 beta and C9 terminal components of complement. Human and mouse properdin sequences show a high (approximately 76%) degree of identity with almost complete conservation of the relatively large number of Cys (44) and Trp (20) residues.