The sequence and topology of human complement component C9

Epitomics: Analysis of Plasma C9 Epitope Heterogeneity in the Plasma of Lung Cancer Patients and Control Subjects

The human proteome is more complex than the genetic code predicts it to be. Epitomics, or protein epitome profiling, is a tool for understanding sub-protein level variation. With the ultimate goal to explore C9 proteoforms and their relevance to lung cancer, here we report plasma C9 epitope-associated molecular heterogeneity in plasma samples of lung cancer patients and control subjects. We show three C9 epitopes (BSI0449, BSI0581, BSI0639) with markedly different association with lung cancer ("unaltered", "upregulated" and "downregulated"). In order to exclude confounding effects, we show first that the three epitope-defining mAbs recognize C9 in purified form and in the natural context, in the human plasma. Then, we present data demonstrating the lack of major epitope interdependence or overlap. The next experiments represent a quest toward the understanding of the molecular basis of apparent disparate association with lung cancer. Using immunochemistry, SDS PAGE and LC-MS/MS technologies, we demonstrate that epitope-specific immunoprecipitates of plasma C9 seem identical regarding peptide sequence. However, we found epitope-specific posttranslational modification and coprecipitated protein composition differences with respect to control and lung cancer plasma. Epitope profiling enabled the classification of hypothetical C9 proteoforms through differential association with lung cancer.

Proteoform Profile Mapping of the Human Serum Complement Component C9 Revealing Unexpected New Features of N-, O-, and C-Glycosylation

The human complement C9 protein (∼65 kDa) is a member of the complement pathway. It plays an essential role in the membrane attack complex (MAC), which forms a lethal pore on the cellular surface of pathogenic bacteria. Here, we charted in detail the structural microheterogeneity of C9 purified from human blood serum, using an integrative workflow combining high-resolution native mass spectrometry and (glyco)peptide-centric proteomics. The proteoform profile of C9 was acquired by high-resolution native mass spectrometry, which revealed the co-occurrence of ∼50 distinct mass spectrometry (MS) signals. Subsequent peptide-centric analysis, through proteolytic digestion of C9 and liquid chromatography (LC)-tandem mass spectrometry (MS/MS) measurements of the resulting peptide mixtures, provided site-specific quantitative profiles of three different types of C9 glycosylation and validation of the native MS data. Our study provides a detailed specification, validation, and quantification of 15 co-occurring C9 proteoforms and the first direct experimental evidence of O-linked glycans in the N-terminal region. Additionally, next to the two known glycosylation sites, a third novel, albeit low abundant, N-glycosylation site on C9 is identified, which surprisingly does not possess the canonical N-glycosylation sequence N-X-S/T. Our data also reveal a binding of up to two Ca²⁺ ions to C9. Mapping all detected and validated sites of modifications on a structural model of C9, as present in the MAC, hints at their putative roles in pore formation or receptor interactions. The applied methods herein represent a powerful tool for the unbiased in-depth analysis of plasma proteins and may advance biomarker discovery, as aberrant glycosylation profiles may be indicative of the pathophysiological state of the patients.

International Union of Basic and Clinical Pharmacology. XCV. Recent advances in the understanding of the pharmacology and biological roles of relaxin family peptide receptors 1-4, the receptors for relaxin family peptides

Relaxin, insulin-like peptide 3 (INSL3), relaxin-3, and INSL5 are the cognate ligands for the relaxin family peptide (RXFP) receptors 1-4, respectively. RXFP1 activates pleiotropic signaling pathways including the signalosome protein complex that facilitates high-sensitivity signaling; coupling to Gα(s), Gα(i), and Gα(o) proteins; interaction with glucocorticoid receptors; and the formation of hetero-oligomers with distinctive pharmacological properties. In addition to relaxin-related ligands, RXFP1 is activated by Clq-tumor necrosis factor-related protein 8 and by small-molecular-weight agonists, such as ML290 [2-isopropoxy-N-(2-(3-(trifluoromethylsulfonyl)phenylcarbamoyl)phenyl)benzamide], that act allosterically. RXFP2 activates only the Gα(s)- and Gα(o)-coupled pathways. Relaxin-3 is primarily a neuropeptide, and its cognate receptor RXFP3 is a target for the treatment of depression, anxiety, and autism. A variety of peptide agonists, antagonists, biased agonists, and an allosteric modulator target RXFP3. Both RXFP3 and the related RXFP4 couple to Gα(i)/Gα(o) proteins. INSL5 has the properties of an incretin; it is secreted from the gut and is orexigenic. The expression of RXFP4 in gut, adipose tissue, and β-islets together with compromised glucose tolerance in INSL5 or RXFP4 knockout mice suggests a metabolic role. This review focuses on the many advances in our understanding of RXFP receptors in the last 5 years, their signal transduction mechanisms, the development of novel compounds that target RXFP1-4, the challenges facing the field, and current prospects for new therapeutics.

Genetic Association of the Porcine C9 Complement Component with Hemolytic Complement Activity

The complement system is a part of the natural immune regulation mechanism against invading pathogens. Complement activation from three different pathways (classical, lectin, and alternative) leads to the formation of C5-convertase, an enzyme for cleavage of C5 into C5a and C5b, followed by C6, C7, C8, and C9 in membrane attack complex. The C9 is the last complement component of the terminal lytic pathway, which plays an important role in lysis of the target cells depending on its self-polymerization to form transmembrane channels. To address the association of C9 with traits related to disease resistance, the complete porcine C9 cDNA was comparatively sequenced to detect single nucleotide polymorphisms (SNPs) in pigs of the breeds Hampshire (HS), Duroc (DU), Berlin miniature pig (BMP), German Landrace (LR), Pietrain (PIE), and Muong Khuong (Vietnamese potbelly pig). Genotyping was performed in 417 F2 animals of a resource population (DUMI: DU×BMP) that were vaccinated with Mycoplasma hyopneumoniae, Aujeszky diseases virus and porcine respiratory and reproductive syndrome virus at 6, 14 and 16 weeks of age, respectively. Two SNPs were detected within the third exon. One of them has an amino acid substitution. The European porcine breeds (LR and PIE) show higher allele frequency of these SNPs than Vietnamese porcine breed (MK). Association of the substitution SNP with hemolytic complement activity indicated statistically significant differences between genotypes in the classical pathway but not in the alternative pathway. The interactions between eight time points of measurement of complement activity before and after vaccinations and genotypes were significantly different. The difference in hemolytic complement activity in the both pathways depends on genotype, kind of vaccine, age and the interaction to the other complement components. These results promote the porcine C9 (pC9) as a candidate gene to improve general animal health in the future.

Relaxin family peptides and their receptors

There are seven relaxin family peptides that are all structurally related to insulin. Relaxin has many roles in female and male reproduction, as a neuropeptide in the central nervous system, as a vasodilator and cardiac stimulant in the cardiovascular system, and as an antifibrotic agent. Insulin-like peptide-3 (INSL3) has clearly defined specialist roles in male and female reproduction, relaxin-3 is primarily a neuropeptide involved in stress and metabolic control, and INSL5 is widely distributed particularly in the gastrointestinal tract. Although they are structurally related to insulin, the relaxin family peptides produce their physiological effects by activating a group of four G protein-coupled receptors (GPCRs), relaxin family peptide receptors 1-4 (RXFP1-4). Relaxin and INSL3 are the cognate ligands for RXFP1 and RXFP2, respectively, that are leucine-rich repeat containing GPCRs. RXFP1 activates a wide spectrum of signaling pathways to generate second messengers that include cAMP and nitric oxide, whereas RXFP2 activates a subset of these pathways. Relaxin-3 and INSL5 are the cognate ligands for RXFP3 and RXFP4 that are closely related to small peptide receptors that when activated inhibit cAMP production and activate MAP kinases. Although there are still many unanswered questions regarding the mode of action of relaxin family peptides, it is clear that they have important physiological roles that could be exploited for therapeutic benefit.

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 and physiologic function of the low-density lipoprotein receptor

The low-density lipoprotein receptor (LDLR) is responsible for uptake of cholesterol-carrying lipoprotein particles into cells. The receptor binds lipoprotein particles at the cell surface and releases them in the low-pH environment of the endosome. The focus of the current review is on biochemical and structural studies of the LDLR and its ligands, emphasizing how structural features of the receptor dictate the binding of low-density lipoprotein (LDL) and beta-migrating forms of very low-density lipoprotein (beta-VLDL) particles, how the receptor releases bound ligands at low pH, and how the cytoplasmic tail of the LDLR interfaces with the endocytic machinery.

Extracellular phosphorylation of C9 by protein kinase CK2 regulates complement-mediated lysis

Ecto-protein kinases (ecto-PK) are expressed on many cell types, both normal and malignant, yet their functions are largely unknown. An ecto-PK capable of phosphorylating the C9 component of the complement system is described. This C9 ecto-PK could be inhibited by TBB, Emodin and DRB, selective inhibitors of protein kinase CK2. Treatment of Raji human B lymphoma cells with these CK2 inhibitors augmented cell killing by Rituximab (anti-CD20 antibodies) and human complement. Analysis of C5b-7-bearing Raji cells showed that extracellular inhibition of the ecto-CK2 enhanced cell lysis by C8 and C9. Blocking of the membrane complement regulator CD59 with monoclonal antibodies further enhanced the effect of the CK2 inhibitors on Raji cell death by complement. C9 ecto-CK2 activity was increased on cancer cells relative to normal fibroblasts and blood cells. Therefore, ecto-CK2 appears to be an additional factor protecting cells from complement-mediated lysis, probably by phosphorylation/inhibition of complement C9.

An abnormal but functionally active complement component C9 protein found in an Irish family with subtotal C9 deficiency

Two independently segregating C9 genetic defects have previously been reported in two siblings in an Irish family with subtotal C9 deficiency. One defect would lead to an abnormal C9 protein, with replacement of a cysteine by a glycine (C98G). The second defect is a premature stop codon at amino acid 406 which would lead to a truncated C9. However, at least one of two abnormal proteins was present in the circulation of the proband at 0.2% of normal C9 concentration. In this study, the abnormal protein was shown to have a molecular weight approximately equal to that of normal C9, and to carry the binding site for monoclonal antibody (mAb) Mc42 which is known to react with an epitope at amino acid positions 412-426, distal to 406. Therefore, the subtotal C9 protein carries the C98G defect. The protein was incorporated into the terminal complement complex, and was active in haemolytic, bactericidal and lipopolysaccharide release assays. A quantitative haemolytic assay indicated even slightly greater haemolytic efficiency than normal C9. Epitope mapping with six antihuman C9 mAbs showed the abnormal protein to react to these antibodies in the same way as normal C9. However, none of these mAbs have epitopes within the lipoprotein receptor A module, where the C98G defect is located. The role of this region in C9 functionality is still unclear. In conclusion, we have shown that the lack of a cysteine led to the production of a protein present in the circulation at very much reduced levels, but which was fully functionally active.

The human complement C9 gene: structural analysis of the 5' gene region and genetic polymorphism studies

C9 is the last of the human complement components creating the membrane attack complex. The single chain serum protein is encoded by a gene located on chromosome 5p13 that is composed of 11 exons. With the aid of inverse PCR, the hitherto unknown regions flanking exon 1 and the 3' part of exon 11 (3'UTR) have been sequenced. A computer-based analysis of the 300-bp region located just upstream of the AUG start codon showed homologies to known DNA modules which affect the transcriptional regulation of certain genes. The most striking of these is a sequence that may substitute the missing TATA box in initiating C9 transcription. In the 3'UTR, three successive polyadenylation signals were found. Although the C9 protein is invariant, four different single nucleotide polymorphisms (SNPs) have been observed at the DNA level by exon-specific PCR and direct sequencing. None of them changes the amino acid composition of the mature protein. Due to a C --> T transition in exon 1 at cDNA position 17, the fifth amino acid of the leader peptide may be either an arginine or a tryptophane. Using either PCR/ RFLP analysis (exons 1 and 11) or allele-specific PCR (intron 1 and exon 4), each polymorphism can be characterized without sequencing. All of the exon 1, intron 1 and exon 11 variants could be detected in small population samples of European, Thai or South American Indian origin. In contrast, the exon 4 C variant was observed only once in a European. The first three SNPs can be combined to designate eight different 'C9 alleles'. Of these, six have actually be found. These data provide strong evidence that several mutation and recombination events occurred in the course of C9 gene evolution.

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.

Vesicle-reconstituted low density lipoprotein receptor. Visualization by cryoelectron microscopy

The low density lipoprotein (LDL) receptor is a key protein for maintaining cellular cholesterol homeostasis by binding cholesterol-rich lipoproteins through their apoB and apoE apoproteins. The LDL receptor is a transmembrane glycoprotein of M(r) approximately 115 kDa; based on its primary sequence, five distinct structural domains have been identified (Yamamoto, T., Davis, C. G., Brown, M. S., Schneider, W. J., Casey, M. L., Goldstein, J. L., and Russell, D. W. (1984) Cell 39, 27-38). As a first step toward providing a structural description of the intact LDL receptor, the receptor has been purified from bovine adrenal cortices, reconstituted into unilamellar egg yolk phosphatidylcholine vesicles, and imaged using cryoelectron microscopy (cryoEM). CryoEM has the advantage of providing images of the reconstituted LDL receptor in its frozen, fully hydrated state. LDL receptor molecules were visualized as elongated, stick-like projections from the vesicle surface with maximum dimensions approximately 120-A length by approximately 45-A width. In some of the images, a short arm (or arms) was visible at the distal end of the stick-like projections. The LDL receptor was labeled via accessible free cysteine residues, probably including that corresponding to Cys-431 of the known full-length sequence of the human LDL receptor. The accessible cysteine was demonstrated using a maleimide-biotin.streptavidin conjugate and confirmed by labeling with monomaleimido-Nanogold. Images obtained by cryoEM showed that the extracellular stick-like domain of the reconstituted LDL receptor was labeled by Nanogold. This combined cryoEM-Nanogold labeling study has provided the first low resolution structural images of the reconstituted, full-length bovine LDL receptor.

Sequence requirements for trafficking of the CRAM transmembrane protein to the flagellar pocket of African trypanosomes

CRAM is a cysteine-rich acidic transmembrane protein, highly expressed in the procyclic form of Trypanosoma brucei. Cell surface expression of CRAM is restricted to the flagellar pocket of trypanosomes, the only place where receptor mediated endocytosis takes place in the parasite. CRAM can function as a receptor and was hypothesized to be a lipoprotein receptor of trypanosomes. We study mechanisms involved in the presentation and routing of CRAM to the flagellar pocket of insect- and bloodstream-form trypanosomes. By deletional mutagenesis, we found that deleting up to four amino acids from the C terminus of CRAM did not affect the localization of CRAM at the flagellar pocket. Shortening the CRAM protein by 8 and 19 amino acids from the C terminus resulted in the distribution of the CRAM protein in the endoplasmic reticulum (ER) (the CRAM protein is no longer uniquely sequestered at the flagellar pocket). This result indicates that the truncation of the CRAM C terminus affected the transport efficiency of CRAM from the ER to the flagellar pocket. However, when CRAM was truncated between 29 and 40 amino acids from the C terminus, CRAM was not only distributed in the ER but also located to the flagellar pocket and spread to the cell surface and the flagellum. Replacing the CRAM transmembrane domain with the invariant surface glycoprotein 65-derived transmembrane region did not affect the flagellar pocket location of CRAM. These results indicate that the CRAM cytoplasmic extension may exhibit two functional domains: one domain near the C terminus is important for efficient export of CRAM from the ER, while the second domain is of importance for confining CRAM to the flagellar pocket membrane.

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.

Phosphorylation of the complement component, C9, by an ecto-protein kinase of human leukemic cells

Ecto-protein kinases (ecto-PK) are surface constituents of many, if not all, animal cell types; normal, transformed or malignant. The occurrence of ecto-PK on the surface of human leukemia cell lines was described [Paas, Y., Fishelson, Z., 1995. Shedding of tyrosine and serine/threonine ecto-PK from human leukemic cells. Arch. Biochem. Biophys. 316 780-788.]. These ecto-PKs have been shown to phosphorylate several exogenous substrates, including the complement C9 protein, an essential component of the terminal complement system. C9 is phosphorylated by ecto-PK of K562 cells on serine residue(s). Phosphorylation occurs in the N-terminal C9a portion produced by cleavage of phosphorylated C9 with human alpha-thrombin. C9 polymers generated upon incubation of C9 with ZnCl2 do not serve as substrates for the K562 ecto-PK. In contrast, unfolded C9, obtained by reduction and alkylation, serves as a superior substrate for the K562 ecto-PK. Native C9 phosphorylation produced a rather low stoichiometry of incorporated phosphate (around 3%) per C9. Despite that, the phosphorylated C9 expressed reduced hemolytic activity. The complement-sensitive variant of K562 (K562/S) did not express the C9 phosphorylating activity. Various PK inhibitors tested failed to block C9 phosphorylation. Only heparin and 2,3-diphosphoglycerate (dpGA) prevented C9 phosphorylation, indicating that the ecto-PK is related to the casein kinase CK2. C9 can be phosphorylated by ecto-PK from other tumor cells, including Jurkat, SK-OV-3 and BT-474. These results suggest that extracellular phosphorylation of C9 may serve as a protective mechanism against complement in tumor cells.

Cloning and characterization of a novel gene encoding a putative transmembrane protein with altered expression in some human transformed and tumor-derived cell lines

Identification and characterization of genes expressed in normal cells and decreased in their malignant counterparts is an important method for detecting candidate tumor suppressors. Using differential display of mRNAs from nontumorigenic infinite life span human fibroblast cell strain MSU-1.1 and an isogenic fibrosarcoma-derived cell line, 6A/SB1, which was derived from chemical carcinogen transformed MSU-1.1 cells, we identified a novel gene, ST7, showing sixfold lower expression in 6A/SB1 cells compared with parental MSU-1.1 cells. Molecular cloning of a near full-length cDNA revealed that the novel gene encodes a putative transmembrane protein composed of 859 amino acids: the 492 N-terminal amino acids including a fivefold cysteine-rich repeat of 40 amino acids homologous to the ligand binding repeat of the known low density lipoprotein receptor, a 24 hydrophobic amino acid stretch spanning the plasma membrane, and a C-terminal domain of 343 residues. ST7 is located on human chromosome 8, band q22.2-23.1, the same locus as the genes involved in acute myeloid leukemia and a locus of high polymorphism in cancer biopsies. The ST7 gene is widely expressed in normal human tissues and is particularly abundant in human heart and skeletal muscle. Northern analysis of 15 tumor cell lines derived from patients and 16 cell lines established from tumors formed in athymic mice by MSU-1.1 cells transformed in culture by various methods showed that 16 of the 31 cell lines have low or undetectable levels of ST7 mRNA. Furthermore, Western blotting analysis using a specific anti-peptide antibody demonstrated that the level of ST7 protein is high in normal fibroblasts and low in 12 sarcoma-derived cell lines tested. Altered expression of ST7 appears to occur at both the transcriptional and post-transcriptional levels. These studies are a first step in characterizing a novel putative receptor protein, whose expression is downregulated in some malignantly transformed cells, and which may play an important role in the transformation process of these cells.

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.

Chiloscyllium plagiosum low-density lipoprotein receptor: evolutionary conservation of five different functional domains

All five functional domains of the low-density lipoprotein (LDL) receptor were assembled in their modern form more than 450 million years ago, as revealed from the cloning and sequencing of an LDL receptor cDNA from Chiloscyllium plagiosum (banded cat shark). The shark LDL receptor has the same overall architecture as the mammalian and amphibian counterparts. Each of the seven cysteine-rich repeats in the ligand binding domain resembles its counterpart in the human LDL receptor more than it does the other repeats in the shark receptor as suggested by the presence of unique "signature" sequences, indicating that these repeats had already acquired their independent structures by the time of shark development. Furthermore, amino acid sequences of the entire ligand binding domain of shark LDL receptor show 35% identity over a stretch of 294 residues with a Lymnaea stagnalis G-protein-linked receptor (LSGLR). The region of homology between these unrelated proteins includes conservation of most of the unique characteristics of the cysteine-rich repeats of LDL receptor at the expected positions in LSGLR. The results presented are consistent with the hypothesis that all seven repeats in the ligand binding domain of LDL receptor may have been lifted directly from an ancestral gene instead of being evolutionary duplications of a single repeat recruited by the primitive LDL receptor from another gene.

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.

Molecular cloning of a gene from Plasmodium falciparum that codes for a protein sharing motifs found in adhesive molecules from mammals and plasmodia

Adhesion of Plasmodium to host cells is an important phenomenon in parasite invasion and in malaria-associated pathology. We report here the molecular cloning of a putative adhesive molecule from P. falciparum that shares both sequence and structural similarities with a sporozoite surface molecule from Plasmodium termed the thrombospondin-related anonymous protein (TRAP) and, to a lesser extent, with the circumsporozoite (CS) protein. The gene, which is present on chromosome 3 as a single copy, was termed CTRP for CS protein-TRAP-related protein. The full-length CTRP encodes a protein containing a putative signal sequence followed by a long extracellular region of 1990 amino acids, a transmembrane domain, and a short cytoplasmic segment. The putative extracellular region of CTRP is defined by two separated adhesive domains. The first domain contains six 210-amino acid-long homologous repeats, the sequence of which is related to the A-type domain found in adhesive molecules including the alpha subunits of several integrins and a number of extracellular matrix glycoproteins. The second domain contains seven repeats of 87-60 amino acids in length, which share similarities with the thrombospondin type 1 domain found in a variety of adhesive molecules. Finally, CTRP also contains consensus motifs found in the superfamily of haematopoietin receptors. Interstrain analysis of eight different parasite isolates revealed that CTRP does not show size polymorphism except in repetitive regions flanking potential adhesive domains.

Genomic organization of human complement protein C8 alpha and further examination of its linkage to C8 beta

Human C8 is one of five complement components (C5b, C6, C7, C8, C9) that interact to form the cytolytic C5b-9 complex on target membranes. It is composed of three nonidentical subunits (C8 alpha, C8 beta, C8 gamma) encoded by separate genes. C8 alpha and C8 beta are linked on chromosome 1p32, whereas C8 gamma is located on 9q22.3-q32. In this study, overlapping genomic clones were isolated and used to decipher the organization of the human C8 alpha gene. The gene contains at least 11 exons spanning approximately 70 kb of DNA. When compared to C6, C8 beta and C9, there is a remarkable similarity in genomic organization, consistent with amino acid sequence comparisons that suggest these proteins are ancestrally related. Regions of each protein that are structurally similar are encoded in exons of correspondingly similar lengths with highly conserved boundaries and phases. Availability of genomic sequence also facilitated a more detailed analysis of C8 alpha and C8 beta linkage. Based on analysis of genomic digests with cDNA probes, the loci were previously reported to be physically linked (< 2.5 kb) and in a 5' alpha-beta 3' orientation. In the present study, results obtained using exon-specific probes indicate the loci are not as closely linked as initially believed. Furthermore, they suggest that cDNA probes used earlier yielded misleading information because they encode exons that are distributed across large segments of genomic DNA.

The functions of the ninth component of human complement are sustained by disulfide bonds with different susceptibilities to reduction

Purified C9 with expected hemolytic and polymerizing activities was found to contain approximately 0.2 mol of sulfhydryl groups/mol of C9. By proteolysis of C9 with labeled SH groups, the SH residues on intact C9 were mapped to Cys-359 and Cys-384 which, presumably, form an intra-domain disulfide bond in the intact molecule. The blocking of these sulfhydryl residues by alkylation, however, had minimal influence on the functions of C9. On the other hand, reduction of C9 by 1 mM dithiothreitol (DTT) (6-fold molar excess over Cys residues) followed by alkylation resulted in a complete block of polymerization activity and a 50% loss of C9 hemolytic activity. In contrast, the ability of C9 to bind EAC1-8 remained largely unaffected. The loss of poly-C9 formation activity correlated with the alkylation of approx. 6 liberated sulfhydryl groups. Hemolytic activity was abolished by treatment with > 5 mM DTT which allowed the liberation of approximately 18 sulfhydryl groups. Most of the DTT-susceptible disulfides were within the C9a fragment (an N-terminal peptide derived by thrombin). Thus, three major functions of C9, EAC1-8 binding, polymerization, and hemolytic activity, are sustained by disulfide bond-dependent conformational motifs with different susceptibility to reducing reagents. The maintenance of the N-terminal C9a region is essential for polymerization, but not EAC1-8 binding activity of C9. Taken together, the results of the present study differentiate in molecular terms several of the functional portions of C9, and stress the significance of intra-chain disulfide linkages in maintaining the structural components necessary for the functions of C9.

Altered glycosylation and selected mutation in recombinant human complement component C9: effects on haemolytic activity

Recombinant wild-type and mutated forms of human complement component C9 have been synthesized in baculovirus-infected insect cells. Wild-type recombinant C9 was indistinguishable from native C9, as judged by haemolytic activity, trypsin and alpha-thrombin digestion, reaction with antibodies to C9, enzymatic deglycosylation to the same core size and polymerization in the presence of Zn2+. Replacement of the native signal peptide with the honey-bee melittin signal peptide, and replacement of Spodoptera frugiperda (Sf9) cells with Trichoplusia ni cells produced yields of 5 micrograms C9/ml supernatant. Three C9 mutants were generated; one mutant, with four acidic residues changed to alanines in a putative calcium-binding site, had the same biological activity as recombinant C9. Another mutant, lacking 23 N-terminal amino acids, previously showing increased polymerization when produced in vitro, polymerized on secretion, rendering it inactive. It was not possible to demonstrate haemolytic activity of the third mutant, cysteines 33 and 36 mutated to alanine, as it was secreted a hundredfold less than the wild-type protein.

The membrane attack complex of complement. Assembly, structure and cytotoxic activity

The membrane attack complex of complement is formed by the molecular fusion of the five terminal complement proteins, C5, C6, C7, C8, and C9. While the assembly process on a target membrane and its modulation by restriction factors present on host cells is now quite well understood the molecular details of the architecture of the complex still need much further clarification. This is especially true for the interaction of the last acting protein C9, which provides the cytotoxic action of the complex, with the precursor C5b-8 complex. Because of this lack of structural details the molecular mechanisms that lead to complement-mediated cell death remain cryptic, however, it is hoped that recent advances in controlling the assembly process and in site-specific modification of the terminal complement proteins by recombinant DNA techniques should change this predicament quickly.

A method for in vitro synthesis of unglycosylated recombinant complement component C9

A method for in vitro synthesis of human complement component C9 has been established in order to generate unglycosylated normal and mutant proteins without the need to sub-clone. One or two step polymerase chain reaction (PCR) was used to add the T7 RNA polymerase promoter and introduce multiple mutations within the cDNA. The cDNA was then transcribed by T7 RNA polymerase and the mRNA translated in a rabbit reticulocyte lysate or wheat germ system. Successful synthesis was confirmed by: the correct size of PCR product DNA on agarose gel electrophoresis, incorporation of [alpha-32P]UTP into mRNA, and formation of [35S]methionine-labelled protein of the correct molecular mass for full length C9. The wheat germ extract generated up to 1.5 micrograms of recombinant C9. This unglycosylated C9 had at least 10% of the haemolytic activity of native C9. Unglycosylated C9 polymerised more readily than the native protein. This spontaneous polymerisation was increased by removal of the first 23 amino acids or mutating two cysteines at positions 33 and 36. This therefore provides a rapid method for screening the effect of multiple mutations on the biological activity and polymerisation of pore forming proteins.

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)

The human complement component C8B gene: structure and phylogenetic relationship

The eighth component of human complement (C8) is a serum protein that consists of three chains (alpha, beta and gamma), encoded by three separate genes, viz., C8A, C8B, and C8G. In serum, the beta-subunit is non-covalently bound to the disulfide-linked alpha-gamma subunit. Using a full-length C8 beta cDNA probe, we isolated several clones from human genomic lambda DNA libraries. Four lambda clones covering the complete cDNA sequence were characterized by TaqI restriction mapping and were "shotgun" subcloned into M13. C8 beta-cDNA-positive clones were partially sequenced to characterize the 12 exons of the gene with sizes from 69 to 347 bp. All intron-exon junctions followed the GT-AG rule. By using polymerase chain reaction (PCR) primers located in the adjacent intron sequences, all 12 exons of the C8B gene could be amplified from genomic DNA. All fragments showed the expected sizes. The sizes of eight introns could be determined by using primer pairs that amplified two exons and the enclosed intron, and by restriction mapping. These analyses and the insert sizes of the genomic lambda clones indicate that the C8B gene has a total size of approximately 40 kb. The polymorphic TaqI site of the C8B gene localized in intron 11 could be demonstrated by direct restriction fragment analysis of a PCR fragment containing exons 11 and 12, and the enclosed intron 11. Homology comparison of the C8B gene with C8A and C9 on the basis of the exon structure confirmed the ancestral relationship known from the protein level.

The size, shape and stability of complement component C9

Electron microscopy of specimens of C9 tilted through 90 degrees visualized this protein to be a globular ellipsoid with dimensions of 77 x 70 x 52 A. To check the congruence of this observation with physical properties of the molecule, hydrodynamic parameters for C9 were determined. From this work a frictional ratio of 1.32 was calculated. C9 was compared with several other proteins of similar frictional ratios whose tertiary structures are known. All examples found of such proteins whose frictional ratios were between 1.26 and 1.37 are either heart-shaped or globular ellipsoids, but none are prolate ellipsoids. By comparison the size and shape of C9 determined by electron microscopy are congruent with its hydrodynamic parameters. Both electron microscopy and physical measurements suggest that the length (110-120 A) of C9 determined by neutron and X-ray scattering experiments is an overestimate. The source of the discrepancy was identified by the demonstration that the high concns of C9 employed in neutron and X-ray scattering work lead to aggregation of the protein. Thus, investigations involving neutron and X-ray scattering were measuring polydisperse solutions of C9. The deduced value of the radius of gyration from that work (33-35 A) is now recognized as being statistical and significantly higher than the correct value of monomeric C9 (26 A), which was calculated from electron microscopy measurements. Also high-resolution electron microscopy clearly visualized poly(C9) to be a barrel-stave construct. These results suggest that monomeric C9 must undergo a major conformational alteration to extend by 55-70 A in order to self-associate laterally in order to fashion the cylindrical poly(C9).

Mutations in the putative lipid-interaction domain of complement C9 result in defective secretion of the functional protein

Complement protein C9 assembles with C5, C6, C7, C8 on the surface of target cells to form the lytic membrane attack complex (MAC). During MAC assembly and insertion into the target membrane, the hydrophilic, globular C9 partially unfolds to expose a hydrophobic lipid interaction domain. Several copies of amphiphilic C9 subsequently polymerize to form the characteristic ring-like MAC. Using a combined photoaffinity label and computer modeling approach, two amphipathic helices in a segment encompassing the amino acids 293-334 have been predicted to interact with membrane lipids. To elucidate the mechanism of C9 lipid binding and insertion, site-directed mutagenesis was used to change the amphipathic character of the helices. While some conservative amino acid replacements such as Thr307 by a Leu were tolerated and yielded fully active C9 when expressed in COS cells, successive changes of Leu305 into Val, Ala, and Glu on the hydrophobic site of the first helix gave rise to only partly or not secreted C9. All non-conservative amino acid replacements introduced on either side of the helices resulted in non-secreted C9 that was subsequently degraded intracellularly, indicating the importance of the correct folding of the presumptive transmembrane domain during biosynthesis. A natural secretion-incompetent mutant was found in which Val293, located in the proposed lipid-binding region, was lacking. Taken together, these findings suggest that the high incidence of homozygous C9 deficiencies may be due to a blockage in intracellular transport and secretion due to point mutations in this 'hot spot' region of the molecule.

Perforin and its role in T lymphocyte-mediated cytolysis

The killing mediated by cytotoxic T lymphocytes (CTL) represents an important mechanism in the immune defence against tumors and virus infections. The lytic mechanism has been proposed to consist of a polarized secretion of granule-stored molecules, occurring on effector-target cell contact. By electron microscopy, membrane deposited, pore-like lesions are detected on the target cell membrane during cytolysis by CTL. These structures resembled strikingly pores formed during complement attack. Granules of CTL isolated by nitrogen cavitation and Percoll gradient centrifugation were shown to retain cytotoxic activity. Further purification of proteins stored in these granules led to the discovery of a membranolytic protein named perforin which was capable of polymerizing into pore-like structures. In addition to this cytolytic protein, a set of serine esterases was found as well as lysosomal enzymes and proteoglycans, whose function are not yet clearly defined. The role of perforin in the cytotoxic process is currently being explored by ablating the active gene in mice.

Analysis of C5b-8 binding sites in the C9 molecule using monoclonal antibodies: participation of two separate epitopes of C9 in C5b-8 binding

C5b-8 binding sites in C9 were examined using mAbs raised against C9. Among 16 mAbs, two, designated P40 and X197, blocked C9-mediated EAC1-8 lysis. C9 pretreated with the mAbs failed to bind to EAC1-8 at 4 degrees C. In addition, the mAbs became inaccessible to the C9 that had been incorporated into EAC1-8 at 4 degrees C. These findings suggest that C9 binding to EAC1-8, but not its membrane spanning or polymerization, is blocked by mAbs. By immunoblotting analysis using alpha-thrombin proteolytic fragments derived from C9 [a N-terminal fragment of mol. wt 25,000 (C9a) and a C-terminal one of mol. wt 37,000 (C9b)] and tryptic fragments of C9 (mol. wts 53,000 (C9a') and 20,000 (C9b')), the epitopes of P40 and X197 were mapped to the N-terminal and C-terminal regions of C9b, respectively. Both P40 and X197 bound to the C9 polymerized with Zn2+ in the fluid phase, whereas X197 but not P40 reacted with the membrane attack complex (MAC) formed on membranes. The results suggest that two distinct epitopes are involved in C9 binding to EAC1-8, and behave in a different manner for globular C9 bound to EAC1-8 at 4 degrees C, C9 assembled in MAC, or poly-C9 induced by Zn2+. These mAbs may be useful in clarifying the conformational states of C9 and in analyzing the molecular interaction between C9 and its inhibitors.

Sequence of the gene encoding an immunodominant microneme protein of Eimeria tenella

A heterodisperse family of antigens, previously detected on sporozoites and merozoites of Eimeria tenella, has been localised to the microneme organelles within the sporozoite. Sequencing of genomic and cDNA clones shows that the gene for this antigen family contains 4 exons separated by 3 short (519, 226 and 156 nucleotides) intervening sequences and that the predicted polypeptide from the longest open reading frame has 4 structural domains. One of these contains 5 copies of the thrombospondin-like motif, previously identified in the partial sequence of the gene, which is conserved in a variety of molecules which have been demonstrated to have adhesive properties. A second domain of the polypeptide has strong similarity to a conserved region that occurs in another group of molecules which have adhesive properties, including the alpha subunits of several integrins, complement factor Bb and a number of extracellular matrix glycoproteins. Overall the antigen resembles the thrombospondin-related anonymous protein identified in the erythrocytic stage of Plasmodium falciparum. The structure of the gene supports a role for this microneme antigen in cell-cell or cell-matrix interactions.