Human complement factor I: analysis of cDNA-derived primary structure and assignment of its gene to chromosome 4

Complement factor I: Regulatory nexus, driver of immunopathology, and therapeutic

Complement factor I (FI) is the nexus for classical, lectin and alternative pathway complement regulation. FI is an 88 kDa plasma protein that circulates in an inactive configuration until it forms a trimolecular complex with its cofactor and substrate whereupon a structural reorganization allows the catalytic triad to cleave its substrates, C3b and C4b. In keeping with its role as the master complement regulatory enzyme, deficiency has been linked to immunopathology. In the setting of complete FI deficiency, a consumptive C3 deficiency results in recurrent infections with encapsulated microorganisms. Aseptic cerebral inflammation and vasculitic presentations are also less commonly observed. Heterozygous mutations in the factor I gene (CFI) have been demonstrated to be enriched in atypical haemolytic uraemic syndrome, albeit with a very low penetrance. Haploinsufficiency of CFI has also been associated with decreased retinal thickness and is a strong risk factor for the development of age-related macular degeneration. Supplementation of FI using plasma purified or recombinant protein has long been postulated, however, technical difficulties prevented progression into clinical trials. It is only using gene therapy that CFI supplementation has reached the clinic with GT005 in phase I/II clinical trials for geographic atrophy.

Role of Supramolecule ErpY-Like Lipoprotein of Leptospira in Thrombin-Catalyzed Fibrin Clot Inhibition and Binding to Complement Factors H and I, and Its Diagnostic Potential

Leptospirosis is one of the most widespread zoonoses caused by pathogenic Leptospira spp. In this study, we report that the LIC11966/ErpY-like lipoprotein is a surface-exposed outer membrane protein exclusively present in pathogenic species of Leptospira The recombinant ErpY (rErpY)-like protein is recognized by the immunoglobulins of confirmed leptospirosis sera of diverse hosts (human, bovine, and canine), suggesting the expression of the native leptospiral surface protein during infection. Circular dichroism of pure rErpY-like protein showed the secondary structural integrity to be uncompromised during the purification process. Analysis of the rErpY-like protein by native polyacrylamide gel electrophoresis, chemical cross-linking, dynamic light scattering, and field emission transmission electron microscopy demonstrated it undergoes supramolecular assembly. The rErpY-like protein can bind to diverse host extracellular matrices, and it presented a saturable and strong binding affinity (dissociation constant [K_D ] of 70.45 ± 4.13 nM) to fibrinogen, a central host plasma component involved in blood clotting. In the presence of the rErpY-like supramolecule, thrombin-catalyzed fibrin clot formation is inhibited up to 7%, implying its role in inhibiting blood coagulation during Leptospira infection. In addition, binding of the rErpY-like supramolecule to complement factors H and I suggests the protein also contributes to Leptospira evading innate host defense during infection by inactivating alternative complement pathways. This study reveals that rErpY-like protein is functionally active in the supramolecular state and performs moonlighting activity under the given in vitro conditions.

Pharmaceutical Machine Learning: Virtual High-Throughput Screens Identifying Promising and Economical Small Molecule Inhibitors of Complement Factor C1s

When excessively activated, C1 is insufficiently regulated, which results in tissue damage. Such tissue damage causes the complement system to become further activated to remove the resulting tissue damage, and a vicious cycle of activation/tissue damage occurs. Current Food and Drug Administration approved treatments include supplemental recombinant C1 inhibitor, but these are extremely costly and a more economical solution is desired. In our work, we have utilized an existing data set of 136 compounds that have been previously tested for activity against C1. Using these compounds and the activity data, we have created models using principal component analysis, genetic algorithm, and support vector machine approaches to characterize activity. The models were then utilized to virtually screen the 72 million compound PubChem repository. This first round of virtual high-throughput screening identified many economical and promising inhibitor candidates, a subset of which was tested to validate their biological activity. These results were used to retrain the models and rescreen PubChem in a second round vHTS. Hit rates for the first round vHTS were 57%, while hit rates for the second round vHTS were 50%. Additional structure⁻property analysis was performed on the active and inactive compounds to identify interesting scaffolds for further investigation.

Consequences of dysregulated complement regulators on red blood cells

The complement system represents the first line of defense that is involved in the clearance of pathogens, dying cells and immune complexes via opsonization, induction of an inflammatory response and the formation of a lytic pore. Red blood cells (RBCs) are very important for the delivery of oxygen to tissues and are continuously in contact with complement proteins in the blood plasma. To prevent complement activation on RBCs, various complement regulatory proteins can be found in plasma and on the cell membrane. RBCs are special cells without a nucleus and having a slightly different make-up of complement regulators than nucleated cells, as membrane cofactor protein (MCP) is not expressed and complement receptor 1 (CR1) is highly expressed. Decreased expression and/or function of complement regulatory proteins may result in unwanted complement activation and accelerated removal of RBCs. This review describes complement regulation on RBCs and the consequences when this regulation is out of balance.

Preliminary genomic survey and sequence analysis of the complement system in non-eutherian mammals

The complement system is a major mediator of the vertebrate immune system, which functions in both innate and specific immune responses. It comprises more than 30 proteins working to remove foreign cells by way of anaphylatoxins, opsonins or the membrane attack complex. Over the last few years, whole genome sequences of non-eutherian mammals (marsupials and a monotreme), the gray short-tailed opossum (Monodelphis domestica), tammar wallaby (Macropus eugenii), Tasmanian devil (Sarcophilus harrisii), koala (Phascolarctos cinereus) and platypus (Ornithorhynchus anatinus), have become publicly available. Using these sequences, we have identified an array of complement components in non-eutherians using online search tools and algorithms. Of 57 complement and complement-related genes investigated, we identified 46 in the gray short-tailed opossum genome, 27 in the tammar wallaby genome, 44 in the Tasmanian devil genome, 47 in the koala genome and 40 in the platypus genome. The results of this study confirm the presence of key complement components in the immune repertoire of non-eutherian mammals and provide a platform for future studies on immune protection in young marsupials.

Complement factor I from flatfish half-smooth tongue (Cynoglossus semilaevis) exhibited anti-microbial activities

Complement factor I (Cfi) is a soluble serine protease which plays a crucial role in the modulation of complement cascades. In the presence of substrate modulating cofactors (such as complement factor H, C4bp, CR1, etc), Cfi cleaves and inactivates C3b and C4b, thereby controlling the complement-mediated processes. In this study, we sequenced and characterized Cfi gene from Cynoglossus Semilaevis (designated as CsCfi) for the first time. The full-length cDNA of CsCfi was 2230 bp in length, including a 98 bp 5'-untranslated region (UTR), a 164 bp 3'-UTR and a 1968 bp open reading frame (ORF). It encoded a polypeptide of 656 amino acids, with a molecular mass of 72.28 kDa and an isoelectric point of 7.71. A signal peptide was defined at N-terminus, resulting in a 626-residue mature protein. Multiple sequence alignment revealed that Cfi proteins were well conserved with the typical modular architecture and identical active sites throughout the vertebrates, which suggested the conserved function of Cfi. Phylogenetic analysis indicated that CsCfi and the homologous Cfi sequences from teleosts clustered into a clade, separating from another clade from the cartilaginous fish and other vertebrates. Tissue expression profile analysis by quantitative real-time PCR (qRT-PCR) showed that CsCfi mRNA constitutively expressed in all tested tissues, with the predominant expression in liver and the lowest in stomach. Temporal expression levels of CsCfi after challenging with Vibrio anguillarum showed different expression patterns in intestine, spleen, skin, blood, head kidney and liver. The recombinant CsCfi (rCsCfi) protein showed broad-spectrum antimicrobial activities against the Gram-positive bacteria Staphylococcus aureus and the Gram-negative bacteria Escherichia coli, Pseudomonas aeruginosa and Shewanella putrefaciens. The research revealed that CsCfi plays an important role in C. Semilaevis immunity.

Molecular characterization and expression analysis of the complement factor I (CpFI) in the whitespotted bamboo shark (Chiloscyllium plagiosum)

Complement factor I (FI) is a plasma serine proteinase that plays an essential role in the modulation of the complement cascade. In the presence of substrate modulating cofactors (Factor H, C4bp, CR1, etc), FI cleaves the activation products of C3 (i.e. C3b) and C4 (i.e. C4b) to limit complement activity. In this study, the full length cDNA of factor I (CpFI) is isolated from the liver of the whitespotted bamboo shark (Chiloscyllium plagiosum). The CpFI cDNA is 2326 bp in length, encoding a protein of 671 amino acids, which shares 72-80% identity with FI molecules of other sharks, higher than the teleosts (37-40%) and mammals (44-47%). The sequence alignment and comparative analysis indicates the FI proteins are well conserved, with the typical modular architecture and identical active sites throughout vertebrate evolution, suggesting the conserved function. However, the additional sequence present between the leader peptide (LP) and the factor I membrane attack complex (FIMAC) domain in other fishes is also found in CpFI, which consists of two kind of tandem repeats. Phylogenetic analysis suggests that CpFI belongs to the elasmobranch clade, in parallel with the higher vertebrates, to form a sister taxa to teleosts. Expression analysis revealed that CpFI is ubiquitously distributed in a variety of tissues, with the constitutive expression in liver, which might reflect the species-specific distribution patterns of FI. Together with earlier reports, the presence of FI in various sharks might suggest the existence of a well-developed complement regulation mechanism in cartilaginous fish.

Complement factor I deficiency: a not so rare immune defect: characterization of new mutations and the first large gene deletion

BACKGROUND

Complement Factor I (CFI) is a serine protease with an important role in complement alternative pathway regulation. Complete factor I deficiency is strongly associated with severe infections. Approximately 30 families with this deficiency have been described worldwide.

PATIENTS AND METHODS

We have studied five new Spanish families suffering from CFI deficiency. From 19 screened people, 7 homozygous, 10 heterozygous and 2 healthy subjects were identified. Clinical, biochemical and genetic descriptions are included.

RESULTS

Molecular studies demonstrated 4 novel mutations in the screened individuals; amongst them, we describe here the first great gene deletion reported in the CFI locus, which includes full exon 2 and part of the large intron 1.

CONCLUSION

CFI deficiency is possibly an underestimated defect and the eventual existence of this deficiency should be tested in those patients exhibiting low C3 and recurrent bacterial infections. We propose a simple diagnostic flowchart to help clinicians in the identification and correct diagnosis of such patients.

A novel blood-based biomarker for detection of autism spectrum disorders

Autism spectrum disorders (ASD) are classified as neurological developmental disorders. Several studies have been carried out to find a candidate biomarker linked to the development of these disorders, but up to date no reliable biomarker is available. Mass spectrometry techniques have been used for protein profiling of blood plasma of children with such disorders in order to identify proteins/peptides that may be used as biomarkers for detection of the disorders. Three differentially expressed peptides with mass-charge (m/z) values of 2020 ± 1, 1864 ± 1 and 1978 ± 1 Da in the heparin plasma of children with ASD that were significantly changed as compared with the peptide pattern of the non-ASD control group are reported here. This novel set of biomarkers allows for a reliable blood-based diagnostic tool that may be used in diagnosis and potentially, in prognosis of ASD.

High complement factor I activity in the plasma of children with autism spectrum disorders

Autism spectrum disorders (ASDs) are neurodevelopmental and behavioural syndromes affecting social orientation, behaviour, and communication that can be classified as developmental disorders. ASD is also associated with immune system abnormality. Immune system abnormalities may be caused partly by complement system factor I deficiency. Complement factor I is a serine protease present in human plasma that is involved in the degradation of complement protein C3b, which is a major opsonin of the complement system. Deficiency in factor I activity is associated with an increased incidence of infections in humans. In this paper, we show that the mean level of factor I activity in the ASD group is significantly higher than in the control group of typically developed and healthy children, suggesting that high activity of complement factor I might have an impact on the development of ASD.

The molecular identification of factor H and factor I molecules in rainbow trout provides insights into complement C3 regulation

The complement system in vertebrates plays a crucial role in the elimination of pathogens. To regulate complement on self-tissue and to prevent spontaneous activation and systemic depletion, complement is controlled by both fluid-phase and membrane-bound inhibitors. One such inhibitor, complement factor I (CFI) regulates complement by proteolytic cleavage of components C3b and C4b in the presence of specific cofactors. Complement factor H (CFH), the main cofactor for CFI, regulates the alternative pathway of complement activation by acting in the breakdown of C3b to iC3b. To gain further insight into the origin of C3 regulation in bony fish we have cloned and characterized the CFI and CFH1 cDNAs in the rainbow trout (Oncorhynchus mykiss). In this study we report the primary sequence, the tissue expression profile, the polypeptide domain architecture and the phylogenetic analysis of trout CFI and CFH1 genes. The deduced amino acid sequences of trout CFI and CFH1 polypeptides exhibit 42% and 32% identity with human orthologs, respectively. RNA expression analysis showed that CFI is expressed differentially in trout tissues, while liver is the main source of CFH1 expression. Our data indicate that factor H and I genes have emerged during evolution as early as the divergence of teleost fish.

The conserved scavenger receptor cysteine-rich superfamily in therapy and diagnosis

The scavenger receptor cysteine-rich (SRCR) superfamily of soluble or membrane-bound protein receptors is characterized by the presence of one or several repeats of an ancient and highly conserved protein module, the SRCR domain. This superfamily (SRCR-SF) has been in constant and progressive expansion, now up to more than 30 members. The study of these members is attracting growing interest, which parallels that in innate immunity. No unifying function has been described to date for the SRCR domains, this being the result of the limited knowledge still available on the physiology of most members of the SRCR-SF, but also of the sequence versatility of the SRCR domains. Indeed, involvement of SRCR-SF members in quite different functions, such as pathogen recognition, modulation of the immune response, epithelial homeostasis, stem cell biology, and tumor development, have all been described. This has brought to us new information, unveiling the possibility that targeting or supplementing SRCR-SF proteins could result in diagnostic and/or therapeutic benefit for a number of physiologic and pathologic states. Recent research has provided structural and functional insight into these proteins, facilitating the development of means to modulate the activity of SRCR-SF members. Indeed, some of these approaches are already in use, paving the way for a more comprehensive use of SRCR-SF members in the clinic. The present review will illustrate some available evidence on the potential of well known and new members of the SRCR-SF in this regard.

Molecular characterization of complement factor I reveals constitutive expression in channel catfish

The complement system in vertebrates plays a crucial role in immune defense via recognition and removal of pathogens. Complement is tightly regulated by a group of both soluble and cell-associated proteins. Complement factor I is a soluble serine protease that regulates multiple pathways in complement activation. In this work, a complement factor I transcript was isolated and sequenced from channel catfish (Ictalurus punctatus) liver after screening expressed sequence tags. The full-length cDNA is comprised of 2284bp in length, encoding a polypeptide of 668 amino acids. The complement factor I protein was found to be well conserved, with similar domain structures and architecture from fish to mammals. The catfish complement factor I exists as a single-copied gene in the catfish genome. Expression analysis revealed that the catfish complement factor I is constitutively expressed in all tissues and leukocyte cell lines tested, indicating its importance as a regulatory enzyme throughout channel catfish. While expression of complement factor I is often found to be in the liver in mammals, it is constitutively expressed in channel catfish and carp throughout in various tissues and organs.

Characterization of shark complement factor I gene(s): genomic analysis of a novel shark-specific sequence

Complement factor I is a crucial regulator of mammalian complement activity. Very little is known of complement regulators in non-mammalian species. We isolated and sequenced four highly similar complement factor I cDNAs from the liver of the nurse shark (Ginglymostoma cirratum), designated as GcIf-1, GcIf-2, GcIf-3 and GcIf-4 (previously referred to as nsFI-a, -b, -c and -d) which encode 689, 673, 673 and 657 amino acid residues, respectively. They share 95% (<or=) amino acid identities with each other, 35.4-39.6% and 62.8-65.9% with factor I of mammals and banded houndshark (Triakis scyllium), respectively. The modular structure of the GcIf is similar to that of mammals with one notable exception, the presence of a novel shark-specific sequence between the leader peptide (LP) and the factor I membrane attack complex (FIMAC) domain. The cDNA sequences differ only in the size and composition of the shark-specific region (SSR). Sequence analysis of each SSR has identified within the region two novel short sequences (SS1 and SS2) and three repeat sequences (RS1-3). Genomic analysis has revealed the existence of three introns between the leader peptide and the FIMAC domain, tentatively designated intron 1, intron 2, and intron 3 which span 4067, 2293 and 2082bp, respectively. Southern blot analysis suggests the presence of a single gene copy for each cDNA type. Phylogenetic analysis suggests that complement factor I of cartilaginous fish diverged prior to the emergence of mammals. All four GcIf cDNA species are expressed in four different tissues and the liver is the main tissue in which expression level of all four is high. This suggests that the expression of GcIf isotypes is tissue-dependent.

Sexual stage adhesion proteins form multi-protein complexes in the malaria parasite Plasmodium falciparum

The sexual phase of the malaria parasite Plasmodium falciparum is accompanied by the coordinated expression of stage-specific adhesive proteins. Among these are six secreted proteins with multiple adhesion domains, termed P. falciparum LCCL domain-containing protein (PfCCp) proteins, which are expressed in the parasitophorous vacuole of the differentiating gametocytes and which are later associated with macrogametes. Although the majority of the PfCCp proteins are implicated in parasite development in the mosquito vector, their functions remain unknown. In the present study we investigated the molecular interactions between the PfCCp proteins during gametocyte development and emergence. Using five different gene-disruptant parasite lines, we show that the lack of one PfCCp protein leads to the loss of other PfCCp family members. Co-immunoprecipitation assays on gametocyte lysates revealed formation of complexes involving all PfCCp proteins, and affinity chromatography co-elution binding assays with recombinant PfCCp domains further indicated direct binding between distinct adhesion domains. PfCCp-coated latex beads bind to newly formed macrogametes but not to gametocytes or older macrogametes 6 or 24 h post-activation. In view of these data, we propose that the PfCCp proteins form multi-protein complexes that are exposed during gametogenesis, thereby mediating cell contacts of macrogametes.

Characterization of mutations in complement factor I (CFI) associated with hemolytic uremic syndrome

Recent studies have identified mutations in the complement regulatory gene factor I (CFI) that predispose to atypical hemolytic uremic syndrome (aHUS). CFI is a two-chain serine protease in which the light chain carries the catalytic domain while the heavy chain's function is unclear. It downregulates the alternative and classical complement pathways by cleaving the alpha' chains of C3b and C4b in the presence of cofactor proteins (known as cofactor activity). Many CFI mutations in aHUS result in low CFI levels with a consequent quantitative defect in complement regulation. In others, the mutant protein is present in normal amounts but the presumed functional deficiency has not yet been defined. In this report we examine the nature of the functional defect in aHUS-associated CFI mutations. The I322T, D501N and D506V mutations reside in the serine protease domain of CFI and result in secreted proteins that lack C3b and C4b cofactor activity. The delTTCAC (1446-1450) mutant leads to a protein that is not secreted. The R299W mutant lies in a region of the CFI heavy chain of no known function. Our assessments demonstrate decreased C3b and C4b cofactor activity, providing evidence that this region is important for cofactor activity. In two other heavy chain mutants and one probable polymorphic variant, no functional deficiency was identified. These defective mutant proteins will result in an inability to appropriately control the complement cascade at sites of endothelial cell injury. The excessive complement activation for a given degree of damage may result in generation of a procoagulant state and aHUS.

Inherited complement regulatory protein deficiency predisposes to human disease in acute injury and chronic inflammatory statesthe examples of vascular damage in atypical hemolytic uremic syndrome and debris accumulation in age-related macular degeneration

In this chapter, we examine the role of complement regulatory activity in atypical hemolytic uremic syndrome (aHUS) and age-related macular degeneration (AMD). These diseases are representative of two distinct types of complement-mediated injury, one being acute and self-limited, the other reflecting accumulation of chronic damage. Neither condition was previously thought to have a pathologic relationship to the immune system. However, alterations in complement regulatory protein genes have now been identified as major predisposing factors for the development of both diseases. In aHUS, heterozygous mutations leading to haploinsufficiency and function-altering polymorphisms in complement regulators have been identified, while in AMD, polymorphic haplotypes in complement genes are associated with development of disease. The basic premise is that a loss of function in a plasma or membrane inhibitor of the alternative complement pathway allows for excessive activation of complement on the endothelium of the kidney in aHUS and on retinal debris in AMD. These associations have much to teach us about the host's innate immune response to acute injury and to chronic debris deposition. We all experience cellular injury and, if we live long enough, will deposit debris in blood vessel walls (atherosclerosis leading to heart attacks and strokes), the brain (amyloid proteins leading to Alzheimer's disease), and retina (lipofuscin pigments leading to AMD). These are three common causes of morbidity and mortality in the developed world. The clinical, genetic, and immunopathologic understandings derived from the two examples of aHUS and AMD may illustrate what to anticipate in related conditions. They highlight how a powerful recognition and effector system, the alternative complement pathway, reacts to altered self. A response to acute injury or chronic debris accumulation must be appropriately balanced. In either case, too much activation or too little regulation promotes undesirable tissue damage and human disease.

Recurrent infections in partial complement factor I deficiency: evaluation of three generations of a Brazilian family

We report here on the evaluation of a factor I-deficient Brazilian family (three generations, 39 members) with strong consanguinity. The complete factor I-deficient patients (n = 3) presented recurrent respiratory infections, skin infections and meningitis; one of them died after sepsis. They presented an impaired total haemolytic activity (CH50), low C3, low factor H and undetectable C3dg/C3d. Partial factor I deficiency was detected in 16 family members (normal low cut-off value was 25 microg/ml). Respiratory infections were the most common clinical occurrence among partial factor I-deficient relatives. Two of them were submitted to nephrectomy following recurrent urinary tract infections. An additional two heterozygous relatives presented with arthritis and rheumatic fever. Apparently, patients with partial factor I deficiency are also at higher risk for recurrent infections. Vaccination against capsulated bacteria and the eventual use of prophylactic antibiotics should be considered individually in this patient group.

The Catalytically Active Serine Protease Domain of Human Complement Factor I

Factor I (fI) is a major regulator of complement. As a protease it has very restricted specificity, cleaving only C3b or C4b in the presence of a cofactor such as factor H (fH). Cleavage of C3b by fI yields iC3b, a major opsonin. The cleavage occurs through the formation of a ternary complex between the enzyme, the substrate, and the cofactor. The catalytic subunit of fI, the SP domain, accommodates substrate recognition and cleavage. The role of the fI heavy chain within the catalysis complex is unknown. Using partial proteolysis and affinity chromatography an intact form of the SP domain was generated and isolated from fI in high yield. fI and the SP domain were found to have similar amidolytic activities but strikingly different proteolytic activities on C3(NH(3)). fI did not cleave C3(NH(3)) in the absence of fH, while in its presence it cleaved C3(NH(3)) rapidly at two sites. The SP domain, however, slowly cleaved C3(NH(3)) in the absence of fH, at more than two sites. Cleavage by the SP domain was inhibited, not stimulated, by fH. Pefabloc SC and antipain inhibited the proteolytic activity of both fI and the SP domain, but suramin inhibited only fI and not the SP domain. The contrast in the proteolytic activities suggests that the heavy chain domains and the cofactor must have roles in orienting the natural substrates and restricting cleavage to the two sites which yield iC3b through a highly specific catalysis.

Human complement factor I does not require cofactors for cleavage of synthetic substrates

Complement factor I (fI) plays a major role in the regulation of the complement system. It circulates in an active form and has very restricted specificity, cleaving only C3b or C4b in the presence of a cofactor such as factor H (fH), complement receptor type 1, membrane cofactor protein, or C4-binding protein. Using peptide-7-amino-4-methylcoumarin derivatives, we investigated the substrate specificity of fI. There is no previous report of synthetic substrate cleavage by fI, but five substrates were found in this study. A survey of 15 substrates and a range of inhibitors showed that fI has specificity similar to that of thrombin, but with much lower catalytic activity than that of thrombin. fI amidolytic activity has a pH optimum of 8.25, typical of serine proteases and is insensitive to ionic strength. This is in contrast to its proteolytic activity within the fI-C3b-fH reaction, in which the pH optimum for C3b cleavage is <5.5 and the reaction rate is highly dependent on ionic strength. The rate of cleavage of tripeptide 7-amino-4-methylcoumarins by fI is unaffected by the presence of fH or C3(NH(3)). The amidolytic activity is inhibited by the synthetic thrombin inhibitor Z-D-Phe-Pro-methoxypropylboroglycinepinanediol ester, consistent with previous reports, and by benzenesulfonyl fluorides such as Pefabloc SC. Suramin inhibits fI directly at concentration of 1 mM. Within a range of metal ions tested, only Cr(2+) and Fe(3+) were found to inhibit both the proteolytic and amidolytic activity of fI.

Expansion of genes encoding complement components in bony fish: biological implications of the complement diversity

The complement system is a major humoral component of vertebrate defenses for tagging and killing target microorganisms. Recent molecular analyses have uncovered a striking feature of bony fish complement, namely that several complement components are encoded by multiple genes. In this review, the structural diversity of C3, C4, C5, factor B, C2, C1r/s and MASP are discussed with special reference to their functional differentiation, mainly focusing on the common carp (Cyprinus carpio), a tetraploidized teleost. In carp, all the members (C3, C4, C5 and a non-complement protein alpha2-macroglobulin) of the thioester-containing protein family are present in multiple isotypes, differing in the primary structures of various functional sites. Three factor B/C2-like isotypes identified in carp showed distinct expression pattern (sites and inducibility), with one behaving as an acute-phase reactant. Two C1r/C1s/MASP2-like isotypes also contain an amino acid substitution that likely affects their substrate specificity. Overall, the present data suggest that the expanded genes of the carp complement system produce more diversified functional components than are known for mammals. The biological significance of this diversity is discussed.

Molecular cloning of the complement regulatory factor I isotypes from the common carp (Cyprinus carpio)

Factor I is a novel serine protease that regulates complement activation. Here we report the complete primary structure of two isotypic factor Is isolated from the common carp ( Cyprinus carpio), a pseudotetraploid teleost. A carp hepatopancreas cDNA library was screened using two RT-PCR-amplified cDNA fragments encoding part of the carp factor I-like serine protease domain. Two distinct cDNA clones, designated FI-A and FI-B, were isolated. Their deduced amino acid sequences share 75.2% identity with each other. FI-A has a typical factor I-like domain organization composed of two disulfide-linked polypeptides (H-chain and L-chain). On the other hand, FI-B contains a novel sequence of 115 amino acids inserted at the N-terminus of the H-chain. Genomic Southern hybridization suggests that FI-A and FI-B are encoded by distinct genes in the carp genome. Expression analysis by RT-PCR revealed that the major site of FI-A expression is the ovary, whereas FI-B expression is detected mainly in the hepatopancreas at a level higher than that of FI-A. The present data, taken together, suggest that carp have duplicated genes coding for factor I, and FI-B with the novel insertion plays a dominant role in the complement system. In addition, homology search of the fugu genome database using the carp FI-A and FI-B sequences identified a putative fugu factor I gene, which has an exon/intron organization different from that of the human orthologue.

Rat complement factor H: molecular cloning, sequencing and quantification with a newly established ELISA

Factor H (FH) is the predominant soluble regulatory protein of the complement system. With a concentration of 300-600 microg/ml in human plasma it acts as a cofactor for the FI-mediated cleavage of the component C3b to iC3b. Furthermore, it competes with factor B for binding to C3b and C3(H2O) and promotes the dissociation of the C3bBb complex (i.e. it has decay accelerating activity). FH is a monomer of about 155 kDa which comprises 20 short consensus repeats (SCR), each of which is composed of nearly 60 amino acid residues. For the screening of a rat liver cDNA library, we used two hybridization probes which had been produced by polymerase chain reaction (PCR). The probes were generated using degenerated primers which corresponded to conserved parts of the human and the murine factor H nucleotide sequences. The entire rat sequence spanned 4240 nucleotides with an open reading frame of 3708 nucleotides. These were preceded by 23 nucleotides of the 5' untranslated region, followed by a stop codon and a 3' untranslated region of 478 nucleotides including the polyadenylation-signal up to the beginning of the poly A tail. Comparison of the rat cDNA-derived coding sequence revealed identities of 74% to the human and 87% to the mouse FH nucleotide sequence. The translation product of rat FH mRNA was 1236 aa in length (leader sequence included) with an identity of 63% to the human and 81.5% to the murine protein. The degree of glycosylation of rat FH-Mr is about 9.5%. To quantitate FH in rat serum and supernatants of primary cultures of rat hepatocytes (HC), a reliable and sensitive sandwich-enzyme-linked immunosorbent assay (ELISA) was established. The concentration of FH in rat serum was calculated to be 238 microg +/- 21 microg/ml (mean +/- SD). Its concentration in the culture supernatants of HC was upregulated about three-fold by interferon (IFN)-gamma (100 U/ml).

Conservation of the modular structure of complement factor I through vertebrate evolution

Mammalian complement factor I plays pivotal roles in the regulation of complement activation and generation of important biological activities from C3. The evolutionary origin of factor I has been unclear except with regard to the molecular cloning of factor I from amphibian Xenopus. Here, we report the identification and characterization of factor I cDNA from the liver of the banded houndshark. The deduced amino acid sequence of shark factor I showed a modular organization that was completely identical to that of mammalian factor I, suggesting the functional conservation of factor I throughout vertebrate evolution. Functionally important amino acid residues such as the basic residues at the processing site and the residues at the active site of the serine protease domain are conserved. Repeated sequences composed of 16 amino acids were inserted at a site between the leader peptide and the factor I/membrane attacking complex module in the shark factor I. This repeat is missing from mammalian and amphibian factor I, and the biological significance of the sequence, if any, is not clear at the moment. There was only one copy of the shark factor I gene, and Northern blotting analysis showed that the shark factor I gene was expressed only in the liver among several organs tested. While the lack of functional data does not exclude the possibility that factor I could have a different function, all these facts, together with the earlier reported data suggest the existence of a well developed complement system in cartilaginous fish.

PSLAP, a protein with multiple adhesive motifs, is expressed in Plasmodium falciparum gametocytes

A gene coding for a protein containing two Scavenger Receptor Cysteine-Rich (SRCR) motifs, four Limulus factor C, Coch-5b2 and Lgl1 (LCCL) motifs; and one Polycystin-1, Lipoxygenase and Alpha Toxin (PLAT) motif was cloned from Plasmodium chabaudi and homologues identified in the P. falciparum and P. yoelii genome data bases. At least one of these sequence motifs (SRCR) has adhesive properties in other proteins, therefore, we propose to name this protein PSLAP for Plasmodium SRCR, LCCL Adhesive-like Protein. Southern blotting and chromosome analysis showed that pslap is a single copy gene on chromosome 14 in P. falciparum 3D7. pslap mRNA is strongly expressed in P. falciparum gametocytes, but was undetectable on Northern blots of RNA from the asexual blood stages. Polyclonal antibodies raised to different parts of PSLAP detected a protein expressed in late gametocytes, but not in the early stages of gametocytogenesis or asexual blood stages of P. falciparum. We suggest that PSLAP functions in the mosquito, for example, in modulation of the invertebrate host immune response or in protection against complement factors in the blood meal.

Proteomic analysis of differential protein expression in human nasopharyngeal carcinoma cells induced by NAG7 transfection

Nasopharyngeal carcinoma (NPC) is a commonly occurring tumor in southern China and south east Asia. A genetic factor has now been recognized to be associated with this cancer. A new gene, named NAG7, was cloned from the common minimal deletion region in 3p25.3-26.3. In order to investigate the function of NAG7 gene, proteomic methods were used to find and identify the differential proteins and expected to elucidate the mechanism of NAG7. The NAG7 eukaryotic expression vector was constructed and transfected into NPC cell line HNE1 with liposome. Twenty-two differential protein spots in transfected cells were found significant and reproducible using high-resolution two-dimensional electrophoresis. Nine proteins that were up-regulated and seven proteins that were down-regulated were identified by matrix assisted laser desorption/ionization time of flight mass spectrometry and database analysis. These proteins included growth arrest specific protein, DNA binding protein, caspase 6, pinch protein and ras-related protein rab-36, which are involved in cell cycling, transcription regulation, signaling pathways and apoptosis. NAG7 may exert its functions by mediating differential expression of these proteins.

Constitutive expression and regulation of rat complement factor H in primary cultures of hepatocytes, Kupffer cells, and two hepatoma cell lines

The 155-kd soluble complement regulator factor H (FH), which consists of 20 short consensus repeats, increases the affinity of complement factor I (FI) for C3b by about 15 times. In addition to its cofactor activity, it prevents factor B from binding to C3b and promotes the dissociation of the C3bBb complex. The primary site of synthesis of FH, as well as of FI, is the liver, but the cell types responsible for the hepatic synthesis of both factors have not yet been clearly identified. In contrast to FI-mRNA, which was detectable only in hepatocytes (HC), FH-specific mRNA was identified in both HC and Kupffer cells (KC). As calculated for equal amounts of mRNA isolated from both cell types, FH-specific mRNA was found to be nearly 10-fold higher in KC than in HC, leading to the conclusion that KC are an abundant source of FH. Of the investigated proinflammatory cytokines IL-6, TNF-alpha, IL-1beta, and IFN-gamma, only IFN-gamma up-regulated FH-specific mRNA up to 6-fold in both primary HC and KC. This was also demonstrable on the protein level. However, FH-specific mRNA was not inducible in the rat hepatoma cell line H4IIE, which did not express FH-specific mRNA and could not be up-regulated in FAO cells that constitutively expressed FH-specific mRNA. This demonstrates that transformed cell lines do not reflect FH regulation in isolated primary HC. In addition to IFN-gamma, the endotoxin lipopolysaccharide (LPS) up-regulated FH-specific mRNA nearly 10-fold in KC after stimulation at concentrations of 10 or 1 ng/ml. In contrast, concentrations of up to 2 microg LPS/ml did not show any effect on HC. Our data suggest that LPS does not regulate the expression of FH in HC.

Complement factor I is upregulated in rat hepatocytes by interleukin-6 but not by interferon-gamma, interleukin-1beta, or tumor necrosis factor-alpha

Complement 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 and thus prevents the assembly of the C3 and C5 convertases. We have investigated the proinflammatory cytokines IL-6, IL-1beta, TNF-alpha and IFN-gamma for their potential role in the regulation of FI expression. Of the investigated cytokines, only IL-6 increased the FI-specific RT-PCR signal in isolated hepatocytes, in the two rat hepatoma-derived cell lines FAO and H4IIE or in HUVECs. Quantitative competitive RT-PCR showed an IL-6 induced upregulation of FI-specific mRNA by about ten-fold. These data are in accord with Northern blot analyses in which the FI-mRNA was upregulated by IL-6 between five- and seven-fold. IL-6, but not IL-1beta, TNF-alpha or IFN-gamma also increased FI-protein levels in cell culture supernatants by about five-fold as determined by a semiquantitative immunoblot using a novel monoclonal antibody specific for rat FI.

Inherited complete factor I deficiency associated with systemic lupus erythematosus, higher susceptibility to infection and low levels of factor H

Here we describe two new cases of complete deficiency of factor I (fI) in two sisters from a consanguineous Brazilian family. The eldest sibling (20-year-old) developed systemic lupus erythematosus (SLE) early during childhood while the youngest had been committed on several occasions owing to repeated infections although she was asymptomatic for auto-immune diseases. We also detected lower concentrations of C3 and factor B in both sisters. Biological functions dependent on complement activation such as the production of opsonins and killing of phagocytozed micro-organisms, chemotactic factors and haemolytic activity were all significantly reduced in both probands. Consistent with the absence of fI and low levels of fH, a deregulated production of C3b was observed by bidimensional electrophoresis in sera of both the probands.

Cloning of a novel scavenger receptor cysteine-rich type I transmembrane molecule (M160) expressed by human macrophages

We report the cloning of a novel human type I cell surface Ag mainly expressed by macrophages. The primary structure was established by molecular cloning, which yielded a 4579-bp cDNA sequence encoding a polypeptide chain of 1453 amino acid residues with 16 potential N:-glycosylation sites. We designated this molecule M160. The domain organization features 12 scavenger receptor cysteine-rich domains followed by a transmembrane region and a cytoplasmic domain that occurs in two forms, a predominant form (M160-alpha) of 71 residues and an alternatively spliced form (M160-ss) of 39 residues. M160-alpha contains three possible phosphorylation sites, which are lost in the alternatively spliced form. RT-PCR analyses showed M160 to be expressed by alveolar macrophages and by the monocyte cell lines HL60, U937, and THP1, but not by Jurkat or Raji cells. Stimulation of U937 cells with phorbol ester resulted in an increased expression of M160 from day 5 onward. RT-PCR analysis of 19 different human tissues showed signals for M160-alpha of varying intensity in all tissues, whereas M160-ss was confined to the spleen. We conclude that M160 is a new member of the scavenger receptor cysteine-rich superfamily expressed by the monocyte/macrophage cell lineage.

Dynamic expression of multiple scavenger receptor cysteine-rich genes in coelomocytes of the purple sea urchin

Coelomocytes, the heterogeneous population of sea urchin putative immune cells, were found to express a complex set of transcripts featuring scavenger receptor cysteine-rich (SRCR) repeats. SRCR domains define a metazoan superfamily of proteins, many of which are implicated in development and regulation of the immune system of vertebrates. Coelomocytes transcribe multiple SRCR genes from among a multigene family encoding an estimated number of 1,200 SRCR domains in specific patterns particular to each individual. Transcription levels for given SRCR genes may range from pronounced to undetectable, yet all tested animals harbor the genomic loci encoding these genes. Analysis of several SRCR genes revealed multiple loci corresponding to each type. In the case of one SRCR type, a cluster of at least three genes was detected within a 133-kb bacterial artificial chromosome insert, and conserved as well as unique regions were identified in sequences of three genomic clones derived from a single animal. Array hybridizations with repeated samples of coelomocyte messages revealed substantial alterations in levels of expression of many SRCR genes, with fluctuations of up to 10-fold in 1 week and up to 30-fold over a period of 3 months. This report is the first demonstration of genomic and transcriptional complexity in molecules expressed by invertebrate coelomocytes. The mechanisms controlling SRCR gene expression and the functional significance of this dynamic system await elucidation.

Function of human factor H and I on xenosurface

The cell membrane-bound forms of mini-factor H with 1-4 short consensus repeats (fH-PI) and factor I (fI-PI) were constructed. Swine endothelial cell (SEC) lines and Chinese hamster ovary (CHO) cell expressing fH-PI or fI-PI were established and confirmed by flow cytometry. The cell lysate of the SEC line expressing fH-PI showed strong cofactor activity for the cleavage of C3b, and fI-PI demonstrated the protease activity for C4b and C3b not only in the fluid phase but also on the cell membrane. In addition, fH-PI blocked human complement-mediated cell lysis by approximately 30-40%. An SEC line with a low expression of fI-PI showed a weak inhibition of cell lysis in human serum, whereas a CHO cell transfectant with a high expression of fI-PI showed over a 60% inhibition of cell lysis. The results suggest that fH-PI and fI-PI have potential for use in clinical xenotransplantation.

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.

C3a is made by proximal tubular HK-2 cells and activates them via the C3a receptor

BACKGROUND

Some individual components of complement are synthesized by the kidney. However, it is not known whether these form functional pathways that are able to mediate more fundamental cellular events. We examined the ability of HK-2 tubular cells to produce an intact alternative pathway of complement and to respond to the C3a fragment thus produced through the C3a receptor.

METHODS

The production of mRNA for alternative pathway components was detected by reverse transcription-polymerase chain reaction, whereas protein synthesis was investigated by probing Western blots of concentrated culture supernatants with polyclonal antisera. Levels of C3a and inositol phosphate produced by HK-2 cells were determined by radioimmunoassay, whereas those of transforming growth factor-beta1 (TGF-beta1) were measured by ELISA. Intracellular tyrosine phosphorylation in response to C3a was evaluated by Western blotting and chemiluminescence.

RESULTS

HK-2 cells produce the complement polypeptides C3a, C3, and factors B and H. They also contain mRNA for all components of the alternative pathway and the C3a receptor. mRNA levels were up-regulated by interleukin-1alpha, interleukin-1beta, and tumor necrosis factor-alpha. Incubation of HK-2 cells with C3a led to an increase in intracellular inositol phosphate and to tyrosine phosphorylation of at least two proteins in a pertussis-toxin-sensitive fashion. C3a and C3a desarg also up-regulated the secretion of TGF-beta1 by these cells.

CONCLUSION

HK-2 cells produce an intact alternative pathway of complement. In addition, both locally produced and urinary C3a have the potential to activate these cells, resulting in inflammatory events such as TGF-beta1 production.

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.

An initiator element and a proximal cis-acting sequence are essential for transcriptional activation of the complement factor I (CFI) gene

Human complement factor I (CFI) is a serine protease which regulates the complement system by inactivation of C3b and C4b in the presence of appropriate cofactors. In this study, we have analyzed the mechanism controlling the constitutive transcriptional activation of the CFI gene. Using deletion analysis and transient CAT expression assays, we have mapped the minimal promoter to the region located between -46 and +160 bp relative to the major transcription start point (tsp), and also shown that cis-acting elements both upstream and downstream of the tsp are important for promoter activity. A silencer element was also found between -71 and -46 bp. The sequence surrounding the tsp was related to the mouse terminal deoxynucleotidyltransferase initiator element (Inr) and point mutations in this sequence, from -3 to +4, drastically reduced CFI promoter activity. Mutations in a -9 to -5 bp CTGAA sequence immediately upstream of the tsp also reduced promoter activity. Gel mobility shift analysis demonstrated the binding of nuclear factors to a CTGAA repeat located at -9 to -5 and +101 to +105. Our results suggest that CFI promoter contains a functional Inr element that is essential for promoter activity, and the interactions of the CTGAA element located between -9 and +5 with nuclear factor(s) may be part of the machinery required for CFI Inr-dependent transcription.

Genomic organization and chromosomal localization of the human CD163 (M130) gene: a member of the scavenger receptor cysteine-rich superfamily

The human protein CD163 (M130) is a member of the scavenger receptor cysteine-rich (SRCR) superfamily, which is exclusively expressed by monocytes and macrophages. Here, we investigated the genomic organization and the chromosomal localization of the human CD163 gene. The CD163 gene is composed of 17 exons and 16 introns and spans over 35 kb. Each of its nine SRCR domains is encoded by a separate exon, which is similar to other members of the group B SRCR subfamily. Two cytoplasmic variants of CD163 arise from alternative splicing of intron 15, while a truncated and an extracellular variant results from alternative splicing of intron 5 or intron 7, respectively. Using fluorescence in situ hybridization we mapped this gene to the human chromosome 12p13. The transcription initiation sites of the CD163 gene were determined and the 5'-flanking region was sequenced. The nucleotide analysis revealed several putative binding sites for transcription factors, which have been shown to play an important role in myeloid specific gene expression. In addition, we identified a L1 element located 1.4 kb upstream of the major transcription initiation site.

Transcriptional and post-transcriptional regulation of complement factor I (CFI) gene expression in Hep G2 cells by interleukin-6

We have investigated the effects of IL-1 and IL-6 on human complement factor I (CFI) production by Hep G2 cells. IL-6 treatment caused a dose- and time-dependent increase in CFI secretion while IL-1 did not demonstrate such effects. The increase in CFI synthesis correlated with increase in CFI mRNA levels. The half-life of CFI mRNA in untreated cells was approx. 23 h and this was increased to 31 h (26% increase) following induction with IL-6. The IL-6 induced increase in CFI gene expression was inhibited by actinomycin D indicating regulatory effects at the level of transcription. Nuclear run-on experiments showed that IL-6 increased the rate of CFI gene transcription 4.2-fold. Transient transfection analysis of chloramphenicol acetyltransferase reporter gene constructs containing truncated segments of the 5'-flanking region of CFI gene showed that the cis-acting sequence(s) controlling the IL-6 inducible transcription resides in an 83 bp region located between -738 bp and -655 bp relative to the transcription start site. Our results indicate that the upregulation of CFI gene expression by IL-6 involves a coordinate effort at the level of transcription and mRNA stability, with the enhanced rate of transcription being the principal mechanism.

Possible arrangement of the five domains in human complement factor I as determined by a combination of X-ray and neutron scattering and homology modeling

Human factor I is a multidomain plasma serine protease with one factor I-membrane attack complex (FIMAC) domain, one CD5 domain, two low-density lipoprotein receptor (LDLr) domains, and one serine protease (SP) domain and is essential for the regulation of complement. The domain arrangement in factor I was determined by X-ray and neutron scattering on serum-derived human factor I (sFI) and recombinant insect cell factor I (rFI). While the radii of gyration of both were the same at 4.05 nm and both had overall lengths of 14 nm, the cross-sectional radii of gyration were different at 1.70 nm for sFI and 1.57 nm for rFI. This difference was attributed to their different means of glycosylation which is complex-type for sFI and high-mannose-type for rFI. Homology models were constructed for the FIMAC, LDLr, and SP domains of factor I using related crystal structures, and CD5 was represented as a globular protein by referencing its electron microscopy dimensions. In these models, 38 of the 40 Cys residues in factor I were predicted to form internal disulfide bridges. The two remaining Cys residues at the N terminus of the FIMAC domain and at the center of the first LDLr domain were potentially not bridged. It was postulated that, if these two Cys residues were bridged to each other, the FIMAC, CD5, and LDLr-1 domains would form a compact triangular arrangement. This hypothesis was tested by automated scattering curve fit searches based on 9600 bilobal models, setting the FIMAC, CD5, and LDLr-1 domains as one lobe and the large SP domain as the other lobe. The searches gave a single small family of similar structures with a separation of 5.9 nm between the centers of the lobes which gave similar good X-ray and neutron fits for both sFI and rFI, despite the different glycosylations of sFI and rFI. These best-fit structures for factor I showed that this domain model is plausible, and suggested that the SP and the CD5 and LDLr-1 domains may present exposed surfaces in factor I whose roles are to interact separately with its substrates C3b and C4b and with cofactor proteins.

A role for scavenger receptors in phagocytosis of protein-coated particles in rainbow trout head kidney macrophages

In macrophages of higher vertebrates, Fc receptors and receptors for complement and other serum factors, are generally known to enhance the phagocytic process. In lower vertebrates like salmonid fishes, none of these or other phagocytic receptors have been thoroughly characterized. The purpose of this study was to elucidate to what extent these and other receptors are involved in the process of phagocytosis in rainbow trout (Oncorhynchus mykiss) head kidney macrophages. We used tosyl activated, paramagnetic dynabeads (2.8 microm in diameter), specifically coated with 125I labeled Atlantic salmon (Salmo salar) IgM or bovine serum albumin (BSA) as phagocytic probes. The effect of complement opsonization was also investigated by incubating the beads in serum. Our results indicate that neither the Fc- nor the complement-receptor(s) were important for phagocytosis of these beads. Our data support the idea that scavenger receptors are involved in phagocytosis in rainbow trout head kidney macrophages, as the use of a competitive scavenger receptor ligand extensively decreased degradation of the labeled protein coat on the beads.

New enzyme lineages by subdomain shuffling

Protein functions have evolved in part via domain recombination events. Such events, for example, recombine structurally independent functional domains and shuffle targeting, regulatory, and/or catalytic functions. Domain recombination, however, can generate new functions, as implied by the observation of catalytic sites at interfaces of distinct folding domains. If useful to an evolving organism, such initially rudimentary functions would likely acquire greater efficiency and diversity, whereas the initially distinct folding domains would likely develop into single functional domains. This represents the probable evolution of the S1 serine protease family, whose two homologous beta-barrel subdomains assemble to form the binding sites and the catalytic machinery. Among S1 family members, the contact interface and catalytic residues are highly conserved whereas surrounding surfaces are highly variable. This observation suggests a new strategy to engineer viable proteins with novel properties, by swapping folding subdomains chosen from among protein family members. Such hybrid proteins would retain properties conserved throughout the family, including folding stability as single domain proteins, while providing new surfaces amenable to directed evolution or engineering of specific new properties. We show here that recombining the N-terminal subdomain from coagulation factor X with the C-terminal subdomain from trypsin creates a potent enzyme (fXYa) with novel properties, in particular a broad substrate specificity. As shown by the 2.15-A crystal structure, plasticity at the hydrophobic subdomain interface maintains activity, while surface loops are displaced compared with the parent subdomains. fXYa thus represents a new serine proteinase lineage with hybrid fX, trypsin, and novel properties.