A functional model of the human C1 complex Emergence of a functional model

The Ehlers-Danlos syndromes

The Ehlers-Danlos syndromes (EDS) are a heterogeneous group of hereditary disorders of connective tissue, with common features including joint hypermobility, soft and hyperextensible skin, abnormal wound healing and easy bruising. Fourteen different types of EDS are recognized, of which the molecular cause is known for 13 types. These types are caused by variants in 20 different genes, the majority of which encode the fibrillar collagen types I, III and V, modifying or processing enzymes for those proteins, and enzymes that can modify glycosaminoglycan chains of proteoglycans. For the hypermobile type of EDS, the molecular underpinnings remain unknown. As connective tissue is ubiquitously distributed throughout the body, manifestations of the different types of EDS are present, to varying degrees, in virtually every organ system. This can make these disorders particularly challenging to diagnose and manage. Management consists of a care team responsible for surveillance of major and organ-specific complications (for example, arterial aneurysm and dissection), integrated physical medicine and rehabilitation. No specific medical or genetic therapies are available for any type of EDS.

Branching Analysis of Multivalent Conjugates Using Size Exclusion Chromatography-Multiangle Light Scattering

Multivalent conjugates (MVCs) (conjugation of multiple proteins to a linear polymer chain) are powerful for improving the bioactivity and pharmacokinetics of a bioactive molecule. Since this effect is highly dependent upon the valency of the conjugated proteins, it is imperative to have a technique for analysis of the conjugation ratio. Studies of MVCs have used size exclusion chromatography-multiangle light scattering (SEC-MALS), which allows for the separate and individual analysis of the protein and biopolymer components based on their specific refractive index increment and UV extinction coefficient constants to determine the number of proteins bound per biopolymer molecule. In this work, we have applied traditional branching analysis to the SEC-MALS data, with the primary assumption that the polymer backbone can be used as the linear counterpart. We demonstrated good agreement between the branching values and the valency determined by traditional analysis, demonstrating that branching analysis can be used as an alternative technique to approximate the valency of MVCs. The branching analysis method also provides a more complete picture of the distribution of the measured values, provides important branching information about the molecules, and lowers the cost and complexity of the characterization. However, since MVC molecules are both conjugate molecules and branched molecules, the most powerful approach to their characterization would be to use both traditional multivalent conjugate analysis and branching analysis in conjunction.

Molecular Basis of Assembly and Activation of Complement Component C1 in Complex with Immunoglobulin G1 and Antigen

The classical complement pathway contributes to the natural immune defense against pathogens and tumors. IgG antibodies can assemble at the cell surface into hexamers via Fc:Fc interactions, which recruit complement component C1q and induce complement activation. Biophysical characterization of the C1:IgG complex has remained elusive primarily due to the low affinity of IgG-C1q binding. Using IgG variants that dynamically form hexamers efficient in C1q binding and complement activation, we could assess C1q binding in solution by native mass spectrometry and size-exclusion chromatography. Fc-domain deglycosylation, described to abrogate complement activation, affected IgG hexamerization and C1q binding. Strikingly, antigen binding by IgG hexamers or deletion of the Fab arms substantially potentiated complement initiation, suggesting that Fab-mediated effects impact downstream Fc-mediated events. Finally, we characterized a reconstituted 2,045.3 ± 0.4-kDa complex of intact C1 bound to antigen-saturated IgG hexamer by native mass spectrometry, providing a clear visualization of a complete complement initiation complex.

Conjugation of proteins to polymer chains to create multivalent molecules

The activation of cellular signaling cascades, critical for regulating cell function and fate, often involves changes in the organization of receptors in the cell membrane. Using synthetic multivalent ligands to control the nanoscale organization of cellular receptors into clusters is an attractive approach to elicit desired downstream cellular responses, since multivalent ligands can be significantly more potent than their corresponding monovalent ligands. Synthetic multivalent ligands can serve as both versatile biological tools and potent nanoscale therapeutics, for example in applications to harness them to control stem cell fate in vitro and in vivo. Here we describe the use of recombinant protein expression and bioconjugate chemistry to synthesize multivalent ligands that have the potential to regulate cell signaling in a variety of cell types.

Deciphering the fine details of c1 assembly and activation mechanisms: "mission impossible"?

The classical complement pathway is initiated by the large (~800 kDa) and flexible multimeric C1 complex. Its catalytic function is triggered by the proteases hetero-tetramer C1r2s2, which is associated to the C1q sensing unit, a complex assembly of 18 chains built as a hexamer of heterotrimers. Initial pioneering studies gained insights into the main architectural principles of the C1 complex. A dissection strategy then provided the high-resolution structures of its main functional and/or structural building blocks, as well as structural details on some key protein–protein interactions. These past and current discoveries will be briefly summed up in order to address the question of what is still ill-defined. On a functional point of view, the main molecular determinants of C1 activation and its tight control will be delineated. The current perspective remains to decipher how C1 really works and is controlled in vivo, both in normal and pathological settings.

Mapping surface accessibility of the C1r/C1s tetramer by chemical modification and mass spectrometry provides new insights into assembly of the human C1 complex

C1, the complex that triggers the classic pathway of complement, is a 790-kDa assembly resulting from association of a recognition protein C1q with a Ca(2+)-dependent tetramer comprising two copies of the proteases C1r and C1s. Early structural investigations have shown that the extended C1s-C1r-C1r-C1s tetramer folds into a compact conformation in C1. Recent site-directed mutagenesis studies have identified the C1q-binding sites in C1r and C1s and led to a three-dimensional model of the C1 complex (Bally, I., Rossi, V., Lunardi, T., Thielens, N. M., Gaboriaud, C., and Arlaud, G. J. (2009) J. Biol. Chem. 284, 19340-19348). In this study, we have used a mass spectrometry-based strategy involving a label-free semi-quantitative analysis of protein samples to gain new structural insights into C1 assembly. Using a stable chemical modification, we have compared the accessibility of the lysine residues in the isolated tetramer and in C1. The labeling data account for 51 of the 73 lysine residues of C1r and C1s. They strongly support the hypothesis that both C1s CUB(1)-EGF-CUB(2) interaction domains, which are distant in the free tetramer, associate with each other in the C1 complex. This analysis also provides the first experimental evidence that, in the proenzyme form of C1, the C1s serine protease domain is partly positioned inside the C1q cone and yields precise information about its orientation in the complex. These results provide further structural insights into the architecture of the C1 complex, allowing significant improvement of our current C1 model.

Identification of the C1q-binding Sites of Human C1r and C1s: a refined three-dimensional model of the C1 complex of complement

The C1 complex of complement is assembled from a recognition protein C1q and C1s-C1r-C1r-C1s, a Ca(2+)-dependent tetramer of two modular proteases C1r and C1s. Resolution of the x-ray structure of the N-terminal CUB(1)-epidermal growth factor (EGF) C1s segment has led to a model of the C1q/C1s-C1r-C1r-C1s interaction where the C1q collagen stem binds at the C1r/C1s interface through ionic bonds involving acidic residues contributed by the C1r EGF module (Gregory, L. A., Thielens, N. M., Arlaud, G. J., Fontecilla-Camps, J. C., and Gaboriaud, C. (2003) J. Biol. Chem. 278, 32157-32164). To identify the C1q-binding sites of C1s-C1r-C1r-C1s, a series of C1r and C1s mutants was expressed, and the C1q binding ability of the resulting tetramer variants was assessed by surface plasmon resonance. Mutations targeting the Glu(137)-Glu-Asp(139) stretch in the C1r EGF module had no effect on C1 assembly, ruling out our previous interaction model. Additional mutations targeting residues expected to participate in the Ca(2+)-binding sites of the C1r and C1s CUB modules provided evidence for high affinity C1q-binding sites contributed by the C1r CUB(1) and CUB(2) modules and lower affinity sites contributed by C1s CUB(1). All of the sites implicate acidic residues also contributing Ca(2+) ligands. C1s-C1r-C1r-C1s thus contributes six C1q-binding sites, one per C1q stem. Based on the location of these sites and available structural information, we propose a refined model of C1 assembly where the CUB(1)-EGF-CUB(2) interaction domains of C1r and C1s are entirely clustered inside C1q and interact through six binding sites with reactive lysines of the C1q stems. This mechanism is similar to that demonstrated for mannan-binding lectin (MBL)-MBL-associated serine protease and ficolin-MBL-associated serine protease complexes.

Trimeric reassembly of the globular domain of human C1q

C1q is a versatile recognition protein which binds to a variety of targets and consequently triggers the classical pathway of complement. C1q is a hetero-trimer composed of three chains (A, B and C) arranged in three domains, a short N-terminal region, followed by a collagenous repeat domain that gives rise to the formation of (ABC) triple helices, each ending in a C-terminal hetero-trimeric globular domain, called gC1q, which is responsible for the recognition properties of C1q. The mechanism of the trimeric assembly of C1q and in particular the role of each domain in the process is unknown. Here, we have investigated if the gC1q domain was able to assemble into functional trimers, in vitro, in the absence of the collagenous domain, a motif known to promote obligatory trimers in other proteins. Acid-mediated gC1q protomers reassembled into functional trimers, once neutralized, indicating that it is the gC1q domain which possesses the information for trimerization. However, reassembly occurred after neutralization, only if the gC1q protomers had preserved a residual tertiary structure at the end of the acidic treatment. Thus, the collagenous domain of C1q might initialize the folding of the gC1q domain so that subsequent assembly of the entire molecule can occur.

Serine proteases of the classical and lectin pathways: Similarities and differences

C1r, C1s, MBL-associated serine protease (MASP)-1, MASP-2 and MASP-3 are mosaic serine proteases of the classical and lectin pathways of complement. They form a family of enzymes with identical domain organization and similar overall structure, but with different enzymatic properties. MASP-2 of the lectin pathway can autoactivate and cleave C4 and C2 components. In the classical pathway two enzymes mediate these functions: C1r autoactivates and activates C1s, while C1s cleaves C4 and C2. The substrate specificity and the biological function of MASP-1 and MASP-3 have not yet been completely resolved. MASP-1 can autoactivate and the activated MASP-1 has more relaxed substrate specificity than the other members of the family. It was demonstrated that MASP-1 can specifically cleave C2, C3 and fibrinogen, but the physiological relevance of these findings has to be proved. We do not know how MASP-3 becomes activated and its biological function is also not clear. In this review, we will summarize current knowledge about the structure and function of these proteases. Special emphasis will be laid on the specificity, autoactivation and evolution of these enzymes.

Functional role of the linker between the complement control protein modules of complement protease C1s

C1s is the modular serine protease responsible for cleavage of C4 and C2, the protein substrates of the first component of C (C1). Its catalytic domain comprises two complement control protein (CCP) modules connected by a four-residue linker Gln340-Pro-Val-Asp343 and a serine protease domain. To assess the functional role of the linker, a series of mutations were performed at positions 340-343 of human C1s, and the resulting mutants were produced using a baculovirus-mediated expression system and characterized functionally. All mutants were secreted in a proenzyme form and had a mass of 77,203-77,716 Da comparable to that of wild-type C1s, except Q340E, which had a mass of 82,008 Da, due to overglycosylation at Asn391. None of the mutations significantly altered C1s ability to assemble with C1r and C1q within C1. Whereas the other mutations had no effect on C1s activation, the Q340E mutant was totally resistant to C1r-mediated activation, both in the fluid phase and within the C1 complex. Once activated, all mutants cleaved C2 with an efficiency comparable to that of wild-type C1s. In contrast, most of the mutations resulted in a decreased C4-cleaving activity, with particularly pronounced inhibitory effects for point mutants Q340K, P341I, V342K, and D343N. Comparable effects were observed when the C4-cleaving activity of the mutants was measured inside C1. Thus, flexibility of the C1s CCP1-CCP2 linker plays no significant role in C1 assembly or C1s activation by C1r inside C1 but plays a critical role in C4 cleavage by adjusting positioning of this substrate for optimal cleavage by the C1s active site.

Elucidation of the Substrate Specificity of the C1s Protease of the Classical Complement Pathway

The complement system is a central component of host defense but can also contribute to the inflammation seen in pathological conditions. The C1s protease of the first complement component, the C1 complex, initiates the pathway. In this study we have elucidated the full specificity of the enzyme for the first time using a randomized phage display library. It was found that, aside from the crucial P(1) position, the S(3) and S(2) subsites (in that order) played the greatest role in determining specificity. C1s prefers Leu or Val at P(3) and Gly or Ala residues at P(2). Apart from the S(2)' position, which showed specificity for Leu, prime subsites did not greatly affect specificity. It was evident, however, that together they significantly contributed to the efficiency of cleavage of a peptide. A peptide substrate based on the top sequence obtained in the phage display validated these results and produced the best kinetics of any C1s substrate to date. The results allow an understanding of the active site specificity of the C1s protease for the first time and provide a basis for the development of specific inhibitors aimed at controlling inflammation associated with complement activation in adverse pathological situations.

A novel human complement-related protein, C1r-like protease (C1r-LP), specifically cleaves pro-C1s

The availability of the human genome sequence allowed us to identify a human complement-related, C1r-like protease gene (c1r-LP) located 2 kb centromeric of the C1r gene (c1r). Compared with c1r, c1r-LP carries a large deletion corresponding to exons 4-8 of c1r. The open reading frame of the C1r-LP cDNA predicts a 50 kDa modular protein displaying 52% amino acid residue identity with the corresponding regions of C1r and 75% identity with a previously described murine C1r-LP. The serine protease domain of C1r-LP, despite an overall similarity with the AGY group of complement serine proteases, has certain structural features characteristic of C2 and factor B, thus raising interesting evolutionary questions. Northern blotting demonstrated the expression of C1r-LP mRNA mainly in the liver and ELISA demonstrated the presence of the protein in human serum at a concentration of 5.5+/-0.9 microg/ml. Immunoprecipitation experiments failed to demonstrate an association of C1r-LP with the C1 complex in serum. Recombinant C1r-LP exhibits esterolytic activity against peptide thioesters with arginine at the P1 position, but its catalytic efficiency (kcat/K(m)) is lower than that of C1r and C1s. The enzymic activity of C1r-LP is inhibited by di-isopropyl fluorophosphate and also by C1 inhibitor, which forms stable complexes with the protease. Most importantly, C1r-LP also expresses proteolytic activity, cleaving pro-C1s into two fragments of sizes identical with those of the two chains of active C1s. Thus C1r-LP may provide a novel means for the formation of the classical pathway C3/C5 convertase.

The Structure of MBL-associated Serine Protease-2 Reveals that Identical Substrate Specificities of C1s and MASP-2 are Realized Through Different Sets of Enzyme–Substrate Interactions

A family of serine proteases mediates the proteolytic cascades of several defense mechanisms in vertebrates, such as the complement system, blood coagulation and fibrinolysis. These proteases usually form large complexes with other glycoproteins. Their common features are their modular structures and restricted substrate specificities. The lectin pathway of complement, where mannose-binding lectin (MBL) recognizes the carbohydrate structures on pathogens, is activated by mannose-binding lectin-associated serine protease-2 (MASP-2). We present the 2.25A resolution structure of the catalytic fragment of MASP-2 encompassing the second complement control protein module (CCP2) and the serine protease (SP) domain. The CCP2 module stabilizes the structure of the SP domain as demonstrated by differential scanning calorimetry measurements. The asymmetric unit contains two molecules with different CCP-SP domain orientations, reflecting increased modular flexibility at the CCP2/SP joint. This flexibility may partly explain the ability of the MASP-2 dimer to perform all of its functions alone, whereas the same functions are mediated by the much larger C1r2-C1s2 tetramer in the C1 complex of the classical pathway. The main scaffold of the MASP-2 SP domain is chymotrypsin-like. Eight surface loops determine the S1 and other subsite specificities. Surprisingly, some surface loops of MASP-2, e.g. loop 1 and loop 2, which form the S1 pocket are similar to those of trypsin, and show significant differences if compared with those of C1s, indicating that the nearly identical substrate specificities of C1s and MASP-2 are realized through different sets of enzyme-substrate interactions.

X-ray structure of the Ca2+-binding interaction domain of C1s. Insights into the assembly of the C1 complex of complement

C1, the complex that triggers the classical pathway of complement, is assembled from two modular proteases C1r and C1s and a recognition protein C1q. The N-terminal CUB1-EGF segments of C1r and C1s are key elements of the C1 architecture, because they mediate both Ca2+-dependent C1r-C1s association and interaction with C1q. The crystal structure of the interaction domain of C1s has been solved and refined to 1.5 A resolution. The structure reveals a head-to-tail homodimer involving interactions between the CUB1 module of one monomer and the epidermal growth factor (EGF) module of its counterpart. A Ca2+ ion is bound to each EGF module and stabilizes both the intra- and inter-monomer interfaces. Unexpectedly, a second Ca2+ ion is bound to the distal end of each CUB1 module, through six ligands contributed by Glu45, Asp53, Asp98, and two water molecules. These acidic residues and Tyr17 are conserved in approximately two-thirds of the CUB repertoire and define a novel, Ca2+-binding CUB module subset. The C1s structure was used to build a model of the C1r-C1s CUB1-EGF heterodimer, which in C1 connects C1r to C1s and mediates interaction with C1q. A structural model of the C1q/C1r/C1s interface is proposed, where the rod-like collagen triple helix of C1q is accommodated into a groove along the transversal axis of the C1r-C1s heterodimer.

The human complement component C1R gene: the exon-intron structure and the molecular basis of allelic diversity

Human C1r is a component of the complement system, which is a major mediator of innate immunity. In this study we investigated the exon-intron organization of the human C1R gene, which spans 11 kb from the initiation codon to the stop codon, and is very similar in exon-intron structure to the C1S gene. Six common and rare alleles, C1R*1, C1R*2, C1R*5, C1R*8, C1R*9, and C1R*13, were characterized by five mutations at amino acid positions 114, 135, 146, 167 and 244, in exons 4, 5 and 7 where the CUB1, EGF and CUB2 domains are encoded, respectively. A comparison with the cDNA of the mouse C1r gene showed that C1R*2is likely to be an ancestral allele. In addition, nine nucleotide substitutions and one length polymorphism were found in introns 2, 3, 4, 8 and 10.

A novel murine complement-related gene encoding a C1r-like serum protein

C1r and C1s are highly specific serine proteases that initiate the classical pathway of complement activation. We recently demonstrated that, in the mouse, the genes encoding these proteins are duplicated. Analysis of the 5'-flanking region of the murine C1rA gene, the homologue of human C1r, revealed the presence of a novel gene encoding a C1r-like protein (c1r-LP). Although this gene carries a large deletion, it shows an overall structure similar to that of c1rA, suggesting that it may have arisen from a duplication of the C1r gene. The c1r-LP gene is expressed primarily in the liver, and is not regulated by lipopolysaccharide. The open reading frame of full-length cDNA clones encodes a pre-protein with a calculated molecular mass of 50.6 kDa which, except for an internal deletion of several modules, has a modular organization similar to that of C1r and shows 51% overall amino acid identity to corresponding regions of C1rA. Western blot analysis demonstrates the presence of C1r-LP in mouse serum. The serine protease domain of C1r-LP displays 60% amino acid residue identity to that of C1rA, however, certain atypical features of the active center, and primarily the absence of the activation/cleavage site, suggest that C1r-LP is either an atypical enzyme, or it lacks proteolytic activity, perhaps serving a regulatory function in the classical pathway.

Monomeric structures of the zymogen and active catalytic domain of complement protease c1r: further insights into the c1 activation mechanism

C1r is the serine protease (SP) that mediates autoactivation of C1, the complex that triggers the classical complement pathway. We have determined the crystal structure of two fragments from the human C1r catalytic domain, each encompassing the second complement control protein (CCP2) module and the SP domain. The wild-type species has an active structure, whereas the S637A mutant is a zymogen. The structures reveal a restricted hinge flexibility of the CCP2-SP interface, and both are characterized by the unique alpha-helical conformation of loop E. The zymogen activation domain exhibits high mobility, and the active structure shows a restricted access to most substrate binding subsites. Further implications relevant to the C1r self-activation process are derived from protein-protein interactions in the crystals.

Structural biology of the C1 complex of complement unveils the mechanisms of its activation and proteolytic activity

C1 is the multimolecular protease that triggers activation of the classical pathway of complement, a major element of antimicrobial host defense also involved in immune tolerance and various pathologies. This 790,000 Da complex is formed from the association of a recognition protein, C1q, and a catalytic subunit, the Ca2+-dependent tetramer C1s-C1r-C1r-C1s comprising two copies of each of the modular proteases C1r and C1s. Early studies mainly based on biochemical analysis and electron microscopy of C1 and its isolated components have allowed for characterization of their domain structure and led to a low-resolution model of the C1 complex in which the elongated C1s-C1r-C1r-C1s tetramer folds into a more compact, "8-shaped" conformation upon interaction with C1q. A major strategy used over the past years has been to dissect the C1 proteins into modular segments to characterize their function and solve their structure by either X-ray crystallography or nuclear magnetic resonance spectroscopy (NMR). The purpose of this review is to focus on this information, with particular emphasis on the architecture of the C1 complex and the mechanisms underlying its activation and proteolytic activity.

Tech.Sight. Antibody design by man and nature

Catalytic antibodies have emerged as being without peer as rationally designed biocatalysts. They have been shown to catalyze an ever-increasing array of chemical reactions with both high substrate specificity and selectivity. Probing the immune repertoire, via an expanding number of techniques, has lead to the production of proteins that can catalyze chemistry that is both difficult to perform using existing chemical methods and that is not catalyzed by endogenous enzymes. Remarkably, recent evidence has pointed to a hitherto unknown catalytic function of all antibodies that seems to be intrinsic to their immunoglobulin structure, the conversion of 1O2* into H2O2. This new catalytic potential of antibodies points to a new 'chemical arm' of the immune system and reveals that the evolution of catalytic antibodies significantly predates their rational design.

The crystal structure of the zymogen catalytic domain of complement protease C1r reveals that a disruptive mechanical stress is required to trigger activation of the C1 complex

C1r is the modular serine protease (SP) that mediates autolytic activation of C1, the macromolecular complex that triggers the classical pathway of complement. The crystal structure of a mutated, proenzyme form of the catalytic domain of human C1r, comprising the first and second complement control protein modules (CCP1, CCP2) and the SP domain has been solved and refined to 2.9 A resolution. The domain associates as a homodimer with an elongated head-to-tail structure featuring a central opening and involving interactions between the CCP1 module of one monomer and the SP domain of its counterpart. Consequently, the catalytic site of one monomer and the cleavage site of the other are located at opposite ends of the dimer. The structure reveals unusual features in the SP domain and provides strong support for the hypothesis that C1r activation in C1 is triggered by a mechanical stress caused by target recognition that disrupts the CCP1-SP interfaces and allows formation of transient states involving important conformational changes.

The role of the individual domains in the structure and function of the catalytic region of a modular serine protease, C1r

The first enzymatic event in the classical pathway of complement activation is autoactivation of the C1r subcomponent of the C1 complex. Activated C1r then cleaves and activates zymogen C1s. C1r is a multidomain serine protease consisting of N-terminal alpha region interacting with other subcomponents and C-terminal gammaB region mediating proteolytic activity. The gammaB region consists of two complement control protein modules (CCP1, CCP2) and a serine protease domain (SP). To clarify the role of the individual domains in the structural and functional properties of the gammaB region we produced the CCP1-CCP2-SP (gammaB), the CCP2-SP, and the SP fragments in recombinant form in Escherichia coli. We successfully renatured the inclusion body proteins. After renaturation all three fragments were obtained in activated form and showed esterolytic activity on synthetic substrates similar to each other. To study the self-activation process in detail zymogen mutant forms of the three fragments were constructed and expressed. Our major statement is that the ability of autoactivation and C1s cleavage is an inherent property of the SP domain. We observed that the CCP2 module significantly increases proteolytic activity of the SP domain on natural substrate, C1s. Therefore, we propose that CCP2 module provides accessory binding sites. Differential scanning calorimetric measurements demonstrated that CCP2 domain greatly stabilizes the structure of SP domain. Deletion of CCP1 domain from the CCP1-CCP2-SP fragment results in the loss of the dimeric structure. Our experiments also provided evidence that dimerization of C1r is not a prerequisite for autoactivation.

Assembly and enzymatic properties of the catalytic domain of human complement protease C1r

The catalytic properties of C1r, the protease that mediates activation of the C1 complex of complement, are mediated by its C-terminal region, comprising two complement control protein (CCP) modules followed by a serine protease (SP) domain. Baculovirus-mediated expression was used to produce fragments containing the SP domain and either 2 CCP modules (CCP1/2-SP) or only the second CCP module (CCP2-SP). In each case, the wild-type species and two mutants stabilized in the proenzyme form by mutations at the cleavage site (R446Q) or at the active site serine residue (S637A), were produced. Both wild-type fragments were recovered as two-chain, activated proteases, whereas all mutants retained a single-chain, proenzyme structure, providing the first experimental evidence that C1r activation is an autolytic process. As shown by sedimentation velocity analysis, all CCP1/2-SP fragments were dimers (5.5-5.6 S), and all CCP2-SP fragments were monomers (3.2-3.4 S). Thus, CCP1 is essential to the assembly of the dimer, but formation of a stable dimer is not a prerequisite for self-activation. Activation of the R446Q mutants could be achieved by extrinsic cleavage by thermolysin, which cleaved the CCP2-SP species more efficiently than the CCP1/2-SP species and yielded enzymes with C1s-cleaving activities similar to their active wild-type counterparts. C1r and its activated fragments all cleaved C1s, with relative efficiencies in the order C1r < CCP1/2-SP < CCP2-SP, indicating that CCP1 is not involved in C1s recognition.

Molecular basis of a selective C1s deficiency associated with early onset multiple autoimmune diseases

We have investigated the molecular basis of selective and complete C1s deficiency in 2-year-old girl with complex autoimmune diseases including lupus-like syndrome, Hashimoto's thyroiditis, and autoimmune hepatitis. This patient's complement profile was characterized by the absence of CH50 activity, C1 functional activity <10%, and undetectable levels of C1s Ag associated with normal levels of C1r and C1q Ags. Exon-specific amplification of genomic DNA by PCR followed by direct sequence analysis revealed a homozygous nonsense mutation in the C1s gene exon XII at codon 534, caused by a nucleotide substitution from C (CGA for arginine) to T (TGA for stop codon). Both parents were heterozygous for this mutation. We used the new restriction site for endonuclease Fok-1 created by the mutation to detect this mutation in the genomic DNA of seven healthy family members. Four additional heterozygotes for the mutation were identified in two generations. Our data characterize for the first time the genetic defect of a selective and complete C1s deficiency in a Caucasian patient.

Antibodies have the intrinsic capacity to destroy antigens

Research throughout the last century has led to a consensus as to the strategy of the humoral component of the immune system. The essence is that, for killing, the antibody molecule activates additional systems that respond to antibody-antigen union. We now report that the immune system seems to have a previously unrecognized chemical potential intrinsic to the antibody molecule itself. All antibodies studied, regardless of source or antigenic specificity, can convert molecular oxygen into hydrogen peroxide, thereby potentially aligning recognition and killing within the same molecule. Aside from pointing to a new chemical arm for the immune system, these results may be important to the understanding of how antibodies evolved and what role they may play in human diseases.

The cleavage of two C1s subunits by a single active C1r reveals substantial flexibility of the C1s-C1r-C1r-C1s tetramer in the C1 complex

The activation of the C1s-C1r-C1r-C1s tetramer in the C1 complex, which involves the cleavage of an Arg-Ile bond in the catalytic domains of the subcomponents, is a two-step process. First, the autolytic activation of C1r takes place, then activated C1r cleaves zymogen C1s. The Arg463Gln mutant of C1r (C1rQI) is stabilized in the zymogen form. This mutant was used to form a C1q-(C1s-C1rQI-C1r-C1s) heteropentamer to study the relative position of the C1r and C1s subunits in the C1 complex. After triggering the C1 by IgG-Sepharose, both C1s subunits are cleaved by the single proteolytically active C1r subunit in the C1s-C1rQI-C1r-C1s tetramer. This finding indicates that the tetramer is flexible enough to adopt different conformations within the C1 complex during the activation process, enabling the single active C1r to cleave both C1s, the neighboring and the sequentially distant one.

Crystal structure of the catalytic domain of human complement c1s: a serine protease with a handle

C1s is the highly specific modular serine protease that mediates the proteolytic activity of the C1 complex and thereby triggers activation of the complement cascade. The crystal structure of a catalytic fragment from human C1s comprising the second complement control protein (CCP2) module and the chymotrypsin-like serine protease (SP) domain has been determined and refined to 1.7 A resolution. In the areas surrounding the active site, the SP structure reveals a restricted access to subsidiary substrate binding sites that could be responsible for the narrow specificity of C1s. The ellipsoidal CCP2 module is oriented perpendicularly to the surface of the SP domain. This arrangement is maintained through a rigid module-domain interface involving intertwined proline- and tyrosine-rich polypeptide segments. The relative orientation of SP and CCP2 is consistent with the fact that the latter provides additional substrate recognition sites for the C4 substrate. This structure provides a first example of a CCP-SP assembly that is conserved in diverse extracellular proteins. Its implications in the activation mechanism of C1 are discussed.

Possible Mechanism for in Vitro Complement Activation in Blood and Plasma Samples: Futhan/EDTA Controls in Vitro Complement Activation

Background: Ongoing in vitro complement (C) activation in citrate or EDTA plasma has prevented an accurate analysis of C-activation products generated in vivo. The aim of this study was to characterize handling and storage conditions required to prevent in vitro C activation in blood and plasma samples collected with Futhan/EDTA.

Methods: BiotrakTM RIAs were used to quantitatively measure C3a and C4a in blood and/or plasma samples from healthy individuals (controls) and from liver transplant patients. Blood samples were routinely drawn into either EDTA (1 g/L) tubes or into tubes containing both EDTA (1 g/L) and Futhan (0.1 g/L) and immediately centrifuged at 2000g for 15 min at 4 °C.

Results: In controls, C4a, but not C3a, in fresh samples (time 0) was higher in EDTA plasma than in Futhan/EDTA plasma (n = 20; P = 0.002). Futhan/EDTA prevented C3a and C4a generation in blood and plasma samples held at room temperature (22–23 °C) for 1 h and in plasma held for 24 h at 4 °C or −70 °C. The mean C3a concentration (1.76 mg/L; n = 19) at time 0 in EDTA plasma samples from liver transplant patients was significantly higher than for controls (0.34 mg/L; n = 11). In these patients, the mean C3a in EDTA samples increased to 13.8 mg/L after 60 min at room temperature, but there was no change in the C3a concentration of an EDTA plasma from a control. In the patients, C3a concentrations were lower in Futhan/EDTA plasma than in EDTA at time 0 and after 60 min at room temperature (1.40 and 2.02 mg/L, respectively). The mean patient C4a was 4.02 mg/L in EDTA plasma at time 0 vs 0.24 mg/L for controls; it increased to 16.9 mg/L after 60 min at room temperature compared with 0.76 mg/L for controls. The mean patient C4a was 0.83 mg/L in Futhan/EDTA plasma at time 0 vs 0.1 mg/L for controls. Neither patient nor control C4a concentrations increased vs time in Futhan/EDTA.

Conclusion: The combination of Futhan (0.1 g/L) and EDTA (1 g/L) eliminates in vitro C activation.

Structure and functions of the interaction domains of C1r and C1s: keystones of the architecture of the C1 complex

C1r and C1s, the proteases responsible for activation and proteolytic activity of the C1 complex of complement, share similar overall structural organizations featuring five nonenzymic protein modules (two CUB modules surrounding a single EGF module, and a pair of CCP modules) followed by a serine protease domain. Besides highly specific proteolytic activities, both proteases exhibit interaction properties associated with their N-terminal regions. These properties include the ability to bind Ca2+ ions with high affinity, to associate with each other within a Ca2+-dependent C1s-C1r-C1r-C1s tetramer, and to interact with C1q upon C1 assembly. Precise functional mapping of these regions has been achieved recently, allowing identification of the domains responsible for these interactions, and providing a comprehensive picture of their structure and function. The objective of this article is to provide a detailed and up-to-date overview of the information available on these domains, which are keystones of the assembly of C1, and appear to play an essential role at the interface between the recognition function of C1 and its proteolytic activity.

One Active C1r Subunit Is Sufficient for the Activity of the Complement C1 Complex: Stabilization of C1r in the Zymogen Form by Point Mutations

The binding of C1 (the first component of complement) to immune complexes leads to the autoactivation of C1r through the cleavage of the Arg463-Ile464 bond in the catalytic domain. Spontaneous activation of C1r (and C1) also occurs in the fluid phase, preventing the characterization of the zymogen form of C1r. To overcome this difficulty, the zymogen form of human C1r was stabilized by mutating the Arg in the Arg463-Ile464 bond to Gln. This mutant was designated as mutant QI. Recombinant C1r (wild type (wt) or mutant) was expressed in insect cells using serum-free medium in functionally pure form; therefore, the cell culture supernatant was suitable to reconstruct C1 for the hemolytic assay. Mutant QI was a stable, nonactivable zymogen and showed no hemolytic activity in reconstituted C1. However, this stable zymogen C1r mutant could form an active mixed dimer with the wt C1r, indicating that one active C1r subunit in the C1 complex is sufficient for the full activity of the entire complex. Our experiments also showed that the exchange of C1r monomers between the C1r dimers is completed in less than 16 h even at pH 7 and 4°C. Two other mutants were also constructed by changing Arg463 to Lys, or Ile464 to Phe, and were designated as mutants KI and RF, respectively. Although these substitutions did increase the stability of the proenzyme in the cell culture supernatant, the mutant proteins retained their ability to autoactivate, and both had a wt-like hemolytic activity.

C1 inhibitor removes the entire C1qr2s2 complex from anti-C1Q monoclonal antibodies with low binding affinities

Evidence is presented for a new C1 Inhibitor (C1 INH) function. C1 INH was capable of dislodging the entire C1qr2s2 complex from C1-activating substances that bound weakly to the globular heads of C1q. Two different mouse IgG1 monoclonal antibodies with different affinities for C1q globular heads were compared for their complement-activating properties in the presence of normal human serum. As expected the higher affinity monoclonal antibody (Qu) was more effective in binding C1q and causing C1-mediated C4b deposition. Unexpectedly, time responses of C1 (C1q) binding to immobilized 3C7 reached a peak then gradually decreased. However, C1q remained constantly bound to immobilized Qu. These results indicated that after C1 activation in human serum, the entire C1 complex (including C1q) was dislodged from 3C7, but not from immobilized Qu. The addition of purified C1 INH to purified C1, which had bound to immobilized 3C7, resulted in removal of C1 (C1q). Removal of the entire C1qr2s2 did not occur when C1 INH preparations were first neutralized by the addition of purified activated C1s. In summary, it is suggested that C1 INH plays a prominent role in dislodging the entire C1qr2s2 from immunoglobulin preparations which have a low binding affinity for the globular heads of C1q.

Two Lineages of Mannose-Binding Lectin-Associated Serine Protease (MASP) in Vertebrates

Mannose-binding lectin-associated serine protease (MASP) is a newly identified member of the serine protease superfamily. MASP is involved in host defense against pathogens through a novel system of complement activation, designated the lectin pathway. To elucidate the origin of the lectin pathway and the molecular evolution of MASP, we cloned six MASP cDNAs from five vertebrate species going from mammal to cyclostome. An alignment of the amino acid sequences deduced from the cDNAs revealed the presence of two different lineages of the MASP gene. This classification was supported by the precise correlation with two types of exon organization for the protease domain. One of the two lineages is unique in that a single exon encodes the protease domain, unlike most other serine proteases. All members of this group, termed the AGY type, have an AGY codon at the active site serine. A phylogenetic tree suggests that the AGY type diverged from another lineage, termed the TCN type, before the emergence of primitive vertebrates. Furthermore, the presence of MASP or MASP-like sequences in most vertebrate species suggests that the lectin pathway functions extensively in vertebrates and that its origin is traced back to the invertebrate stage.

Evolutionary conserved rigid module-domain interactions can be detected at the sequence level: the examples of complement and blood coagulation proteases

Several extracellular modular proteins, including proteases of the complement and blood coagulation cascades, are shown here to exhibit conserved sequence patterns specific for a particular module-domain association. This was detected by comparative analysis of sequence variability in different multiple sequence alignments, which provides a new tool to investigate the evolution of modular proteins. A first example deals with the proteins featuring a common complement control protein (CCP) module-serine protease (SP) domain pattern at their C-terminal end, defined here as the CCP-SP sub-family. These proteins include the complement proteases C1r, C1s and MASPs, the Limulus clotting factor C, and the proteins of the haptoglobin family. A second example deals with blood coagulation factors VII, IX and X and protein C, all featuring a common epidermal growth factor (EGF)-SP C-terminal assembly. Highly specific motifs are found at the connection between the CCP or EGF module and the activation peptide of the SP domain: [P/A]-x-C-x-[P/A]-[I/V]-C-G-x-[P/S/K] in the case of the CCP-SP proteins, and C-x-[P/S]-x-x-x-[Y/F]-P-C-G in the case of the EGF-SP proteins. Each motif is strictly conserved in the whole sub-family and it is detected in no more than one other known protein sequence. Strikingly, most of the conserved residues specific to each sub-family appear to be clustered at the interface between the SP domain and the CCP or EGF module. We propose that a rigid module-domain interaction occurs in these proteins and has been conserved through evolution. The functional implications of these assemblies, underlined by such evolutionary constraints, are discussed.

Structural and functional studies on C1r and C1s: new insights into the mechanisms involved in C1 activity and assembly

C1r and C1s, the enzymes responsible for the activation and proteolytic activity of the C1 complex of complement, are modular serine proteases featuring similar overall structural organizations, yet expressing very distinct functional properties within C1. This review will initially summarize available information on the structure and function of the protein modules and serine protease domains of C1r and C1s. It will then focus on the regions of both proteases involved in: (i) assembly of C1s-C1r-C1r-C1s, the Ca(2+)-dependent tetrameric catalytic subunit of C1; (ii) expression of C1 catalytic activities. Particular emphasis will be aid on recent structural and functional studies that provide new insights into the complex mechanisms involved in the assembly, activation, and proteolytic activity of C1.

Structure and function of the serine-protease subcomponents of C1: protein engineering studies

Our protein engineering studies on human C1r and C1s revealed important characteristics of the individual domains of these multidomain serine-proteases, and supplied evidence about the cooperation of the domains to create binding sites, and to control the activation process. We expressed the recombinant subcomponents in the baculovirus-insect cell system and checked the biological activity. Deletions and point mutants of C1r were constructed and C1r-C1s chimeras were also produced. Our deletion mutants demonstrated that the N-terminal CUB domain and the EGF-like domain of C1r together are responsible for the calcium dependent C1r-C1s interaction. It seems very likely that these two modules form the calcium-binding site of the C1r alpha-fragment and participate in the tetramer formation. The deletion mutants also demonstrated that the N-terminal region of the C1r molecule contains essential elements involved in the control of activation of the serine-protease module. The substrate specificity of the serine-protease is also determined by the five N-terminal noncatalytic domain of C1r/C1s chimera, which contains the catalytic domain of C1s preceded by the N-terminal region of C1r, could replace the C1r in the hemolytically active C1 complex. The C1s/C1r chimera, in which the alpha-fragment of the C1r was replaced for that of the C1s exibits both C1r- and C1s-like characteristics. We stabilized the zymogen form of human C1r by mutating the Arg(463)-Ile(464) bond. Using our stable zymogen C1r we showed that one active C1r in the C1 complex is sufficient for the full activity of the entire complex. Further experiment with this mutant could provide us with important information about the structure of the C1 complex.

Activation of complement by human IgG1 and human IgG3 antibodies against the human leucocyte antigen CD52

Activation of the complement cascade by immunoglobulin G (IgG) plays a major role in the host defense against pathogens. Using recombinant human antibodies specific for the leucocyte antigen CD52, different allotypes of human IgG1 subclass were compared for their ability to activate human complement. In addition the roles of the different length hinge regions of IgG1 and IgG3 were investigated. It was found that the naturally occurring allotypes G1m(a,z) and G1m(f), and one artificially created isoallotype, G1m(null), did not significantly differ in their overall ability to cause cell lysis. However, some differences in binding of individual components of the classical activation pathway were detected. More of the complement component C1s seemed to be associated with the allotype G1m(f), although this did not result in an overall improvement in lytic potency. In this system the wild-type IgG3 was found to be less effective in complement lysis than IgG1. By shortening the hinge region of IgG3 to resemble that of an IgG1 antibody, increased complement binding was observed compared with that of wild-type IgG3 and the IgG1 allotypes. The overall lytic potency of the antibody was also improved compared with wild type IgG3 and it was also slightly more effective than the IgG1 allotypes.

A newly discovered function for C1 inhibitor, removal of the entire C1qr2s2 complex from immobilized human IgG subclasses

A new function for C1 inhibitor (C1 INH) is reported. C1 inhibitor dislodged the entire activated C1 complex (C1qr2s2) from immobilized human IgG. C1 binding to doses of immobilized human IgG3, IgG1, or IgG2 was quantified as a function of time. When human serum, as a source of C1qr2s2, was added to relatively low doses of immobilized IgG, C1q binding peaked at 1.0 min then gradually decreased. However when purified C1q was applied to immobilized IgG, C1q binding did not diminish with time. The removal of C1q was duplicated by adding purified C1 INH to C1qr2s2 which had been bound to immobilized IgG. The dislodgement of C1q from immobilized IgG required the presence of intact C1qr2s2 and of C1 INH. This removal of C1q by purified C1 INH was prevented when activated C1s was used to neutralize C1 INH function or when relatively high levels of IgG were immobilized.

Baculovirus-mediated expression of truncated modular fragments from the catalytic region of human complement serine protease C1s. Evidence for the involvement of both complement control protein modules in the recognition of the C4 protein substrate

C1s is the modular serine protease responsible for cleavage of C4 and C2, the protein substrates of the first component of complement. Its catalytic region (gamma-B) comprises two complement control protein (CCP) modules, a short activation peptide (ap), and a serine protease domain (SP). A baculovirus-mediated expression system was used to produce recombinant truncated fragments from this region, deleted either from the first CCP module (CCP2-ap-SP) or from both CCP modules (ap-SP). The aglycosylated fragment CCP2-ap-SPag was also expressed by using tunicamycin. The fragments were produced at yields of 0.6-3 mg/liter of culture, isolated, and characterized chemically and then tested functionally by comparison with intact C1s and its proteolytic gamma-B fragment. All recombinant fragments were expressed in a proenzyme form and cleaved by C1r to generate active enzymes expressing esterolytic activity and reactivity toward C1 inhibitor comparable to those of intact C1s. Likewise, the activated fragments gamma-B, CCP2-ap-SP, and ap-SP retained C1s ability to cleave C2 in the fluid phase. In contrast, whereas fragment gamma-B cleaved C4 as efficiently as C1s, the C4-cleaving activity of CCP2-ap-SP was greatly reduced (about 70-fold) and that of ap-SP was abolished. It is concluded that C4 cleavage involves substrate recognition sites located in both CCP modules of C1s, whereas C2 cleavage is affected mainly by the serine protease domain. Evidence is also provided that the carbohydrate moiety linked to the second CCP module of C1s has no significant effect on catalytic activity.

Expression and characterization of a 159 amino acid, N-terminal fragment of human complement component C1s

A 159 residue, N-terminal fragment of the human C1s complement component, C1s alpha(159), was expressed in the baculovirus, insect cell system. The protein was abundantly produced 3 days after infection, reaching levels as high as 40 microg/ml in cell culture media. It had a molecular weight of 18,100 (+/-4.9) Da by laser desorption mass spectrometry, close to the theoretical value of 18,111 Da, confirmed by sequencing. Sedimentation equilibrium and gel filtration column chromatography showed that C1s alpha(159) was a monomer in the presence of EDTA, and a dimer in the presence of Ca2+. The C1s alpha(159)2 dimer had a sedimentation coefficient of 3.1 S. When the C1s alpha(159)2 was mixed with Clq, there was little or no interaction. Likewise, unactivated C1r2 dimer had a sedimentation coefficient of 6.8 S, and when mixed with C1q little or no interaction was observed. When C1s alpha(159)2 was mixed with the 6.8 S C1r2 in Ca2+, a 7.5 S complex was formed, presumably the C1s alpha(159) x C1r x C1r x C1s alpha(159) tetramer. When C1q, which migrated at 10.1 S was mixed with C1s alpha(159)2 and C1r2 in the presence of Ca2+, a C1-like complex, but containing C1s alpha(159) instead of C1s, was formed which migrated at 14.0 S. This C1-like molecule remained unactivated unless challenged with an ovalbumin-antiovalbumin immune complex. In the presence of immune complex, the C1r became activated. This suggested that the presence of the 159 amino acid C1s alpha domain, which held the C1r to the C1q, was sufficient to permit activation by an immune complex, even though the catalytic domains of C1s were not present.

Probing the structure of C1 with an anti-C1s monoclonal antibody: the possible existence of two forms of C1 in solution

Anti-human C1s monoclonal antibody H1532, a mouse gamma-1-immunoglobulin elicited by a C1r2C1s2 immunogen, appeared to bind to the beta-domain of C1s by electron microscopy. In agreement with this observation, Western blotting demonstrated good binding to unreduced C1s, but no binding to the alpha or gamma-B domains. When added to solutions of the C1r2C1s2 tetramer, HI532 converted the 8.7 S tetramer into an 18 S complex, which was seen by electron microscopy to be a dimer of parallel C1s x C1r x C1r x C1s molecules cross-linked by two bivalent monoclonal antibodies. If increasing amounts of HI532 were added to C1r2C1s2 followed by addition of equivalent C1q, there was a progressive loss of hemolytic activity, which became zero when two equivalents of antibody HI532 were added. When two equivalents of HI532 were added to serum or C1 reconstituted overnight from purified subcomponents, there was an immediate loss of approximately 50% of the hemolytic activity; thereafter, activity decayed slowly and even after 24 hr, 10-30% of the activity remained. The rapid loss of only 50% of the activity would be readily explained by the existence of two conformations of C1, one of which was rapidly disassembled by antibody, and the other was resistant to disassembly. These two conformations may correspond to two previously proposed structures for the C1 complex.

Chemical synthesis and characterization of the epidermal growth factor-like module of human complement protease C1r

C1r is one of the two serine proteases of C1, the first component of complement, in which it is associated in a calcium-dependent manner to the homologous serine protease C1s. This interaction is mediated by the N-terminal region of C1r, which comprises a single epidermal growth factor (EGF)-like module containing the consensus sequence required for calcium binding, surrounded by two CUB modules. With a view to determine the structure of the EGF-like module of C1r and evaluate its contribution to calcium binding, this module [C1r(123-175)] was synthesized by automated solid-phase methodology using the Boc strategy. A first synthesis using the Boc-His(Z) derivative gave very low yield, due to partial deprotection of His residues leading to chain termination by acetylation, and to insertion of glycine residues. This could be circumvented by using the Boc-His(DNP) derivative and by condensation of appropriate glycine-containing segments. The synthetic peptide was efficiently folded under redox conditions to the species with three correct disulfide bridges, as determined by mass spectrometry and N-terminal sequence analyses of thermolytic fragments. The homogeneity of the synthetic peptide was assessed by reversed-phase HPLC and electrospray mass spectrometry. One-dimensional 1H NMR spectroscopic analysis provided evidence that the EGF-like module had a well defined structure, and was able to bind calcium with an apparent Kd of 10 mM. This value, comparable to that found for the isolated EGF-like modules of coagulation factors IX and X, is much higher than that measured for native C1r. As already proposed for factors IX and X, it is suggested that neighbouring module(s), most probably the N-terminal CUB module, contribute(s) to the calcium binding site.

Identification of a cryptic protein kinase CK2 phosphorylation site in human complement protease Clr, and its use to probe intramolecular interaction

Treatment of human (activated)C1r by CK2 resulted in the incorporation of [32P]phosphate into the N-terminal alpha region of its non-catalytic A chain. Fragmentation of 32P-labelled (activated)C1r followed by N-terminal sequence and mass spectrometry analyses allowed identification of Ser189 as the phosphorylation site. Accessibility of Ser189 was low in intact C1r, due in part to the presence of one of the oligosaccharides borne by the alpha region, further reduced in the presence of calcium, and abolished when C1r was incorporated into the C1s-C1r-C1r-C1s tetramer or the C1 complex. In contrast, phosphorylation was enhanced in the isolated alpha fragment and insensitive to calcium. Taken together, these data provide support for the occurrence of a (Ca2+)-dependent interaction between the alpha region and the remainder of the C1r molecule.

Functional effects of domain deletions in a multidomain serine protease, C1r

The C1r subcomponent of the first component of complement is a complex, multidomain glycoprotein containing five regulatory or binding modules in addition to the serine protease domain. To reveal the functional role of the N-terminal regulatory domains, two deletion mutants of C1r were constructed. One mutant comprises the N-terminal half of domain I joined to the second half of the highly homologous domain III, resulting in one chimeric domain in the N-terminal region, instead of domains I-III. In the second mutant most of the N-terminal portion of domain I was deleted. Both deletion mutants were expressed in the baculovirus-insect cell expression system with yields typical of wild type C1r. Both mutants maintained the ability of the wild type C1r to dimerize. The folding and secretion of the recombinant proteins was not affected by these deletions, and C1-inhibitor binding was not impaired. The stability of the zymogen was significantly decreased however, indicating that the N-terminal region of the C1r molecule contains essential elements involved in the control of activation of the serine protease module. Tetramer formation with C1s in the presence of Ca2+ was abolished by both deletions. We suggest that the first domain of C1r is essential for tetramer formation, since the deletion of domain I from C1r impairs this interaction.

Developmentally regulated expression of a cell surface protein, neuropilin, in the mouse nervous system

Neuropilin (previously A5) is a cell surface glycoprotein that was originally identified in Xenopus tadpole nervous tissues. In Xenopus, neuropilin is expressed on both the presynaptic and postsynaptic elements in the visual and general somatic sensory systems, suggesting a role in neuronal cell recognition. In this study, we identified a mouse homologue of neuropilin and examined its expression in developing mouse nervous tissues. cDNA cloning and sequencing revealed that the primary structure of the mouse neuropilin was highly similar to that of Xenopus and that the extracellular segment of the molecule possessed several motifs that were expected to be involved in cell-cell interaction. Immunohistochemistry and in situ hybridization analyses in mice indicated that the expression of neuropilin was restricted to particular neuron circuits. Neuropilin protein was localized on axons but not on the somata of neurons. The expression of neuropilin persisted through the time when axons were actively growing to form neuronal connections. These observations suggest that neuropilin is involved in growth, fasciculation, and targeting for a particular groups of axons.

HIV-1 rsgp41 depends on calcium for binding of human c1q but not for binding of gp120

Human immunodeficiency virus type 1 activates the complement cascade via the classical pathway by direct binding of C1q through specific sites in the TM surface protein, gp41. In this paper we investigated the divalent cation dependence of the interaction between HIV-1 gp41 and C1q or gp120. A solid phase radioimmunoassay was used to investigate the interaction between a recombinant soluble form of HIV-1 gp41 (rsgp41) and C1q and an enzyme linked immunoassay was used to investigate the interaction between rsgp41 and gp120. The interaction between C1q and rsgp41, but not between C1q and immune complexes, was dependent upon the presence of calcium. Calcium could not be replaced by larger cations such as strontium, barium, lead or smaller ions such as magnesium and manganese. Zinc increased binding to 22% of binding achieved with calcium. The interaction between rsgp41 and gp120 was not dependent upon the presence of divalent ions. Thus, calcium is required for the interaction between rsgp41 and C1q, whereas the interaction between rsgp41 and gp120 is independent of divalent cations.