Classical complement pathway components C1r and C1s: purification from human serum and in recombinant form and functional characterization.

C1r and C1s are the proteases responsible for the activation and proteolytic activity of the C1 complex of the classical complement pathway, respectively. They are assembled into a Ca(2+)-dependent C1s-C1r-C1r-C1s tetramer which in turn associates with the recognition protein C1q. The C1 complex circulates in serum as a zymogen and is activated upon binding of C1q to appropriate targets, such as antigen-antibody complexes.

The serine protease domain of MASP-3: enzymatic properties and crystal structure in complex with ecotin.

Mannan-binding lectin (MBL), ficolins and collectin-11 are known to associate with three homologous modular proteases, the MBL-Associated Serine Proteases (MASPs). The crystal structures of the catalytic domains of MASP-1 and MASP-2 have been solved, but the structure of the corresponding domain of MASP-3 remains unknown. A link between mutations in the MASP1/3 gene and the rare autosomal recessive 3MC (Mingarelli, Malpuech, Michels and Carnevale,) syndrome, characterized by various developmental disorders, was discovered recently, revealing an unexpected important role of MASP-3 in early developmental processes.

Expression of recombinant human complement C1q allows identification of the C1r/C1s-binding sites.

Complement C1q is a hexameric molecule assembled from 18 polypeptide chains of three different types encoded by three genes. This versatile recognition protein senses a wide variety of immune and nonimmune ligands, including pathogens and altered self components, and triggers the classical complement pathway through activation of its associated proteases C1r and C1s. We report a method for expression of recombinant full-length human C1q involving stable transfection of HEK 293-F mammalian cells and fusion of an affinity tag to the C-terminal end of the C chain.

Structure and properties of the Ca(2+)-binding CUB domain, a widespread ligand-recognition unit involved in major biological functions.

CUB domains are 110-residue protein motifs exhibiting a β-sandwich fold and mediating protein-protein interactions in various extracellular proteins. Recent X-ray structural and mutagenesis studies have led to the identification of a particular CUB domain subset, cbCUB (Ca(2+)-binding CUB domain). Unlike other CUB domains, these harbour a homologous Ca(2+)-binding site that underlies a conserved binding site mediating ionic interaction between two of the three conserved acidic Ca(2+) ligands and a basic (lysine or arginine) residue of a protein ligand, similar to the interactions mediated by the low-density lipoprotein receptor family.

Enzymatically modified low-density lipoprotein is recognized by c1q and activates the classical complement pathway.

Several studies suggest that the complement system is involved in atherogenesis. To further investigate this question, we have studied the ability of native and modified forms of LDL to bind and activate C1, the complex protease that triggers the classical pathway of complement. Unlike native LDL, oxidized (oxLDL) and enzymatically modified (E-LDL) derivatives were both recognized by the C1q subunit of C1, but only E-LDL particles, obtained by sequential treatment with a protease and then with cholesterol esterase, had the ability to trigger C1 activation.

M-ficolin interacts with the long pentraxin PTX3: a novel case of cross-talk between soluble pattern-recognition molecules.

Ficolins and pentraxins are soluble oligomeric pattern-recognition molecules that sense danger signals from pathogens and altered self-cells and might act synergistically in innate immune defense and maintenance of immune tolerance. The interaction of M-ficolin with the long pentraxin pentraxin 3 (PTX3) has been characterized using surface plasmon resonance spectroscopy and electron microscopy. M-ficolin was shown to bind PTX3 with high affinity in the presence of calcium ions.

X-ray structure of the human calreticulin globular domain reveals a peptide-binding area and suggests a multi-molecular mechanism.

In the endoplasmic reticulum, calreticulin acts as a chaperone and a Ca(2+)-signalling protein. At the cell surface, it mediates numerous important biological effects. The crystal structure of the human calreticulin globular domain was solved at 1.

Investigations on the C1q-calreticulin-phosphatidylserine interactions yield new insights into apoptotic cell recognition.

Both C1q and calreticulin (CRT) are involved in the recognition of apoptotic cells. CRT was initially characterized as a receptor for the C1q collagen-like fragment (CLF), whereas C1q was shown to bind apoptotic cells through its globular region (GR). Using purified CRT and recombinant CRT domains, we now provide unambiguous experimental evidence that, in addition to its CLF, the C1q GR also binds CRT and that both types of interactions are mediated by the CRT globular domain.

Proteins of the innate immune system crystallize on carbon nanotubes but are not activated.

The classical pathway of complement is an essential component of the human innate immune system involved in the defense against pathogens as well as in the clearance of altered self-components. Activation of this pathway is triggered by C1, a multimolecular complex comprising a recognition protein C1q associated with a catalytic subunit C1s-C1r-C1r-C1s. We report here the direct observation of organized binding of C1 components C1q and C1s-C1r-C1r-C1s on carbon nanotubes, an ubiquitous component in nanotechnology research.

The human c1q globular domain: structure and recognition of non-immune self ligands.

C1q, the ligand-binding unit of the C1 complex of complement, is a pattern recognition molecule with the unique ability to sense an amazing variety of targets, including a number of altered structures from self, such as apoptotic cells. The three-dimensional structure of its C-terminal globular domain, responsible for its recognition function, has been solved by X-ray crystallography, revealing a tightly packed heterotrimeric assembly with marked differences in the surface patterns of the subunits, and yielding insights into its versatile binding properties. In conjunction with other approaches, this same technique has been used recently to decipher the mechanisms that allow this domain to interact with various non-immune self ligands, including molecules known to provide eat-me signals on apoptotic cells, such as phosphatidylserine and DNA.

Structural and functional characterization of the recombinant human mitochondrial trifunctional protein.

The α and β subunits of the human mitochondrial trifunctional protein (TFP), the multienzyme complex involved in fatty acid β-oxidation, were coexpressed in Escherichia coli and purified to homogeneity by nickel affinity chromatography. The resulting α/His-β construct was analyzed by gel filtration, sedimentation velocity, and electron microscopy, indicating a predominance of α(2)β(2) and α(4)β(4) complexes, with higher order oligomers. Electron microscopy indicated that the elementary species α(2)β(2) had overall structural similarity with its bacterial homologue.

Direct interaction between CD91 and C1q.

C1q-mediated removal of immune complexes and apoptotic cells plays an important role in tissue homeostasis and the prevention of autoimmune conditions. It has been suggested that C1q mediates phagocytosis of apoptotic cells through a receptor complex assembled from CD91 (alpha-2- macroglobulin receptor, or low-density lipoprotein receptor-related protein) and calreticulin, with CD91 being the transmembrane part and calreticulin acting as the C1q-binding molecule. In the present study, we observe that C1q binds cells from a CD91 expressing monocytic cell line as well as monocytes from human blood.

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.

Cutting edge: C1q binds deoxyribose and heparan sulfate through neighboring sites of its recognition domain.

C1q, the recognition subunit of the C1 complex of complement, is an archetypal pattern recognition molecule with the striking ability to sense a wide variety of targets, including a number of altered self-motifs. The recognition properties of its globular domain were further deciphered by means of x-ray crystallography using deoxy-D-ribose and heparan sulfate as ligands. Highly specific recognition of deoxy-D-ribose, involving interactions with Arg C98, Arg C111, and Asn C113, was observed at 1.

Complement protein C1q forms a complex with cytotoxic prion protein oligomers.

A growing number of studies have investigated the interaction between C1q and PrP, but the oligomeric form of PrP involved in this interaction remains to be determined. Aggregation of recombinant full-length murine PrP in the presence of 100 mm NaCl allowed us to isolate three different types of oligomers by size-exclusion chromatography. In contrast to PrP monomers and fibrils, these oligomers activate the classical complement pathway, the smallest species containing 8-15 PrP protomers being the most efficient.

Structural insights into the recognition properties of human ficolins.

Innate immunity relies upon the ability of a variety of recognition molecules to sense pathogens through conserved molecular signatures that are often carbohydrates. Ficolins are oligomeric proteins assembled from collagen-like stalks and fibrinogen-like domains that have the ability to sense these molecular patterns on both pathogens and apoptotic cell surfaces. Three ficolins, termed L, H and M, have been identified in humans.

Functional characterization of the recombinant human C1 inhibitor serpin domain: insights into heparin binding.

Variants of the human C1 inhibitor serpin domain containing three N-linked carbohydrates at positions 216, 231, and 330 (C1inhDelta97), a single carbohydrate at position 330 (C1inhDelta97DM), or no carbohydrate were produced in a baculovirus/insect cells system. An N-terminally His-tagged C1inhDelta97 variant was also produced. Removal of the oligosaccharide at position 330 dramatically decreased expression, precluding further analysis.

Complement protein C1q recognizes enzymatically modified low-density lipoprotein through unesterified fatty acids generated by cholesterol esterase.

We previously reported that enzymatically modified low-density lipoprotein (E-LDL) particles obtained by LDL treatment with trypsin and then cholesterol esterase are recognized by C1q and activate the C1 complex of complement. The objective of this study was to identify the E-LDL component(s) recognized by C1q. In addition to trypsin, plasmin, thrombin, tryptase, and matrix metalloprotease-2 each yielded E-LDL particles with high C1-activating efficiency, and the C1 activation extent was strictly dependent on cholesterol esterase treatment in all cases.

Carbohydrate recognition properties of human ficolins: glycan array screening reveals the sialic acid binding specificity of M-ficolin.

Ficolins are oligomeric innate immune recognition proteins consisting of a collagen-like region and a fibrinogen-like recognition domain that bind to pathogen- and apoptotic cell-associated molecular patterns. To investigate their carbohydrate binding specificities, serum-derived L-ficolin and recombinant H- and M-ficolins were fluorescently labeled, and their carbohydrate binding ability was analyzed by glycan array screening. L-ficolin preferentially recognized disulfated N-acetyllactosamine and tri- and tetrasaccharides containing terminal galactose or N-acetylglucosamine.

Analysis of human C1q by combined bottom-up and top-down mass spectrometry: detailed mapping of post-translational modifications and insights into the C1r/C1s binding sites.

C1q is a subunit of the C1 complex, a key player in innate immunity that triggers activation of the classical complement pathway. Featuring a unique structural organization and comprising a collagen-like domain with a high level of post-translational modifications, C1q represents a challenging protein assembly for structural biology. We report for the first time a comprehensive proteomics study of C1q combining bottom-up and top-down analyses.

Human astrovirus coat protein binds C1q and MBL and inhibits the classical and lectin pathways of complement activation.

Human astroviruses (HAstVs) constitute a family of non-enveloped, RNA viruses which cause infantile gastroenteritis. We have previously demonstrated that purified HAstV coat protein (CP), multiple copies of which compose the viral capsid, bind C1q resulting in inhibition of classical complement pathway activity. The objective of this study was to further analyze the mechanism by which CP inhibits C1 activation.

How phagocytes track down and respond to apoptotic cells.

The uptake of apoptotic cells by macrophages and dendritic cells or nonprofessional phagocytes is crucial for development and tissue homeostasis. This is of special importance because deficiencies in the recognition or removal of apoptotic cells may result in autoimmune diseases. The efficient elimination of an apoptotic cell involves contact between the altered cell and the phagocyte, specific recognition, and phagocytosis of the target.

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.

Residue Lys57 in the collagen-like region of human L-ficolin and its counterpart Lys47 in H-ficolin play a key role in the interaction with the mannan-binding lectin-associated serine proteases and the collectin receptor calreticulin.

L- and H-ficolins are serum oligomeric defense proteins consisting of a collagen-like region and a fibrinogen-like recognition domain that bind to pathogen- and apoptotic cell-associated molecular patterns. They share with mannan-binding lectin (MBL) the ability to associate with MBL-associated serine proteases (MASP)-1, -2, -3, and protein MAp19 and to trigger the lectin complement pathway through MASP-2 activation. Recent studies have revealed the essential role of Lys(55) in the collagenous region of MBL in the interaction with the MASPs and calreticulin (CRT).