The Human C1q Globular Domain: Structure and Recognition of Non-Immune Self Ligands

Groupe IRPAS, Institut de Biologie Structurale Grenoble, France.
Frontiers in Immunology 11/2011; 2:92. DOI: 10.3389/fimmu.2011.00092
Source: PubMed

ABSTRACT 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. These investigations provide evidence for a common binding area for these ligands located in subunit C of the C1q globular domain, and suggest that ligand recognition through this area down-regulates C1 activation, hence contributing to the control of the inflammatory reaction. The purpose of this article is to give an overview of these advances which represent a first step toward understanding the recognition mechanisms of C1q and their biological implications.

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Available from: Nicole M Thielens, Jul 19, 2015
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    • "3.7. IgG4-Fc can assemble into an Fc-Fc mediated hexamer The C1q component of complement has a hexameric arrangement , and activates the classical complement pathway through binding to IgM and IgG immune complexes (Burton, 1990; Ghai et al., 2007; Gaboriaud et al., 2012). An IgM hexamer (or pentamer with J-chain) can activate complement (Randall et al., 1990). "
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    ABSTRACT: The Fc region of IgG antibodies, important for effector functions such as antibody-dependent cell-mediated cytotoxicity, antibody-dependent cellular phagocytosis and complement activation, contains an oligosaccharide moiety covalently attached to each CH2 domain. The oligosaccharide not only orients the CH2 domains but plays an important role in influencing IgG effector function, and engineering the IgG-Fc oligosaccharide moiety is an important aspect in the design of therapeutic monoclonal IgG antibodies. Recently we reported the crystal structure of glycosylated IgG4-Fc, revealing structural features that could explain the anti-inflammatory biological properties of IgG4 compared with IgG1. We now report the crystal structure of enzymatically deglycosylated IgG4-Fc, derived from human serum, at 2.7Å resolution. Intermolecular CH2-CH2 domain interactions partially bury the CH2 domain surface that would otherwise be exposed by the absence of oligosaccharide, and two Fc molecules are interlocked in a symmetric, open conformation. The conformation of the CH2 domain DE loop, to which oligosaccharide is attached, is altered in the absence of carbohydrate. Furthermore, the CH2 domain FG loop, important for Fcγ receptor and C1q binding, adopts two different conformations. One loop conformation is unique to IgG4 and would disrupt binding, consistent with IgG4's anti-inflammatory properties. The second is similar to the conserved conformation found in IgG1, suggesting that in contrast to IgG1, the IgG4 CH2 FG loop is dynamic. Finally, crystal packing reveals a hexameric arrangement of IgG4-Fc molecules, providing further clues about the interaction between C1q and IgG.
    Molecular Immunology 06/2014; 62(1):46-53. DOI:10.1016/j.molimm.2014.05.015 · 3.00 Impact Factor
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    • "In some of these cases work is underway (e.g. (Dobo et al., 2009; Gaboriaud et al., 2011; Garlatti et al., 2010; Gout et al., 2011; Jenkins et al., 2006; Lang et al., 2010; Teillet et al., 2008) that is likely to give insight into the higher order assembly and perhaps its is this that will provide the structural focus at the meeting in 2014. Other questions remaining include generating a molecular understanding of how MAC pore formation is regulated by CD59, how the different convertases achieve specificity of activity and regulation and, particularly with the wealth of GWAS and other genetic studies, what are the molecular bases of the many genetic susceptibilities now associated with complement proteins? "
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    Immunobiology 11/2012; 217(11):1117-21. DOI:10.1016/j.imbio.2012.07.005 · 3.18 Impact Factor
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    ABSTRACT: Complement C1q subcomponent subunit A (C1qA) is one of the three components of C1q molecule. Functional C1q is composed of eighteen polypeptide chains: six C1qA chains, six C1qB chains, and six C1qC chains, which are arranged as six heterotrimers of ABC: (ABC)6. Each of the individual C1q polypeptide chain consists of a N-terminal region and a C-terminal globular region (gC1q), of ~135 residues. Each N-terminal consists of 2-11 amino acid segments containing a half-cysteine residue that is involved in formation of inter-chain disulphide bonds, followed by a collagen-like region (CLR) consisting of ~81 residues. The collagen-like regions in A, B and C chains of each heterotrimer come together to form a triple helical collagen like structure. Further, A and B chains in each heterotrimer are bound by a disulphide bond, while C chain forms a disulphide bond with a C chain from the adjoining heterotrimer. Therefore the eighteen subunits come together to form six globular heads (gC1q), which are clusters of 3 independently folded C-terminal domains of the A, B and C chain. These globular domains recognize an array of self, non-self and altered-self ligands. C1q associates with the proenzymes C1r and C1s (2 molecules of each, in the molar ratio of 1:2:2 in a calcium dependent manner) to yield an active C1 complex, the first component of the serum complement system. C1r, upon binding of gC1q to an inciting stimulus, autoactivates itself and catalyzes breakage of a C1s ester bond, resulting in C1s activation and subsequent cleavage of C2 and C4 into their respective “a” and “b” fragments. Recognition of ligands by C1q molecule also defines C1q as a pattern recognition molecule (PRM). C1q recognizes distinct structures either directly on microbial structures and apoptotic cells, or indirectly after their recognition by antibodies or C-reactive protein (CRP). C1q in turn binds to multiple receptors (such as cC1qR (calreticulin), integrin α2β1 or other molecules on the surface of specific cell types of either myeloid or endothelial cell orgin) and shows regulated broad physiological functions beyond complement activation.
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