Interaction of C1q with IgG1, C-reactive Protein and Pentraxin 3:  Mutational Studies Using Recombinant Globular Head Modules of Human C1q A, B, and C Chains

Abstract

C1q is the first subcomponent of the classical complement pathway that can interact with a range of biochemically and structurally diverse self and nonself ligands. The globular domain of C1q (gC1q), which is the ligand-recognition domain, is a heterotrimeric structure composed of the Cterminal regions of A (ghA), B (ghB), and C (ghC) chains. The expression and functional characterization of ghA, ghB, and ghC modules have revealed that each chain has specific and differential binding properties toward C1q ligands. It is largely considered that C1q-ligand interactions are ionic in nature; however, the complementary ligand-binding sites on C1q and the mechanisms of interactions are still unclear. To identify the residues on the gC1q domain that are likely to be involved in ligand recognition, we have generated a number of substitution mutants of ghA, ghB, and ghC modules and examined their interactions with three selected ligands: IgG1, Creactive protein (CRP), and pentraxin 3 (PTX3). Our results suggest that charged residues belonging to the apex of the gC1q heterotrimer (with participation of all three chains) as well as the side of the ghB are crucial for C1q binding to these ligands, and their contribution to each interaction is different. It is likely that a set of charged residues from the gC1q surface participate † This study was supported by the National Science Foundation of Bulgaria, Grant MY-K-1303 to L.T.R. and L-1000 to M.S.K. R.G.is sponsored by the Deutsche Forschungsgemeinschaft through the Graduiertenkolleg GK370. U.K. is funded by the European Commission, University of Oxford, and the Alexander von Humboldt Foundation. A.M. and B.B. are funded by TELETHON, MIUR (FIRB Fund), the European Commission, and AIRC.© 2006 American Chemical Society * Corresponding author. Phone: +44-1865+44- -222325. Fax: +44-1865 +49-641-9941259. ukishore@hotmail.comu.kishore@rediffmail.com. NIH Public Access Author ManuscriptBiochemistry. Author manuscript; available in PMC 2013 December 29. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript via different ionic and hydrogen bonds with corresponding residues from the ligand, instead of forming separate binding sites. Thus, a recently proposed model suggesting the rotation of the gC1q domain upon ligand recognition may be extended to C1q interaction with CRP and PTX3 in addition to IgG1.C1q is the first subcomponent of the classical complement pathway and its binding to IgGor IgM-containing immune complexes leads to the autoactivation of C1r, which in turn, activates C1s. C1r and C1s, the two serine protease proenzymes, together with C1q constitute C1, the first component of the classical pathway. The activation of the C1 complex (C1q + C1r2 + C1s2) subsequently leads to the activation of the C2−C9 components of the classical pathway and the formation of the terminal-membrane-attack complex (MAC) (1, 2). C1q is a versatile innate immune molecule that can bind a diverse range of self and nonself ligands, ranging from proteins to lipids ...

Atypical Hemolytic Uremic Syndrome-Associated Variants and Autoantibodies Impair Binding of Factor H and Factor H-Related Protein 1 to Pentraxin 3

“…2C). This is in accordance with a previous study showing pH-independent binding of C1q to PTX3 (53).…”

Binding of the Long Pentraxin PTX3 to Factor H: Interacting Domains and Function in the Regulation of Complement Activation

“…In addition, the interaction of PTX3 with C1q and its role in the activation of the classical complement pathway have been reported (15,39,44,47), while the role of PTX3 in the alternative pathway has not been investigated. In the present study, we report a novel interaction of PTX3 with factor H, the main soluble regulator of the alternative pathway of complement.…”

HDL in Infectious Diseases and Sepsis

“…In addition to PTX3, C1q recognizes and binds to ficolin-2 and mannose-binding lectin (MBL), thus modulating the classical and the lectin pathways of complement activation (Bottazzi et al 1997). The best described and characterized ligand of PTX3 is the first component of the classical complement system C1q (Bottazzi et al 1997;Nauta et al 2003); PTX3 interacts with the globular head of the protein (Roumenina et al 2006) thus resulting in the activation of the classical complement cascade only when C1q is plastic immobilized, a situation that mimics C1q bound to a microbial surface. Anti-inflammatory molecules were shown to modulate PTX3 expression.…”