Structural basis for surface activation of the classical complement cascade by the short pentraxin C-reactive protein

Abstract

Human C-reactive protein (CRP) is a pentameric complex involved in defence against pathogens and regulation of autoimmunity. CRP is also a therapeutic target, with both administration and depletion of serum CRP being pursued as a possible treatment for autoimmune and cardiovascular diseases, among others. CRP binds to phosphocholine (PC) moieties on membranes in order to activate the complement system via the C1 complex, but it is unknown how CRP, or any pentraxin, binds to C1. Here, we present a cryo-electron tomography (cryoET)-derived structure of CRP bound to PC ligands and the C1 complex. To gain control of CRP binding, a synthetic mimotope of PC was synthesised and used to decorate cell-mimetic liposome surfaces. Structure-guided mutagenesis of CRP yielded a fully-active complex able to bind PC-coated liposomes that was ideal for cryoET and subtomogram averaging. In contrast to antibodies, which form Fc-mediated hexameric platforms to bind and activate the C1 complex, CRP formed rectangular platforms assembled from four laterally-associated CRP pentamers that bind only four of the six available globular C1 head groups. Potential residues mediating lateral association of CRP were identified from interactions between unit cells in existing crystal structures, which rationalised previously unexplained mutagenesis data regarding CRP-mediated complement activation. The structure also enabled interpretation of existing biochemical data regarding interactions mediating C1 binding, and identified additional residues for further mutagenesis studies. These structural data therefore provide a possible mechanism for regulation of complement by CRP, which limits complement progression and has consequences for how the innate immune system influences autoimmunity.

C-reactive protein: structure, function, regulation, and role in clinical diseases

“…Subsequently, they applied the Cryo-ET resolved the structure of CRP binding to PC after binding to C1 complex. Revealed new interfaces, interpreting previously contradictory biochemical data, and clarifying the possible mechanism by which CRP regulates the complement system ( 25 ).…”

“…Furthermore, CRP acting as an innate pattern recognition receptor ( 23 ), can precipitate the somatic C-polysaccharide of Streptococcus pneumoniae on microorganisms in the presence of calcium, then triggers the innate immunity through the classical complement pathway by interacting with the global head region of C1q or other cell surface glycoproteins such as FcγR receptors ( 16 , 24 , 25 ). The structural analysis of the CRP binding phosphocholine (PC) ligand and C1 complex using Cryo-Electron Tomography (Cryo-ET) revealed a novel mechanism.…”

“…Contrary to previous understanding, it was observed that CRP does not form an Fc-mediated hexamer antibody platform to bind and activate the C1 complex. Instead, CRP forms a rectangular platform assembled by tetrameric CRP to effectively bind and activate complement ( 25 ).…”