Structural basis of the C1q/C1s interaction and its central role in assembly of the C1 complex of complement activation

Abstract

Complement component C1, the complex that initiates the classical pathway of complement activation, is a 790-kDa assembly formed from the target-recognition subcomponent C1q and the modular proteases C1r and C1s. The proteases are elongated tetramers that become more compact when they bind to the collagen-like domains of C1q. Here, we describe a series of structures that reveal how the subcomponents associate to form C1. A complex between C1s and a collagen-like peptide containing the C1r/C1s-binding motif of C1q shows that the collagen binds to a shallow groove via a critical lysine side chain that contacts Ca 2+ -coordinating residues. The data explain the Ca 2+ -dependent binding mechanism, which is conserved in C1r and also in mannan-binding lectin-associated serine proteases, the serine proteases of the lectin pathway activation complexes. In an accompanying structure, C1s forms a compact ring-shaped tetramer featuring a unique head-to-tail interaction at its center that replicates the likely arrangement of C1r/C1s polypeptides in the C1 complex. Additional structures reveal how C1s polypeptides are positioned to enable activation by C1r and interaction with the substrate C4 inside the cage-like assembly formed by the collagenous stems of C1q. Together with previously determined structures of C1r fragments, the results reported here provide a structural basis for understanding the early steps of complement activation via the classical pathway.

Effective Neutrophil Phagocytosis of <b><i>Aspergillus</i></b> <b><i>fumigatus</i></b> Is Mediated by Classical Pathway Complement Activation

“…The classical pathway is initiated by the binding of antibodies followed by the binding of C1 [13]. The C1 complex (C1qr 2 s 2 ) consists of C1q, which binds to antibody-antigen complexes, and the serine proteases C1r and C1s, that initially processes C4 and subsequently C2 to generate the C3 convertase (C4b2a) [13,14]. C1q can also recognize and bind microbial surfaces directly or indirectly via the interaction of pentraxins to activate the classical pathway independent of antibodies [15].…”

Bacterial expression and preliminary crystallographic studies of a 149-residue fragment of human Caprin-1

“…The sequences of HR1 and HR2 do not conform to any consensus sequence for known protein domains suggestive of functions. Caprin-2 has an identifiable structural domain at its C-terminus (CRD; C1q-related domain) which is homologous to the globular head domain of the complement protein C1q (Shapiro & Scherer, 1998;Gaboriaud et al, 2003;Garlatti et al, 2010;Venkatraman Girija et al, 2013;Miao et al, 2014). Caprin-1 and Caprin-2 also contain RGG boxes and RG-rich sequences characteristic of RNA-binding proteins near the C-terminal region of HR2 (Fig.…”

Structure and activation of C1, the complex initiating the classical pathway of the complement cascade

“…1A). In the C1r 2 s 2 tetramer, the two C1s molecules dimerize through their CUB1-EGF domains and a similar head-to-tail organization, involving also CUB1-EGF domains, has been proposed for the C1r-C1s interface (3,5). The contacts between the C1r 2 s 2 tetramer and the six C1q collagen stems are centered on interactions between specific C1q lysine side chains with negative side chains organized around the Ca 2+ sites in C1r CUB1, C1r CUB2, and C1s CUB1 domains, resulting in a total of six collagen-binding sites within the C1r 2 s 2 tetramer (6, 7).…”

“…The C1 complex is formed by the PRM C1q and the heterotetramer C1r 2 s 2 containing two C1s proteases located in the center and one C1r protease on the outside of each C1s molecule (2,3). C1q consists of six heterotrimers each containing the C1qA, C1qB, and C1qC chains.…”