Abstract
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., Thielens, N. M., Arlaud, G. J., Fontecilla-Camps, J. C., and Gaboriaud, C. (2003) J. Biol. Chem. 278, 32157-32164). To identify the C1q-binding sites of C1s-C1r-C1r-C1s, a series of C1r and C1s mutants was expressed, and the C1q binding ability of the resulting tetramer variants was assessed by surface plasmon resonance. Mutations targeting the Glu(137)-Glu-Asp(139) stretch in the C1r EGF module had no effect on C1 assembly, ruling out our previous interaction model. Additional mutations targeting residues expected to participate in the Ca(2+)-binding sites of the C1r and C1s CUB modules provided evidence for high affinity C1q-binding sites contributed by the C1r CUB(1) and CUB(2) modules and lower affinity sites contributed by C1s CUB(1). All of the sites implicate acidic residues also contributing Ca(2+) ligands. C1s-C1r-C1r-C1s thus contributes six C1q-binding sites, one per C1q stem. Based on the location of these sites and available structural information, we propose a refined model of C1 assembly where the CUB(1)-EGF-CUB(2) interaction domains of C1r and C1s are entirely clustered inside C1q and interact through six binding sites with reactive lysines of the C1q stems. This mechanism is similar to that demonstrated for mannan-binding lectin (MBL)-MBL-associated serine protease and ficolin-MBL-associated serine protease complexes.
Highlights
The classical pathway of complement, a major component of innate immune defense against pathogens and altered self, is triggered by C1, a 790-kDa Ca2ϩ-dependent complex assembled from a recognition protein C1q and C1s-C1r-C1r-C1s, a tetramer of two modular proteases, C1r and C1s, that respectively mediate activation and proteolytic activity of the complex [1,2,3]
C1r and C1s are homologous modular proteases each comprising, starting from the N-terminal end, a C1r/C1s, sea urchin EGF2, bone morphogenetic protein (CUB) module [5], an epidermal growth factor (EGF)-like module [6], a second CUB module, two complement control protein modules [7], and a serine protease domain. This modular structure is shared by the mannan-binding lectin-associated serine proteases (MASPs), a group of enzymes that associate with mannan-binding lectin (MBL) and the ficolins and thereby trigger activation of the lectin pathway of complement [8]
The structures of human C1s CUB1-EGF, human mannan-binding lectin-associated protein 19 (MAp19), human MASP-1/3 CUB1-EGF-CUB2, and rat MASP-2 CUB1-EGF-CUB2 have been solved by x-ray crystallography (16 –19), revealing that these domains all associate as head-to-tail homodimers through a highly conserved interface involving interactions between the CUB1 module of one monomer and the EGF module of its counterpart
Summary
The classical pathway of complement, a major component of innate immune defense against pathogens and altered self, is triggered by C1, a 790-kDa Ca2ϩ-dependent complex assembled from a recognition protein C1q and C1s-C1r-C1r-C1s, a tetramer of two modular proteases, C1r and C1s, that respectively mediate activation and proteolytic activity of the complex [1,2,3]. C1r and C1s are homologous modular proteases each comprising, starting from the N-terminal end, a C1r/C1s, sea urchin EGF2 (uEGF), bone morphogenetic protein (CUB) module [5], an EGF-like module [6], a second CUB module, two complement control protein modules [7], and a serine protease domain. This modular structure is shared by the mannan-binding lectin-associated serine proteases (MASPs), a group of enzymes that associate with mannan-binding lectin (MBL) and the ficolins and thereby trigger activation of the lectin pathway of complement [8]. Our data rule out our previous interaction model and provide evidence that C1 assembly involves the same basic Ca2ϩ-dependent mechanism as demonstrated in the case of MBL-MASP and ficolinMASP complexes
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
