Identification of the C1q-binding Sites of Human C1r and C1s: A REFINED THREE-DIMENSIONAL MODEL OF THE C1 COMPLEX OF COMPLEMENT*
AUTOR(ES)
Bally, Isabelle
FONTE
American Society for Biochemistry and Molecular Biology
RESUMO
The C1 complex of complement is assembled from a recognition protein C1q and C1s-C1r-C1r-C1s, a Ca2+-dependent tetramer of two modular proteases C1r and C1s. Resolution of the x-ray structure of the N-terminal CUB1-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 Glu137-Glu-Asp139 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 Ca2+-binding sites of the C1r and C1s CUB modules provided evidence for high affinity C1q-binding sites contributed by the C1r CUB1 and CUB2 modules and lower affinity sites contributed by C1s CUB1. All of the sites implicate acidic residues also contributing Ca2+ 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 CUB1-EGF-CUB2 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.
ACESSO AO ARTIGO
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2740559Documentos Relacionados
- A porin from Klebsiella pneumoniae: sequence homology, three-dimensional model, and complement binding.
- A complex prediction: three-dimensional model of the yeast exosome
- gC1q-R/p32, a C1q-binding protein, is a receptor for the InlB invasion protein of Listeria monocytogenes
- Three-dimensional model for the hormone binding domains of steroid receptors.
- Identification of a gC1q-binding protein (gC1q-R) on the surface of human neutrophils. Subcellular localization and binding properties in comparison with the cC1q-R.