6z6v
Globular head of C1q in complex with the nanobody C1qNb75
Structural highlights
DiseaseC1QC_HUMAN Defects in C1QC are a cause of complement component C1q deficiency (C1QD) [MIM:613652. A rare defect resulting in C1 deficiency and impaired activation of the complement classical pathway. C1 deficiency generally leads to severe immune complex disease with features of systemic lupus erythematosus and glomerulonephritis.[1] FunctionC1QC_HUMAN C1q associates with the proenzymes C1r and C1s to yield C1, the first component of the serum complement system. The collagen-like regions of C1q interact with the Ca(2+)-dependent C1r(2)C1s(2) proenzyme complex, and efficient activation of C1 takes place on interaction of the globular heads of C1q with the Fc regions of IgG or IgM antibody present in immune complexes. Publication Abstract from PubMedThe classical pathway of complement is important for protection against pathogens and in maintaining tissue homeostasis, but excessive or aberrant activation is directly linked to numerous pathologies. We describe the development and in vitro characterization of C1qNb75, a single domain antibody (nanobody) specific for C1q, the pattern recognition molecule of the classical pathway. C1qNb75 binds to the globular head modules of human C1q with sub-nanomolar affinity and impedes classical pathway mediated hemolysis by IgG and IgM. Crystal structure analysis revealed that C1qNb75 recognizes an epitope primarily located in the C1q B-chain that overlaps with the binding sites of IgG and IgM. Thus, C1qNb75 competitively prevents C1q from binding to IgG and IgM causing blockade of complement activation by the classical pathway. Overall, C1qNb75 represents a high-affinity nanobody-based inhibitor of IgG- and IgM-mediated activation of the classical pathway and may serve as a valuable reagent in mechanistic and functional studies of complement, and as an efficient inhibitor of complement under conditions of excessive CP activation. Functional and Structural Characterization of a Potent C1q Inhibitor Targeting the Classical Pathway of the Complement System.,Laursen NS, Pedersen DV, Gytz H, Zarantonello A, Bernth Jensen JM, Hansen AG, Thiel S, Andersen GR Front Immunol. 2020 Jul 17;11:1504. doi: 10.3389/fimmu.2020.01504. eCollection, 2020. PMID:32849513[2] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. See AlsoReferences
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