Complement C1QA

Complement C1q subcomponent subunit A (C1QA) is a constituent of the complement component C1q (a subunit of the first complement component C1). Human C1QA is a 28 kDa polypeptide chain with 244 amino acids forming a lollipop-shaped structure, of which 81 amino acids form a long collagen-like N-terminal "stalk" and 136 amino acids form a characteristic spherical "head". C1QA polypeptide chain is connected to C1QB chain by disulfide bond forming an A-B or B-A doublet, whereas, C1QC is connected to adjacent C chain by a disulfide bond forming a C-C doublet. These disulfide-linked doublets are noncovalently linked to form a ‘bouquet-of-flowers’ like hexamer structure, that is intact complement component C1q.

C1QA functions as a sub-chain of complement C1q, which together with C1q form the complement C1 complex. When antigen-antibody complexes are formed, the 6 triple helical strands of C1q can bind to the Fc domains of antibodies, such as IgM and IgG (except IgG4), leading to the activation of the complement classical pathway. C1QA abnormality causes C1q deficiency, complement dysfunction, and immunodeficiency.

Fig. 1 Structure and assembly of C1q subunit.¹

Reference

1. Ain, Danyaal, et al. "The role of complement in the tumor microenvironment." Faculty Reviews 10 (2021).