Complement C1q, a 460 kDa hexameric glycoprotein, is the first recognition subcomponent of the complement classical pathway. This protein is made up of 18 polypeptide chains, consisting of 9 non-covalently linked subunits having 6 of A, B, and C chains (Fig.1). These chains contain three different domains which are a short N-terminal region (3-9 residues), a collagen region having 81 residues, and a C-terminal globular (gC1q) domain of about 185 residues. C1q is able to bind many antibodies and ligands, including IgG, IgM, CRP, phosphatidylserine (PS), HIV-1, HTLV-1, as well as some receptors such as integrin α2β1 and calreticulin-CD91.
C1q has a variety of complement and non-complement functions. Firstly, C1q can bind a range of ligands derived from self, non-self, and altered self and modulate the functions of immune and non-immune cells including dendritic cells and microglia. Moreover, C1q is associated with the clearance of apoptotic cells and subsequent B cell tolerance. Previous research has demonstrated that C1q plays a crucial role in pregnancy where its deficiency and dysregulation can have adverse effects, resulting in missed abortion, preeclampsia, or spontaneous loss, and infections. In addition, C1q is generated in the central nervous system and has a protective role against pathogens and possible inflammatory functions while interacting with aggregated proteins leading to neurodegenerative diseases.
Fig.1 Structure of C1q. (Kouser, 2015)
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Reference
Kouser, L., et al. Emerging and novel functions of complement protein C1q. Frontiers in immunology. 2015, 6, 317.
Fig.1 Structure of C1q. (Kouser, 2015)