Complement component 1q (C1q) (C1q)

Complement component 1q (C1q) is the recognition subcomponent of the C1 complex that initiates the classical complement pathway of the innate immune system. It binds antigen–antibody complexes—principally IgM and IgG subclasses except IgG4—thereby activating the associated serine proteases C1r and C1s to trigger downstream complement activation[1]. Structurally, C1q is a ~460 kDa hexamer composed of 18 chains (six each of A, B, and C) forming six collagen-like heterotrimers that converge into a central stalk with six C-terminal globular heads (gC1q); the A, C, and B genes (C1QA, C1QC, C1QB) are clustered on chromosome 1 in A–C–B order[1][7][3]. The gC1q domain is a heterotrimeric β-sandwich with a Ca2+-binding site that underpins versatile ligand recognition and Ca2+-dependent interactions with C1r/C1s in the assembled C1 complex[8][6][2]. Beyond complement activation, C1q functions as a pattern-recognition molecule binding diverse self and non-self ligands (e.g., microbial surfaces, apoptotic cells, phosphatidylserine, CRP) and engages cell receptors to regulate phagocytosis and immune signaling, with additional roles described in CNS aging and tumor biology[7][5][4].

Other names

Complement C1qC1q complexC1qA/C1qB/C1qC chainsC1q globular head (gC1q)

For ligands that activate C1q: multivalent binding of gC1q to clustered Fc regions on IgM/IgG immune complexes triggers conformational changes that enable Ca2+-dependent assembly/activation of C1r–C1s within C1, leading to cleavage of C4 and C2 and formation of C3 convertase (C4b2a). For non-complement interactions: gC1q binding to ligands/receptors modulates cellular responses (e.g., phagocytosis via calreticulin–CD91; integrin-mediated effects).