gC1q

gC1q, short for globular C1q domain, is a conserved protein domain of about 140 amino acids found in the C1q subcomponent of the classical complement pathway and in a broader family of related proteins known as the C1q/TNF-related (CTRP) superfamily. The globular C1q domain is named for its compact, globular structure, in contrast to the collagen-like stalks that characterize C1q.

In the classical complement pathway, the globular heads of C1q mediate recognition by binding to antibodies within immune complexes and to a variety of non-immunoglobulin ligands. This recognition triggers the activation of the associated serine proteases C1r and C1s, leading to downstream proteolysis and the cascade that drives opsonization, inflammation, and lysis. The gC1q domain is the principal ligand-binding module of C1q and contributes to the versatility of the molecule in detecting diverse targets.

Beyond C1q, gC1q domains are found in many other extracellular proteins, including adiponectin and other CTRP family members. In these proteins, the domain participates in multimerization, receptor interactions, and signaling processes that link innate immunity with metabolism and inflammation. Structurally, the gC1q domain belongs to the C1q/TNF-related superfamily and typically adopts a beta-sandwich fold that is evolutionarily conserved across species.

Clinical relevance includes the role of C1q in immune surveillance and the association between C1q deficiency and autoimmune diseases such as systemic lupus erythematosus. Alterations affecting gC1q-containing proteins can influence inflammatory and metabolic pathways, making the domain a focus of research in immunology and related fields.