C1Q Rabbit

C1q is a multimeric protein composed of 18 polypeptide chains, which include six A-chains, six B-chains, and six C-chains . These chains form a hexameric structure with a collagen-like triple-helical region and six globular head regions . The collagen-like region is crucial for the multivalent binding of the globular heads to various targets, including immunoglobulins and non-immunoglobulin activators .

The globular heads of C1q are structurally similar to domains found within the tumor necrosis factor (TNF) superfamily of proteins and can bind to a wide range of ligands . This binding capability allows C1q to interact with both immunoglobulin and non-immunoglobulin activators of the complement system, thereby initiating the classical pathway of complement activation .

C1q plays an essential recognition role in both adaptive and innate immunity . It interacts with the proenzyme complex of C1r2–C1s2 to form the C1 complex, which is the first component of the classical pathway of the complement system . The activation of this pathway leads to a cascade of events that result in the opsonization of pathogens, recruitment of inflammatory cells, and lysis of target cells .

In addition to its role in infection and immunity, C1q is involved in the clearance of cell debris and apoptotic cells . This function is vital for maintaining tissue homeostasis and preventing autoimmune responses .

Early chemical studies of human and rabbit C1q provided indirect evidence of collagen-like structures within C1q, as it was reported to have an unusually high glycine content and to contain hydroxylysine and hydroxyproline residues . These studies suggested that C1q might play a structural role in the extracellular matrix, in addition to its role in the activation of the serum complement system .

Recent research has focused on the development and characterization of inhibitors targeting C1q to modulate the classical pathway of the complement system . For example, a potent C1q inhibitor, C1qNb75, has been developed to bind to the globular head modules of human C1q with high affinity, preventing C1q from binding to immunoglobulins and thereby blocking complement activation . Such inhibitors may serve as valuable tools in mechanistic and functional studies of complement and as potential therapeutic agents in conditions of excessive complement activation .