IgCAM

IgCAM, or immunoglobulin complement activation module, is a protein domain found in many immune system proteins, including immunoglobulins, complement proteins, and certain cell surface receptors. This module is responsible for the activation of the complement system, a crucial part of the innate immune response. The IgCAM domain consists of approximately 100 amino acids and is characterized by a beta-sheet sandwich structure. It contains a conserved cysteine residue that is essential for the formation of disulfide bonds, which stabilize the domain's structure.

The IgCAM domain is found in various proteins, including the constant regions of immunoglobulins, the complement proteins C1q, C4, and C2, and the cell surface receptors CD46 and CD59. In immunoglobulins, the IgCAM domain is responsible for the binding of the Fc region to the complement proteins C1q and C4, initiating the classical pathway of complement activation. In complement proteins, the IgCAM domain is involved in the formation of the C1 complex and the C3 convertase, which are essential for the amplification and propagation of the complement response.

The IgCAM domain is also found in certain cell surface receptors, such as CD46 and CD59, which are involved in the regulation of complement activation. CD46 is a membrane-bound protein that inhibits the classical pathway of complement activation by binding to C3b and C4b, preventing their further processing. CD59 is a glycosylphosphatidylinositol (GPI)-anchored protein that inhibits the membrane attack complex (MAC) of the complement system, preventing the formation of membrane pores in target cells.

In summary, the IgCAM domain is a versatile protein module that plays a crucial role in the activation and regulation of the complement system. Its presence in various immune system proteins highlights its importance in the host's defense against pathogens and the maintenance of immune homeostasis.