Complement C2

Complement C2 is an important protein in the complement system which regulates the complement activation of classical and MBL pathways. C2 can bind to membrane-bound C4b and is then cleaved by C1s or MASP2, forming C2b and C4bC2a (a C3-convertase). This is a key step in complement activation. C3 proteins are cleaved by C3 convertase and lead to the generation of crucial effector molecules – C3a and C3b. Besides, C2 is related to first-line defense against microbial infection that is important for detection and clearance of the invading pathogens.

Fig. 1 Complement C2 structure.¹

In general, the activation of the alternative pathway is C3 dependent, which is usually intact in C2 deficiency and can trigger the production of the membrane attack complex (MAC) independently of C2. Nevertheless, in some situations, in the absence of C2, C3 is not efficiently cleaved leading to a limited deposition of C3 fragments on immune complexes and the surface of apoptotic cells. Thus, circulating apoptotic cells turned into a source of self-antigen for auto-antibodies that participate in the generation of immune complexes. The immune complexes are deposited throughout the body, potentially resulting in localized inflammatory reactions in joints and kidneys, and finally causing renal disease from chronic activation of the complement system.

Reference

1. From Wikipedia: By Emw - Own work, CC BY-SA 3.0 https://commons.wikimedia.org/wiki/File:Protein_C2_PDB_2i6q.png