As is known to all, the complement system can lyse the target cells by forming the membrane attack complex (MAC) on the surface of pathogen cell membranes. The MAC is a large poroid protein complex consisting of complement C5b, C6, C7, C8, and C9. Complement component 8 (C8) is a ~150 kDa glycosylated heterotrimer protein composed of 3 polypeptide chains: α chain, β chain, and γ chain, namely C8A, C8B, and C8G, respectively. The α chain and β chain are noncovalently held together, and the γ chain binds to the α chain by a disulfide bond. The α chain and the β chain are highly homologous to C6, C7, and C9, which contain a MAC perforin domain and several other modules, whereas, the γ chain is a single domain polypeptide with a lipocalin fold. The α chain and β chain of C8 play essential roles in the formation of MAC. The MAC perforin domain in the β chain is responsible for integrating C8 with the previously formed C5b-7 complex. Once the C8B is bound to C5b-7, the MAC perforin domain of the α chain undergoes a conformational change to insert into the membrane and also serves as the binding site of C9 to form MAC. C8 is also a critical component of the soluble terminal complex component sC5b-9.
C8 deficiency is usually caused by mutations in the C8A or C8B gene, leading to an increased risk of recurrent bacterial infections, especially neisserial infections, and immunodeficiency.
Fig. 1 Structure of human complement component C8 complex.1
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Reference
From Wikipedia: By SchauderCM - Own work, CC BY-SA 3.0 https://commons.wikimedia.org/wiki/File:3OJY.png
Fig. 1 Structure of human complement component C8 complex.1