The membrane-attack complex (MAC) of the complement system forms transmembrane channels. These channels disrupt the phospholipid bilayer of target cells, leading to cell lysis and death. A number of proteins participate in the assembly of the MAC. Freshly activated C5b binds to C6 to form a C5b-6 complex, then to C7 forming the C5b-7 complex. The C5b-7 complex binds to C8, which is composed of three chains (alpha, beta, and gamma), thus forming the C5b-8 complex. C5b-8 subsequently binds to C9 and acts as a catalyst in the polymerisation of C9. Active MAC has a subunit composition of C5b-C6-C7-C8-C9{n}. Perforin is a protein found in cytolytic T-cell and killer cells. In the presence of calcium, perforin polymerises into transmembrane tubules and is capable of lysing, non-specifically, a variety of target cells. There are a number of regions of similarity in the sequences of complement components C6, C7, C8-alpha, C8-beta, C9 and perforin. The X-ray crystal structure of a MACPF domain reveals that it shares a common fold with bacterial cholesterol dependent cytolysins (Pfam:PF01289) such as perfringolysin O. Three key pieces of evidence suggests that MACPF domains and CDCs are homologous: Functional similarity (pore formation), conservation of three glycine residues at a hinge in both families and conservation of a complex core fold [1].

1: A common fold mediates vertebrate defense and bacterial attack. Rosado CJ, Buckle AM, Law RH, Butcher RE, Kan WT, Bird CH, Ung K, Browne KA, Baran K, Bashtannyk-Puhalovich TA, Faux NG, Wong W, Porter CJ, Pike RN, Ellisdon AM, Pearce MC, Bottomley SP, Emsley J, Smith AI, Rossjohn J, Hartland EL, Voskoboinik I, Trapani Science. 2007;317:1548-1551. PMID:17717151

2: Structure of C8alpha-MACPF reveals mechanism of membrane attack in complement immune defense. Hadders MA, Beringer DX, Gros P; Science. 2007;317:1552-1554. PMID:17872444