The membrane attack complex (MAC), is a potent killing molecule of the complement system that assembles sequentially on the membrane of gram negative bacteria and inserts a large beta barrel. Structurally, the MAC is similar to the CDC class of pore forming toxins produced by gram positive bacteria and the MACPF toxin, pleurotolysin produced by oyster mushrooms. These toxins contain two helical bundles, which flank a beta sheet structure. It is these helices that undergo a large conformational change to form the barrel of the pore by forming two transmembrane hairpins (TMHs). Whilst these MACPF/CDC have limited sequence identity, many have been shown to contain a conserved GG motif near the TMH2 region that is important for pore formation.
The assembled MAC consists of seven individual components: C5b, C6, C7, C8α, C8β, C8γ, and between 12-18 copies of C9. Interestingly, C9 polymerises independently of these components in the presence of zinc to form a tubular ring (polyC9) that resembles the structure of a MAC. However, the role of zinc in this polymerisation process is poorly understood.
Using a surface entropy reduction algorithm to improve crystallisability of C9 a mutant was identified that lost its propensity for zinc induced polymerisation. This mutant was demonstrated to retain full activity in the presence of the C5b-8 and shown to assemble on membranes. Furthermore, homology modelling indicate a close juxtaposition of the mutated residues with the GG motif found in a helix-turn-helix. Taken together, these results indicate a region important for zinc induced polymerisation of C9, which may trigger polyC9 formation.