The complement cascade is an innate immune response that involves an orchestrated catalytic reaction of complement proteins that serves to eliminate pathogenic cells as well as augment the adaptive response. The membrane attack complex (MAC) is one of the end stage effectors of this cascade. The MAC punches holes in a wide variety of different cells by forming a hetero-oligomeric pore. This pore is initiated by the C5b6 complex, followed successive specific binding of C7, C8, and completing the pore with 18 units C9. Central to pore formation is the canonical unravelling of two transmembrane hairpins of all components except C5b and C8γ.
C7 plays a crucial role in MAC-mediated lysis by anchoring C5b6 to form a stable surface-bound C5b-7. As this step exclusively occurs during MAC formation, anti-C7 inhibitors may serve as an alternative therapeutic against MAC-specific autoimmune diseases. Thus, elucidating the kinetics behind this integral step will provide invaluable insight to the development of novel anti-C7 inhibitors. Here, the dynamics behind C5b-7 formation were explored by constructing a C7 mutant lacking the FIM domains, and a C7 mutant with a disulfide lock in the first transmembrane hairpin (TMH1), designed to prevent its conformational change. Using surface-plasmon resonance, both C7 mutants surprisingly revealed tight binding to C5b6, although the resulting complex is less stable relative to the wild-type. In parallel, the lytic assay showed a dramatic reduction in capacity to lyse sheep red blood cells.
Put together, the results highlighted that firstly, the C7 FIMs were important for stabilisation but not initial binding of C5b-7 and secondly, formation of a stable C5b-7 does not strictly require unravelling of TMH1 of C7. Altogether, novel future inhibitors that intervene these kinetic steps may serve as potential therapeutics to impair MAC formation in autoimmune diseases.