Complement C9 is a terminal complement system protein that can assemble in multiple copies as part of the membrane attack complex. Recruitment follows the C5b–complement C6–complement C7–complement C8 assembly. The repeated copies contribute much of the membrane pore, while the initiating components remain part of the complex.[ref]
Structure of complement C6 suggests a mechanism for initiation and unidirectional, sequential assembly of membrane attack complex (MAC) by Alexander E. Aleshin and others. Introduction: terminal assembly and repeated C9 recruitment
A 2016 experiment assembled pores on liposomes and checked function through fluorescent-dye release. The complexes were isolated and imaged as frozen-hydrated particles by cryo-electron microscopy. The reconstructed complete pore contained18 complement C9 copies. That experimental end-product count is not a universal threshold for membrane leakage.[ref]
Structural basis of complement membrane attack complex formation by Marina Serna and others. Results: Cryo-EM structure of the MAC; Molecular architecture; Figure1
For an original accounting model, suppose ten suitable initiators and126 competent complement C9 molecules are available, and stipulate18 copies per completed modeled product. The C9 supply permits at most such products. This is a conditional inventory calculation, not a claim that the assay cannot detect leakage from incomplete assemblies.
The experiment links two levels of evidence. A structural reconstruction describes the arrangement of the isolated particles; dye release tests a membrane consequence in the chosen liposome preparation. Neither readout alone measures the concentration of freely circulating complement C9. Defining the molecular species and the functional endpoint is therefore part of a quantitative interpretation.
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