Complement C7 is a protein of the terminal complement system, recruited after C5b has associated with complement C6. It helps form the membrane-associated precursor to the membrane attack complex. It is one component of an ordered assembly, rather than a complete pore by itself.[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-complement assembly sequence
A 2016 cryo-electron microscopy study placed complement C7 between the early precursor and complement C8. Contacts on different parts of complement C7 connected it with both neighbors, explaining how one component can help propagate assembly. Its presence was also checked by mass spectrometry and functional evidence.[ref]
Structural basis of complement membrane attack complex formation by Marina Serna and others. Results: Molecular architecture of the MAC; Auxiliary domains and MAC assembly
An original supply model makes the sequential dependency tangible. Suppose eight competent early precursors are available but only five complement C7 molecules, and each completed intermediate requires one. At most five such intermediates can form. Supplying more complement C9 would not remove the shortage at this earlier step. The count is an upper bound under the stipulated model, not a predicted yield of functioning pores.
A useful functional comparison would reconstitute a defined set of components with and without complement C7, measuring both assembled material and membrane leakage. Detecting the protein, detecting a complex and detecting leakage answer different questions. A failed leakage assay alone would not identify which assembly step failed.
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