The membrane attack complex is an assembly of complement system proteins that can form an opening through a target lipid bilayer. Assembly begins with C5b and recruits complement C6, complement C7, complement C8 and multiple copies of complement C9. The resulting structure connects a soluble protein cascade to a physical change in membrane permeability.[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. Abstract, Journal of Biological Chemistry 287 (2012), pp.10210–10222; DOI10.1074/jbc.M111.327809.
The scientific puzzle is how proteins carried in solution become a membrane-spanning assembly. A 2012 study determined the crystal structure of complement C6 and compared it with related structures. The authors proposed that coordinated conformational changes expose membrane-inserting regions as components join the growing complex. The directly measured C6 structure and the proposed assembly pathway are distinct levels of evidence.[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. Abstract and Figures1,6–7: measured C6 structure and explicitly proposed assembly models.
An illuminating comparison is between detecting complement proteins near a membrane and demonstrating a completed membrane-spanning structure. Recruitment is one step; assembly and insertion are further steps. Counting a precursor or one bound component alone therefore cannot establish that a fully formed membrane attack complex has changed the membrane's transport properties.
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