= Membrane attack complex
The \b[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.<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.]{ref}
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.<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.]{ref}
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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