= Complement C6
<Complement C6> is a plasma <protein> involved in assembly of the <membrane attack complex>, a multiprotein structure capable of creating a pore in a target <cell membrane>. It participates downstream of <complement C5> activation, when the larger fragment <C5b> supplies an assembly platform. The intact circulating protein and the assembled pore are different molecular objects.<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 components and assembly sequence]{ref}
A 2012 study determined a full-length <complement C6> <crystal structure> and compared it with related pore-forming proteins. The authors used those structural relationships to propose a mechanism for sequential assembly. The measured structure and the proposed sequence of conformational changes are different kinds of result; a structure of isolated <complement C6> is not a direct recording of a complete pore assembling.<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 and Results, Overall Structure of C6: crystal structure and proposed mechanism]{ref}
For an original stoichiometric bound, assume a model with one <complement C6> per <C5b>-containing assembly. Nine available <C5b> molecules but only six available <complement C6> molecules allow at most six such assemblies. This count does not guarantee six finished pores: later components, binding competence and assembly conditions also matter.<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: one copy of C6 in the mature complex]{ref}
A complementary functional readout is <hemolysis>. The 1965 isolation study of <complement C5> found that its reported lytic function depended on <complement C6>. Combining component-dependence assays with structural evidence connects a molecular participant to a biological effect without equating protein amount with completed pore number.<Isolation of beta 1F-globulin from human serum and its characterization as the fifth component of complement by U. R. Nilsson and others>[Abstract: C6-dependent hemolytic function]{ref}
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