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A membrane attack complex attached to a pathogenic cell.

The membrane attack complex (MAC) is typically formed on the surface of intruding pathogenic bacterial cells as a result of the activation of the complement system, and it is one of the weapons of the immune system.


Structure and function

It is composed of a complex of four complement proteins (C5b, C6, C7, and C8) which bind to the outer surface of the plasma membrane, and many copies of a fifth protein (C9) that hookup to one another, forming a ring in the membrane. C6-C9 all contain a common MACPF domain. [1] This region is homologous to cholesterol dependent cytolysins from Gram positive bacteria.[2]

The ring structure formed by C9 is a pore in the membrane that allows free diffusion of molecules in and out of the cell. If enough pores form, the cell is no longer able to survive.

Initiation: C5-C7

Membrane attack complex.

The membrane attack complex is initiated when the complement protein, C5 convertase, cleaves C5 into C5a and C5b.

Another complement protein, C6, binds to C5b.

The C5bC6 complex is bound by C7.

This junction alters the configuration of the protein molecules exposing a hydrophobic site on C7 that allows the C7 to insert into the phospholipid bilayer of the pathogen.

Polymerization: C8-C9

Similar hydrophobic sites on C8 and C9 molecules are exposed when they bind to the complex, so they can also insert into the bilayer.

C8 is a complex made of two proteins C8 beta and C8 alpha-gamma.

C8 alpha-gamma has the hydrophobic area that inserts into the bilayer. C8 alpha-gamma induces the polymerization of 10-16 molecules of C9 into a pore-forming structure known as the membrane attack complex.

  • It has a hydrophilic internal face to allow the passage of water.


CD59 acts to inhibit the complex.


  1. ^ Tschopp J, Masson D, Stanley KK (1986). "Structural/functional similarity between proteins involved in complement- and cytotoxic T-lymphocyte-mediated cytolysis". Nature 322 (6082): 831–4. doi:10.1038/322831a0. PMID 2427956.  
  2. ^ Carlos J. Rosado, Ashley M. Buckle, Ruby H. P. Law, Rebecca E. Butcher, Wan-Ting Kan, Catherina H. Bird, Kheng Ung, Kylie A. Browne, Katherine Baran, Tanya A. Bashtannyk-Puhalovich, Noel G. Faux, Wilson Wong, Corrine J. Porter, Robert N. Pike, Andrew M. Ellisdon, Mary C. Pearce, Stephen P. Bottomley, Jonas Emsley, A. Ian Smith, Jamie Rossjohn, Elizabeth L. Hartland, Ilia Voskoboinik, Joseph A. Trapani, Phillip I. Bird, Michelle A. Dunstone, and James C. Whisstock (2007). "A Common Fold Mediates Vertebrate Defense and Bacterial Attack". Science 317: 1548. doi:10.1126/science.1144706. PMID 17717151.  

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