Component of the MAC: binds to C5b,6,7,8 and polymerizes to form membrane pores
cascade, C6, C7, C8, and C9, are structurally related proteins without enzymatic activity. C5b transiently maintains a conformation capable of binding the next proteins in the cascade, C6 and C7. The C7 component of the resulting C5b,6,7 complex is hydrophobic, and it inserts into the lipid bilayer of cell membranes, where it becomes a high-affinity receptor for the C8 molecule. The C8 protein is a trimer composed of three distinct chains, one of which binds to the C5b,6,7 complex and forms a cova-lent heterodimer with the second chain; the third chain inserts into the lipid bilayer of the membrane. This stably inserted C5b,6,7,8 complex (C5b-8) has a limited ability to lyse cells. The formation of a fully active MAC is accomplished by the binding of C9, the final component of the complement cascades, to the C5b-8 complex. C9 is a serum protein that polymerizes at the site of the bound C5b-8 to form pores in plasma membranes. These pores are about 100 A in diameter, and they form channels that allow free movement of water and ions. The entry of water results in osmotic swelling and rupture of the cells on whose surface the MAC is deposited. The pores formed by polymerized C9 are similar to the membrane pores formed by perforin, the cytolytic granule protein found in cytotoxic T lymphocytes and NK cells (see Chapter 10), and C9 is structurally homologous to perforin.
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