Proteins of the C Attack Mechanism

The individual proteins of the C attack mechanism have been purified and characterized, and some of their physical properties are shown in Table III. 

C5 is 200,000 molecular weight glycoprotein composed of two dissimilar polypeptide chains linked by one or more disulfide bonds. Its molecular weight has been variously estimated at between 180,000 and 220,000 (Nilsson and Miiller-Eberhard, 1965 Nilsson et al., 1972, 1975). The cleavage of C5 by various proteolytic enzymes is the only known enzymatic step of the C attack sequence. 

000 molecular weight fragment generated on cleavage from the amino-terminal end of the heavy (a) chain of C5 (Nilsson et al., 1975), is a potent chemotactic factor (Snyderman et al., 1969) and ana-phylatoxin (Cochrane and Miiller-Eberhard, 1968 Wissler, 1972 Feman- 

C6 is composed of a single polypeptide chain with a molecular weight estimated to be between 111,000 and 128,000, and a sedimentation coefficient of 5.7 S (Podack et al., 1976a). It has the ability to bind to and stabilize C5b and does not appear to be cleaved during this or any subsequent reaction. 

the key molecule of the C attack mechanism, has electrophoretic mobility, a molecular weight of 163,000, and a sedimentation coefficient of 8 S (Kolb and Muller-Eberhard, 1976). The binding of C8 to a cell-bound C567 complex results in membrane damage, and C9 serves to accelerate this action (Stolfi, 1968 Tamura et al., 1972). C8 is composed of three polypeptide chains, with the 77,000 molecular weights y chain bound to the 13,700 a chain by one or more disulfide bonds, while the 63,000 j8 chain is bonded to the a-y complex by noncovalent forces (Kolb and Miiller-Eberhard, 1976). 

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