The fluid-phase assembly of the membrane attack complex of complement

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Abstract
The dynamics and protein stoichiometry of the fluid-phase assembly of the membrane attack complex of complement were characterized by using light-scattering intensity measurements. The assembly proceeded in an ordered manner with generation of stable and highly reproducible intermediates. In the absence of phospholipid or C8, mixtures of C5b-6 and C7 self-associated to fluid phase-C5b-7 which had a weight-average molecular weight of (4.1 .+-. 0.2) .times. 106. This corresponded to an average of nine C5b-7 complexes per particle. The particles appeared heterodisperse on sucrose gradients with s20,w values ranging from 21 to 39 S. Addition of C8 and C9 caused no further aggregation or disassembly of the particles. When excess C8 was added to the aggregated C5b-7, the ratio of C8 incorporated per C5b-7 moiety was 0.98 .+-. 0.03. At saturating levels of C9, the C9/C5b-8 ratio in the particles was 7.2 .+-. 0.6. Incorporation of C8 caused a small increase in the Z-averaged particle diffusion coefficient [(9.9-10.3) .times. 10-8 cm2/s], indicating that it added in a manner that "filled in the gaps" in the C5b-7 particles. C9 caused only small decreases in the particle diffusion coefficient and substantially decreased the f/fmin ratio. The time course for C9 incorporation into fluid phase-C5b-8 indicated an initial rapid phase followed by a slow phase. The rapid phase corresponded to the incorporation of about one C9 for every two C5b-8 complexes. This suggested that one C9 binding site was accessible on about half of the C5b-8 complexes. This may imply that only about half of the C5b-8 complexes were capable of C9 polymerization so that the ratio of C9 incorporated per functional C5b-8 was (14 .+-. 2)/1. The initial velocity of the slow phase of C9 addition gave an activation energy of 37 kcal/mol. The activation energy for c5b-8-independent polymerization of C9 had a similar value of 41 kcal/mol. Light-scattering intensity measurements seemed to be a highly reliable method for quantitative characterization of the fluid-phase assembly.