Vitronectin (S Protein) binds to membrane attack complexes that fail to insert into membranes. The result is a soluble complex of S Protein and C5b-9 and this has been named SC5b-9 (Dodds, A.W. and Sim, R.B. (1997)). Activation of complement in the absence of bilipid layer membranes results in most of the activated C5 forming SC5b-9 complexes. This can occur with activating particles such as immune complexes, yeast and bacterial cell walls. The SC5b-9 complexes can have more than one C9 (usually three) and more than one S Protein (usually three) per complex. Thus, the typical molecular weight is ~1,030,000 Da.
One C5b-7 complex can bind up to three molecules of S Protein. If C8 or C8 and C9 have already bound to the C5b-7 complex three S Protein molecules will bind to these complexes as well. The C5b6 and C5b-7 complexes sometimes diffuse away from the target cell and enter the membrane of a nearby cell. This is called bystander lysis or “reactive lysis” and can be a significant source of pathology. Binding of S Protein blocks this non-specific lysis. Diffusion of these early complexes away from the activating surface may result in addition of C8 and C9 to the complex prior to S Protein binding. Once S Protein binds to these complexes it prevents their membranolytic functions, thus creating soluble complexes that are subsequently cleared from circulation.
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