Degradation of human complement component C4b in the presence of the C4b-binding protein-protein S complex

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Abstract
Vitamin K-dependent protein S and the higher MW form of C4b-binding protein (C4bp-high) interact, forming a 1:1 complex with a KD of .apprx. 1 .times. 10-7 M. The effect of protein S on the degradation of C4b by factor I (C3b inactivator) and C4bp was investigated both in fluid phase and on cell surfaces, with the use of highly purified components. Fluid-phase degradation of C4b was monitored on sodium dodecyl sulfate/polyacrylamide-slab-gel electrophoresis, and the effect on surface-bound C4b was estimated by hemolytic assay. No effect of protein S could be demonstrated in any of the systems used. Although bound to C4bp, protein S is neither involved in, nor does it affect, the interaction between C4bp and C4b. The binding sites on the C4bp molecule for protein S and for C4b are independent and different.