Assembly of protein S and C4b-binding protein on membranes

The interaction of protein S with membranes and subsequent combination with complement C4b-binding protein (C4BP) was studied. Protein S interacted with phospholipid vesicles in a calcium-dependent manner typical of other vitamin K-dependent proteins. Association of C4BP with protein S showed no apparent selectivity for membrane-bound or solution phase protein S. When bound to the membrane, the protein complexes projected out from the vesicle surface and induced vesicle radius changes of 11.4 nm for tightly packed protein S alone and 17.5 nm for the protein S-C4BP complex. Due to a low density of the protein S-C4BP on the membrane at saturation, the actual projection of this complex out from the membrane surface would be much greater than 17.5 nm. A low saturation density suggested that the protein complex had a large two-dimensional hydrodynamic radius in the plane of the membrane that prevented tight packing of protein. In the presence of calcium, the protein-protein interaction was rapid (k_a ≥ 1·10⁶ M^-1 s^-1) and had very high affinity (K_D ≤ 10^-10 M). The dissociation rate was slow with an estimated rate constant of ≤ 2·10^-4 s ^-1 at 25°C. Protein-protein interaction was much slower in the absence of calcium with an estimated association rate constant of only 2·10⁴ M^-1 s^-1. Consequently, the protein-protein interaction was greatly enhanced by calcium. The very high affinity interaction between protein S and C4BP suggested specificity and an important function for the protein S-C4BP complex in blood. In this regard it was important that C4BP which was bound to protein S on the phospholipid surface could interact with complement protein C4b. These results suggested that protein S may serve an important role in localizing C4BP to negatively charged phospholipid. This would provide regulation of complement activation at sites where the coagulation system is activated such as on the surface of activated platelets.