Specific membrane binding of factor VIII is mediated by O-phospho-L-serine, a moiety of phosphatidylserine

Phosphatidylserine, a negatively charged lipid, is exposed on the platelet membrane following cell stimulation, correlating with the expression of factor VIII receptors. We have explored the importance of the negative electrostatic potential of phosphatidylserine vs chemical moieties of phosphatidylserine for specific membrane binding of factor VIII. Fluorescein-labeled factor VIII bound to membranes containing 15% phosphatidic acid, a negatively charged phospholipid, with low affinity compared to phosphatidylserine-containing membranes. Binding was not specific as it was inhibited by other proteins in plasma. Factor VIII bound to membranes containing 10% phosphatidylserine in spite of a varying net charge provided by 0-15% stearylamine, a positively charged lipid. The soluble phosphatidylserine moiety, O-phospho-L-serine, inhibited factor VIII binding to phosphatidylserine-containing membranes with a Ki of 20 mM, but the stereoisomer, O-phospho-D-serine, was 5-fold less effective. Furthermore, binding of factor VIII to membranes containing synthetic phosphatidyl-D-serine was 5-fold less than binding to membranes containing phosphatidyl-L-serine. Membranes containing synthetic phosphatidyl-L-homoserine, differing from phosphatidylserine by a single methylene, supported high-affinity binding, but it was not specific as factor VIII was displaced by other plasma proteins. O-Phospho-L-serine also inhibited the binding of factor VIII to platelet-derived microparticles with a Ki of 20 mM, and the stereoisomer was 4-fold less effective. These results indicate that membrane binding of factor VIII is mediated by a stereoselective recognition O-phospho-L-serine of phosphatidylserine and that negative electrostatic potential is of lesser importance.