Surprising phospholipid specificity of two blood clotting proteins: factor VII and protein C

Phospholipid membranes are an important part of the blood clotting cascade and can alter rates of clotting reactions depending on their phospholipid composition. Seven proteins in blood clotting bind reversibly to phospholipid membranes through γ‐carboxyglutamate‐rich (GLA) domains. Although the GLA domains of these proteins are very similar structurally, their membrane binding affinities vary by almost three orders of magnitude. We employed surface plasmon resonance binding studies and enzymatic assays to systematically investigate the phospholipid specificity of these seven GLA domains. It has long been thought that GLA domains bind preferentially to bilayers containing phosphatidylserine (PS), but we found, surprisingly, that two of the GLA domain‐containing blood clotting proteins (factor VII and protein C) bound preferentially to membranes containing phosphatidic acid (PA) or phosphatidylinositol phosphate (PIP), compared to PS. Furthermore, PA and PIP strongly enhanced the enzymatic activities of factor VIIa and activated protein C. Incidentally, of the seven blood clotting proteins with GLA domains, factor VII and protein C are known to have the lowest binding affinities for PS‐ membranes. Our findings provide new insights into the membrane binding mechanism for these two medically important GLA domain‐containing clotting proteins, through PA‐ and PIP‐specific binding interactions.