Binds activated protein C. Enhances protein C activation by the thrombin-thrombomodulin complex; plays a role in the protein C pathway controlling blood coagulation. Expressed strongly in the endothelial cells of arteries and veins in heart and lung, less intensely in capillaries in the lung and skin, and not at all in the endothelium of small vessels of the liver and kidney. N-glycosylated. A soluble form exists; probably released by a metalloprotease. Seems to have the same activity as the membrane-bound form.