A functionally defective protein C (PC)-Mie, detected the plasma of a patient with hereditary thrombophilia, has Lys substituted for gamma-carboxyglutamic acid (Gla)26 residue. The activation rate of PC-Mie by Protac or thrombin in the absence of Ca2+ and that by thrombin with native thrombomodulin (TM), recombinant soluble truncated TM or on cultured endothelial cells in the presence of Ca2+ were all apparently lower than that of normal PC. The anticoagulant activity of Protac-activated PC (APC)-Mie on the plasma clotting time and the rate of inactivation of factor Va by APC-Mie in the presence of phospholipids were lower than those seen with normal APC. APC-Mie and normal APC bound equally to protein S and to biotinyl-factor Va. However, neither PC-Mie nor APC-Mie bound to phospholipids and to cultured human endothelial cells. It was similar to that observed with Gladomainless PC/APC, but different from that seen with normal PC/APC. These results suggest that Gla26-dependent conformation is required for the binding of PC/APC to phospholipids, TM and the surface of endothelial cell PC/APC receptor, but not to protein S and factor Va.