THE GLA(26) RESIDUE OF PROTEIN-C IS REQUIRED FOR THE BINDING OF PROTEIN-C TO THROMBOMODULIN AND ENDOTHELIAL-CELL PROTEIN-C RECEPTOR, BUT NOT TO PROTEIN-S AND FACTOR VA

A functionally defective protein C (PC)-Mie, detected in the plasma of a patient with hereditary thrombophilia, has Lys substituted for gamm a-carboxyglutamic acid (Gla)(26) residue. The activation rate of PC-Mi e by Protac or thrombin in the absence of Ca2+ and that by thrombin wi th 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-ac tivated PC (APC)-Mie on the plasma clotting time and the rate of inact ivation 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 eq ually to protein S and to biotinyl-factor Va. However, neither PC-Mie nor APC-Mie bound to phospholipids and to cultured human endothelial c ells. It was similar to that observed with Gla-domainless PC/APC, but different from that seen with normal PC/APC. These results suggest tha t Gla(26)-dependent conformation is required for the binding of PC/APC to phospholipids, TM and the surface of endothelial cell PC/APC recep tor, but not to protein S and factor Va.