AMINO-ACID-RESIDUES IN THROMBIN-SENSITIVE REGION AND FIRST EPIDERMAL GROWTH-FACTOR DOMAIN OF VITAMIN-K-DEPENDENT PROTEIN-S DETERMINING SPECIFICITY OF THE ACTIVATED PROTEIN-C COFACTOR FUNCTION

Human protein S (PS) potentiates the anticoagulant activity of human b ut not bovine activated protein C (APC), whereas bovine PS is a cofact or to APC from both species. The structural requirements for the speci ficity of the APC cofactor function of human PS are located in its thr ombin-sensitive region (TSR) and the first epidermal growth factor (EG F1)-like module. To elucidate which residues in these two modules dete rmine the specificity of the APC cofactor activity, 41 human PS mutant s were expressed. All mutants were cofactors to human APC and some als o to bovine APC, Residues in TSR (positions 49 and 52) and EGF1 (resid ues 97 and 106) together determined the specificity of the APC cofacto r function, whereas substitution of individual residues did not change specificity. Bovine PS, and mutants expressing cofactor activity to b ovine APC, stimulated phospholipid binding of bovine APC, In contrast, human PS and mutants lacking cofactor activity to bovine APC failed t o support binding of bovine APC to phospholipids. These data indicate that residues in TSR and EGF1 cause the specificity of the APC cofacto r activity and support the concept that key residues in these two modu les interact with APC on the phospholipid surface.