Identification of Sulfo-Tyrosines of Factor V.

The factor Va molecule is the essential cofactor of the prothrombinase complex. This complex composed of factor Xa and factor Va assembled on a platelet membrane-surface in the presence of Ca²⁺ ions converts membrane-bound prothrombin to thrombin. Single chain factor V does not bind factor Xa. Single-chain factor V is cleaved by thrombin first at Arg709 followed by cleavages at Arg1018 and Arg1545 to produce the heavy and light chains of the active cofactor (factor Va) and two activation peptides. Efficient thrombin cleavage and activation of factor V is essential for cofactor function and requires tyrosine sulfation. Tyrosine sulfation of factor V also appears to regulate its activity. Seven tyrosine residues in factor V, Tyr665, Tyr696, Tyr698, Tyr1494, Tyr1510, Tyr1515, and Tyr1565 have been identified as potential sites of sulfation. However, which residues are sulfated and their contribution to procofactor activation and cofactor function still remain to be elucidated. Two of the sulfation sites Tyr696 and Tyr698 are located in the acidic amino acid region near to the first required thrombin cleavage site at Arg709. Recent data demonstrated that these residues are essential for factor V activation and cofactor activity. Another acidic amino acid region, 1490–1520 is adjacent to the thrombin cleavage site at Arg1545 required for light chain formation. This region also contains three potential sulfation sites at residues 1494, 1510, and 1515 and was shown to be required for optimum procofactor activation. To ascertain which of these three residues is important for procofactor activation, site-directed mutagenesis was used to create recombinant factor V molecules with mutations ¹⁴⁹³DY¹⁴⁹⁴→AF, ¹⁵⁰⁸DDY¹⁵¹⁰→AAF and ¹⁵¹⁴DY¹⁵¹⁵→AF. The clotting and cofactor activity of the ¹⁴⁹³DY¹⁴⁹⁴→AF and ¹⁵¹⁴DY¹⁵¹⁵→AF mutants was similar to the clotting activity observed with the wild type recombinant factor Va molecule following activation by thrombin or RVV-V activator. In contrast, under similar experimental conditions recombinant factor V with the substitution ¹⁵⁰⁸DDY¹⁵¹⁰→AAF was deficient in its clotting activity and had impaired cofactor activity. Moreover, following prolonged incubation with thrombin, no light chain formation was observed in the factor V molecule bearing the ¹⁵⁰⁸DDY¹⁵¹⁰→AAF mutation. Thus, amino acid residues 1508–1510 of factor V are required for thrombin interaction with the procofactor which in turn appears necessary for cleavage at Arg1545. Studies of sulfated proteins have shown that the effect of sulfo-tyrosines on protein structure/function can be preserved by replacing them with glutamic acid. To explicitly identify the sulfated tyrosines on the factor V molecule, we mutated Tyr696, Tyr698 and Tyr1510 to glutamic acid and transfected them into COS-7L cells. Expression was performed in the presence of media containing or devoid of sulfate. In the presence of sulfate, the cofactor and clotting activities of the DY⁶⁹⁶DY⁶⁹⁸→DEDE and DDY¹⁵¹⁰→DDE mutants, separately were similar to the wild type recombinant factor Va molecule. However, in the absence of sulfate, the wild type and the mutant recombinant factor V molecules had both impaired cofactor activity and clotting activity following their activation with thrombin. However, their respective activity was higher than the activity of the factor V molecule bearing the ¹⁵⁰⁸DDY¹⁵¹⁰→AAF mutation. Our data suggest that residues 696, 698, and 1510 of factor V appear to be sulfated and might be important for procofactor activation and cofactor function.