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Identification and Partial Characterization of Factor Va Heavy Chain Kinase from Human Platelets*

Factor Va, the essential cofactor for prothrombinase, is phosphorylated on the acidic COOH terminus of the heavy chain of the cofactor, at Ser692, by a platelet membrane-associated casein kinase II (CKII). Consistent with this observation, phosphorylation of the factor Va heavy chain by the platelet kinase was inhibited by heparin. The membrane-associated platelet CKII kinase was partially purified using heparin-agarose, phosphocellulose, and ion exchange chromatography. CKII antigen was monitored using a polyclonal antibody to the α-subunit, and kinase activity in the various fractions was confirmed using human factor Va as a substrate. Immunoblotting experiments using polyclonal antibodies raised against synthetic peptides mimicking a portion of the deduced amino acid sequence of the α-, α′-, and β-subunits of human CKII demonstrated the coexistence of both α- and α′-subunits in platelets and suggested that the platelet CKII kinase may exist in part as an αα′β2 complex. It is also possible that there are two distinct populations of CKII in platelets, one that is αα/ββ and one that is α′α′/ββ. In the presence of the purified platelet-derived CKII, human factor Va incorporates between 0.8 and 1.3 mol of phosphate/mol of factor Va depending on the concentration of the β-subunit in the kinase preparation. A peptide mimicking the sequence 687–705 of the human factor V molecule incorporates radioactivity in the presence of purified CKII and inhibits factor Va heavy chain phosphorylation by the platelet CKII. In contrast, a peptide with an alanine instead of a serine at position 692 neither incorporates phosphate nor inhibits factor Va phosphorylation by the platelet CKII. Human factor Va is inactivated by activated protein C following three cleavages of the heavy chain at Arg506, Arg306, and Arg679. Cleavage at Arg506 is necessary for efficient exposure of the inactivating cleavage site at Arg306. The phosphorylated cofactor has increased susceptibility to inactivation by activated protein C, since phosphorylated factor Va was found to be inactivated approximately 3-fold faster than its native counterpart. Acceleration of the inactivation process of the phosphorylated cofactor occurs because of acceleration of the rate of cleavage at Arg506. These data suggest a critical role for factor Va phosphorylation on the surface of platelets in regulating cofactor activity.