Factor V stored in platelets is an important source of factor Va for the prothrombinase complex. Investigations of potential platelet factor Va-binding proteins, using factor Va light chain affinity chromatography, identified a disulfide-linked multimeric protein with a reduced mobility of 155 kDa in the column eluate. Immunodepletion and immunoblotting indicated that this protein was multimerin. Multimerin specifically bound factors V and Va and the isolated factor Va light chain, but not the heavy chain of factor Va. Factor V stored in platelets, but not plasma factor V, was found to be complexed with multimerin. Multimerin immunodepletion of resting platelet lysates was associated with the removal of factor V and the loss of factor V coagulant activity. Immunoelectron microscopic studies colocalized factor V with multimerin in the -granules of resting platelets. With thrombin-induced platelet activation, we observed dissociation of factor Va-multimerin complexes, multimerin-independent membrane binding of factor Va, and prothrombinase activity that was not inhibitable by multimerin antibodies. This study indicates that platelet factor V is stored as a complex with multimerin and suggests a possible role for multimerin as a carrier protein for factor V stored in platelets.

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