FACTOR-V IS COMPLEXED WITH MULTIMERIN IN RESTING PLATELET LYSATES ANDCOLOCALIZES WITH MULTIMERIN IN PLATELET ALPHA-GRANULES

Factor V stored in platelets is an important source of factor Va for t he prothrombinase complex. Investigations of potential platelet factor Va-binding proteins, using factor Va light chain affinity chromatogra phy, identified a disulfide-linked multimeric protein with a reduced m obility of 155 kDa in the column eluate, Immunodepletion and immunoblo tting indicated that this protein was multimerin. Multimerin specifica lly 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 n ot plasma factor V, was found to be complexed with multimerin, Multime rin immunodepletion of resting platelet lysates was associated with th e removal of factor V and the loss of factor V coagulant activity. Imm unoelectron microscopic studies colocalized factor V with multimerin i n the alpha-granules of resting platelets. With thrombin-induced plate let activation, we observed dissociation of factor Va-multimerin compl exes, multimerin-independent membrane binding of factor Va, and prothr ombinase activity that was not inhibitable by multimerin antibodies. T his study indicates that platelet factor V is stored as a complex with multimerin and suggests a possible role for multimerin as a carrier p rotein for factor V stored in platelets.