A novel stimulatory monoclonal antibody (Mab) termed Mab.F11 induces granular secretion and subsequent aggregation of human platelets. Mab.F11 recognizes a unique 32 and 35 kDa protein duplex on the platelet membrane surface, called the F11 receptor; binding of Mab.F11 to its receptor results in increased intracellular phosphorylation of P47, the known protein kinase C (PKC) substrate pleckstrin. In order to determine whether the mechanism of action of Mab.F11 involves direct activation of PKC, two types of functional assays for measuring PKC activity were performed. Measurement of PKC activity in digitonin-permeabilized platelets revealed that Mab.F11 produced a rapid, 2-3 fold increase in the control value in the phosphorylation of the PKC peptide substrate, PKC(19-31) Ser25. The increase in PKC activity induced by Mab.F11 was found to be associated with the platelet membrane; a 1.6-fold control value increase in membrane PKC activity occurred rapidly, within 10 s of the addition of Mab.F11. The translocation from the cytoplasm to the membrane induced by Mab.F11 in PKC isoenzymes alpha and zeta was reversible, whereas translocation of the PKC isoenzymes delta, beta, eta' and theta was irreversible, with PKC levels remaining elevated in the membrane for at least 15 min. Taken together, our results demonstrate that in the initial stages of platelet activation by this stimulatory antibody, the enhanced membrane PKC activity reflects the presence of all six isoenzymes. At later stages, PKC activity is reflective of four isoenzymes. These results demonstrate that separate groups of PKC isoenzymes must be involved in different aspects of platelet activation. The long lag period and prolonged activation time of platelets by Mab.F11 renders this agonist most suitable for identifying the isoenzymes and their specific endogenous protein substrates involved in platelet secretion and aggregation induced by platelet membrane protein antibodies.

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