Preliminary functional characterization of a 24,000 dalton platelet surface protein involved in platelet activation

We have recently investigated three monoclonal antibodies, FMC 8, FMC 48 and FMC 56, directed against a 24,000 dalton (24K) cell surface protein on human platelets and pre-B progenitor cells. These monoclonal antibodies (at 0.1-10 micrograms/ml) were found to be potent inducers of platelet aggregation with normal and with aspirin-treated platelets. FMC 8 Fab' and FMC 56 (Fab')2 fragments blocked aggregation induced by intact antibody. The Fab' and (Fab')2 fragments, however, were ineffective by themselves (at 100 micrograms/ml) as platelet agonists but augmented (at 10-25 micrograms/ml) the aggregation response of normal and aspirin-treated platelets to threshold concentrations of ADP. It has been suggested that the 24K cell surface protein may be identical to a 24K cAMP-dependent phosphoprotein involved in regulating transmembrane calcium flux. To examine this, 32P-phosphate-loaded platelets were treated with prostaglandin E1 to raise intracellular cAMP levels and then analysed on two-dimensional O'Farrell gels. No labelled phosphoprotein corresponded in mobility to the 24K cell surface protein. Further, FMC 56 failed to immunoprecipitate a 24K phosphoprotein. The combined results suggest that the three monoclonal antibodies activate platelets via a prostaglandin-independent and as yet undefined mechanism which requires the intact bivalent antibody for full platelet response. Fab' fragments (monovalent) and (Fab')2 fragments (altered Fab-Fab configuration) are ineffective as platelet agonists but can augment the response of other stimuli. Thus, further studies with this system should allow delineation of those intracellular events necessary for partial and full platelet responses.