PHYSICAL PROXIMITY AND FUNCTIONAL INTERPLAY OF THE GLYCOPROTEIN IB-IX-V COMPLEX AND THE FC RECEPTOR FC-GAMMA-RIIA ON THE PLATELET PLASMA-MEMBRANE

Although the glycoprotein (GP) Ib-IX-V complex and Fc gamma RIIA are d istinct platelet membrane receptors, previous studies have suggested t hat these structures may be co-localized. To determine more directly t he proximity of GP Ib-IX-V and Fc gamma RIIA, we assessed the effects of anti-GP Ib alpha monoclonal antibodies on Fc gamma RIIA-mediated pl atelet aggregation and on the direct binding of polymeric IgG to human platelets. In addition, we directly examined the proximity of Fc gamm a RII and GP Ib-IX-V using flow cytometric fluorescence energy transfe r and immunoprecipitation studies. Preincubation of platelets with eit her of two monoclonal antibodies (AN51 or SZ2) directed against GP Ib alpha completely blocked platelet aggregation by polymeric IgG. Simila rly, these antibodies totally inhibited platelet aggregation by two st rains of viridans group streptococci known to induce aggregation via F c gamma RIIA, In addition, AN51 and SZ2 significantly reduced the bind ing of polymeric IgG to washed fixed platelets, When assessed by flow cytometry, significant levels of bidirectional energy transfer were de tected between Fc gamma RIIA and GP Ib alpha, indicating a physical pr oximity of less than 10 nm between these receptors, This energy transf er was not due to high receptor density, because no homoassociative en ergy transfer was seen, Moreover, immunoprecipitation of Fc gamma RIIA from platelet lysates also co-precipitated GP Ib alpha. These results indicate that GP Ib alpha and Fc gamma RIIA are co-localized on the p latelet membrane and that this association is not random.