Distribution of ligand-occupied alpha IIb beta 3 in resting and activated human platelets determined by expression of a novel class of ligand-induced binding site recognized by monoclonal antibody AP6

The sequence beta (3)203–228 is involved, in a yet undetermined manner, in alpha IIb beta 3 function. We now show that murine monoclonal antibody (MoAb) AP6, specific for beta (3)211–221, binds to alpha IIb beta 3 on adenosine diphosphate (ADP)-activated platelets only when the receptor is occupied by intact fibrinogen. The ligand-induced binding- site reported by AP6 is unique in that it is not expressed following occupancy by either RGD peptides or the gamma-chain carboxy-terminal dodecapeptide. Binding of AP6 to platelets coincides temporally with the binding of the MoAb 9F9, specific for a receptor-induced binding site on fibrinogen. Thus, AP6 reports the binding of fibrinogen to the recognition pocket of alpha IIb beta 3. Its binding to thrombin- stimulated washed platelets correlates with secretion as determined using an MoAb to P-selectin. When ultrathin sections of nonactivated platelets were examined by immunogold staining and electron microscopy, AP6 identified a pool of alpha IIb beta 3 colocalizing with P-selectin and suggesting the presence of alpha IIb beta 3-ligand complexes in the alpha-granule membrane. There was little binding of AP6 to surface alpha IIb beta 3 of unstimulated platelets. After ADP-induced activation, AP6 was abundantly distributed over the entire platelet surface, including pseudopods, but only when fibrinogen was present in the medium. ADP had little effect on AP6 reactivity within platelets. This contrasted with washed platelets and thrombin, where extensive AP6 binding was observed within internal membrane pools as early as 10 to 15 seconds after stimulation. Surface labeling with AP6 followed slower kinetics. Flow cytometry on Triton X-100 permeabilized fixed platelets confirmed AP6 binding to alpha IIb beta 3 within the platelet. Thus, our results provide evidence of (1) a pool of alpha-granule alpha IIb beta 3 occupied by ligand in nonactivated platelets; (2) thrombin- induced activation of alpha IIb beta 3 within the platelet, and (3) thrombin-induced mobilization of ligand-bound alpha IIb beta 3 to the surface.