Complement proteins C5b-9 initiate secretion of platelet storage granules without increased binding of fibrinogen or von Willebrand factor to newly expressed cell surface GPIIb-IIIa.

The functional and conformational activation of cell surface glycoproteins IIb-IIIa (GPIIb-IIIa) was probed in platelets stimulated to secrete by complement proteins C5b-9. Gel-filtered human platelets exposed to the purified human C5b-9 proteins exhibited non-lytic secretory release of both alpha- and dense granule storage pools with only a small increase in total binding of 125I-fibrinogen (less than 3000 molecules/cell) to the cell surface. By contrast to ADP- or thrombin-activated platelets, increased 125I-fibrinogen bound to C5b-9 platelets was not inhibited by Arg-Gly-Asp-containing peptides, suggesting that the high affinity membrane receptor for fibrinogen is not expressed under these conditions. C5b-9-stimulated platelets also failed to bind 125I-von Willebrand factor (less than 1 ng/10(8) platelets), confirming that the adhesive protein receptor function of cell surface GPIIb-IIIa is not expressed in these cells. Although specific binding of 125I-fibrinogen or 125I-von Willebrand factor did not significantly increase after C5b-9 assembly, these proteins elicited de novo expression of the GPIIb-IIIa activation-associated epitope recognized by monoclonal antibody PAC-1, and binding of this antibody to C5b-9 platelets was fully competed by Arg-Gly-Asp-containing peptides. These data suggest that the metabolic events which trigger granule secretion after C5b-9 insertion into the plasma membrane cause cell surface GPIIb-IIIa to be expressed in an activation-associated but functionally incompetent conformation.