Involvement of GPIIb-IIIa on human platelets in phosphotyrosine-specific dephosphorylation

Washed platelets from either normal donors or patients with thrombasthenia lacking in integrin GPIIb-IIIa were stimulated by thrombin or STA2 with stirring and their kinetics of protein-tyrosine phosphorylation were compared. The early increase in protein-tyrosine phosphorylation on 115 and 75 kDa protein bands was observed within 10 s after stimulation in both normal and thrombasthenic platelets. While both 115 and 75 kDa tyrosine-phosphorylated protein bands were quickly dephosphorylated in normal platelets, thrombin-induced 115 kDa or STA2-induced 115 and 75 kDa protein bands were not dephosphorylated in thrombasthenic platelets. The delay of phosphotyrosine-specific dephosphorylation on those protein bands was observed when thrombin- or STA2-induced aggregation of normal platelets was inhibited by RGDS, an inhibitor of fibrinogen binding to GPIIb-IIIa. These data indicate that fibrinogen binding to GPIIb-IIIa is involved in the regulation of phosphotyrosine-specific dephosphorylation on certain protein bands.