Exposure of ligand-binding sites on platelet integrin alpha(IIB)/beta(3) by phosphorylation of the beta(3) subunit

The exposure of ligand-binding sites for adhesive proteins on platelet integrin alpha(IIB)/beta(3) (glycoprotein IIB/IIIA) by platelet-activating factor (PAF) is transient, whereas sites exposed by alpha-thrombin remain accessible. The same difference is seen in the phosphorylation of the beta(3) subunit. Inhibition of protein kinases (1 mu M staurosporine) and protein kinase C (10 mu M bisindolylmaleimide) closes binding sites exposed by both agonists and induces dephosphorylation of beta(3). Inhibition of Tyr-kinases (20 mu M Herbimycin A) has only a slight effect. Inhibition of Ser/Thr-phosphatases (1 mu M okadaic acid, 30 s preincubation) changes the transient exposure and beta(3) phosphorylation by PAF into the 'permanent' patterns induced by alpha-thrombin. Inhibition of Tyr-phosphatases (100 mu M vanadate) has little effect. Preincubation with okadaic acid makes exposed binding sites and phosphorylated beta(3) insensitive to staurosporine, resulting in exposed alpha(IIB)/beta(3) independent of concurrent phosphorylation/ dephosphorylation. The stoichiometry of beta(3) phosphorylation by a-thrombin is 0.80+/-0.10. Thus, one of the mechanisms that regulates exposure and closure of ligand-binding sites on the alpha(IIB)/beta(3) is phosphorylation/dephosphorylation of a Ser/Thr-residue in the beta(3) subunit.