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-activ ating factor (PAF) is transient, whereas sites exposed by alpha-thromb in remain accessible. The same difference is seen in the phosphorylati on of the beta(3) subunit. Inhibition of protein kinases (1 mu M staur osporine) and protein kinase C (10 mu M bisindolylmaleimide) closes bi nding sites exposed by both agonists and induces dephosphorylation of beta(3). Inhibition of Tyr-kinases (20 mu M Herbimycin A) has only a s light effect. Inhibition of Ser/Thr-phosphatases (1 mu M okadaic acid, 30 s preincubation) changes the transient exposure and beta(3) phosph orylation by PAF into the 'permanent' patterns induced by alpha-thromb in. Inhibition of Tyr-phosphatases (100 mu M vanadate) has little effe ct. Preincubation with okadaic acid makes exposed binding sites and ph osphorylated beta(3) insensitive to staurosporine, resulting in expose d alpha(IIB)/beta(3) independent of concurrent phosphorylation/ dephos phorylation. The stoichiometry of beta(3) phosphorylation by a-thrombi n 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 phosp horylation/dephosphorylation of a Ser/Thr-residue in the beta(3) subun it.