DUAL MECHANISM OF PROTEIN-TYROSINE PHOSPHORYLATION IN CONCANAVALIN A-STIMULATED PLATELETS

Treatment of human platelets with the lectin Concanavalin A (Con A) re sulted in the tyrosine phosphorylation of several proteins with molecu lar masses 65, 80, 85, 95, 120, 135, and 150 kDa. These proteins were divided in two groups: the first group included the 65-, 85-, 95-, and 120-kDa bands, which were tyrosine phosphorylated also in thrombin-st imulated platelets; the second group (80-, 135-, and 150-kDa bands) in cluded proteins whose tyrosine phosphorylation was exclusively promote d by Con A, but not by thrombin. Members of the second group were rapi dly dephosphorylated when the lectin was displaced from the cell surfa ce by methyl alpha-D-mannopyranoside. Pretreatment of intact platelets with the prostacyclin analog iloprost, inhibited Con A-induced tyrosi ne phosphorylation of the first group of proteins, but had no effect o n the tyrosine phosphorylation of the proteins of the second group. Su ccinyl-Con A, a dimeric derivative of the lectin, which binds to the p latelet surface but does not promote clustering of the receptor, did n ot induce tyrosine phosphorylation of the second group of proteins, al though phosphorylation of some members of the first group was observed . Our results demonstrate the presence of two different mechanisms lea ding to protein-tyrosine phosphorylation in Con A-stimulated platelets , and identify a new signal transduction pathway, promoted by the clus tering of membrane glycoproteins, that produces tyrosine phosphorylati on of specific substrates. This new pathway may be activated by platel et interaction with multivalent ligands, such as adhesive proteins, du ring adhesion, spreading, and aggregation. (C) 1995 Wiley-Liss, Inc.