Phosphoinositide 3-kinase and p72syk noncovalently associate with the low affinity Fc gamma receptor on human platelets through an immunoreceptor tyrosine-based activation motif. Reconstitution with synthetic phosphopeptides.

Previously, we have demonstrated that the cytoplasmic tyrosine kinase p72syk is coupled to the platelet Fc receptor for IgG (Fc gamma RIIA) (Chacko, G. W., Duchemin, A. M., Coggeshall, K. M., Osborne, J. M., Brandt, J. T., and Anderson, C. L. (1994) J. Biol. Chem. 269, 32435-32440). Further analysis of the platelet activation by Fc gamma RIIA demonstrated that Fc gamma RIIA is also inducibly coupled to the serine/threonine and lipid kinase, phosphoinositide 3-kinase (PI 3-K). activation of platelets with anti-Fc gamma RIIA antibodies resulted in the noncovalent association of PI 3-K with Fc gamma RIIA as well as an increase in Fc gamma RIIA-associated PI 3-K activity. Binding of both p72syk and PI 3-K to Fc gamma RIIA was reconstituted with synthetic phosphopeptides corresponding to the sequence of the atypical immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic domain of Fc gamma RIIA. Our findings demonstrate that coupling of both p72syk and PI. 3-K activities to Fc gamma RIIA is regulated by tyrosine phosphorylation of the ITAM, and we speculate that p72syk might act as an adapter to recruit PI 3-K to activated Fc gamma RIIA.