REASSESSMENT OF PROTEIN-TYROSINE PHOSPHORYLATION IN THROMBASTHENIC PLATELETS - EVIDENCE THAT PHOSPHORYLATION OF CORTACTIN AND A 64-KD PROTEIN IS DEPENDENT ON THROMBIN ACTIVATION AND INTEGRIN ALPHA(IIB)BETA(3)

Tyrosine phosphorylation of a number of platelet proteins is dependent on platelet integrin alpha(III),beta(3) (also termed GPIIb-IIIa) and its engagement in aggregation, For instance, in type I thrombasthenic platelets, which lack alpha(IIb)beta(3) and do not aggregate, several substrates are either poorly or not phosphorylated. We have compared t hrombasthenic platelets of type I, type II (15% alpha(IIb)beta(3), fun ctional), and variant type (50% alpha(IIb)beta(3), no fibrinogen bindi ng). The platelets from the three patients exhibited the same low tyro sine phosphorylation profiles, confirming the key role of functional a lpha(IIb)beta(3) in initiating protein tyrosine phosphorylation. We no ted that in addition to the characteristic absence of the 100 to 105 k D doublet, a 77 to 80 kD doublet and to a lesser extent a 64-kD band, exhibited low phosphorylation kinetics, but with normal initial phosph orylation rates (up to 60 seconds). Similar results were obtained by i nhibition of thrombin aggregation of control platelets by alpha(IIb)be ta(3) antagonists (the RGDS peptide or the monoclonal antibody 10E5), or in the absence of stirring (fibrinogen binding, but no aggregation) , These results suggest that tyrosine phosphorylation of the 77 to 80 kD doublet, identified by immunoprecipitation as the cytoskeletal prot ein cortactin, and the 64 kD band are dependent both on thrombin activ ation during early steps and on the late steps of alpha(IIb)beta(3) en gagement in aggregation. Implications as to involvement of step-specif ic kinase and/or phosphatase activities are discussed. (C) 1997 by The American Society of Hematology.