IDENTIFICATION OF THE SRC FAMILY KINASES, LCK AND FGR IN PLATELETS - THEIR TYROSINE PHOSPHORYLATION STATUS AND SUBCELLULAR-DISTRIBUTION COMPARED WITH OTHER SRC FAMILY MEMBERS

We have identified the Src family members, Lck and Fgr in resting huma n and rodent platelets and compared their subcellular distributions an d tyrosine phosphorylation status to those of the other Src family kin ases to gain insights into the signal transduction pathways active in maintaining platelets in the circulation. Like Fyn, Lyn, and Yes, most of Fgr and Lck was detergent-insoluble in human and rat platelets. In comparison, Src showed higher detergent solubility than the Src-relat ed kinases. Most all human platelet Src was detergent-soluble, while t hat of rodent platelets was present in all detergent fractions. We als o compared the tyrosine-phosphorylation status of Lck and Fgr to other Src family members in resting platelets using immunoprecipitation and immunoblotting. All of these Src family members except Fgr exhibited substantial phosphotyrosine antibody labeling. The partitioning of the se kinases, with the exception of Src, with the detergent-insoluble fr action, their tyrosine-phosphorylation status, and co-localization wit h endocytotic vesicles lead us to hypothesize that the Src family kina ses are involved in signaling events that drive cytoskeletal reorganiz ation and active endocytosis of plasma proteins by circulating platele ts.