THE ASSOCIATION OF THE PROTEIN-TYROSINE KINASES P56(LCK) AND P60(FYN) WITH THE GLYCOSYL PHOSPHATIDYLINOSITOL-ANCHORED PROTEINS THY-1 AND CD48 IN RAT THYMOCYTES IS DEPENDENT ON THE STATE OF CELLULAR ACTIVATION

Cell surface glycoproteins anchored to the plasma membrane via glycosylphosphatidylinositol (GPI) structures, and hence having no cytoplasmic domains, can nevertheless transmit activation signals in lymphocytes. By immunoprecipitation from detergent lysates and in vitro immune complex kinase reactions the GPI-anchored molecules Thy-1 and CD48 are shown to be associated with multimolecular complexes of phosphoproteins including the protein tyrosine kinases p56lck and p60fyn in both rat and mouse thymocytes. Moreover, the kinase activity associated with Thy-1 on rat thymocytes is shown to be dependent on the activation state of the cells, with stimulation by the lectin, concanavalin A, producing a marked decrease in Thy-1-associated kinase activity. In such activated cells, there is an increased association of kinase activity with CD48, but this may be explained in terms of increased surface expression of CD48 and of increased total kinase activity. Additional phosphoproteins of 85, 36 and 32 kDa were consistently seen as components of the complexes.