AN EPIDERMAL GROWTH-FACTOR RECEPTOR-LEUKOCYTE TYROSINE KINASE CHIMERIC RECEPTOR GENERATES LIGAND-DEPENDENT GROWTH SIGNALS THROUGH THE RAS SIGNALING PATHWAY

Leukocyte tyrosine kinase (LTK) is a receptor tyrosine kinase that bel ongs to the insulin receptor family. LTR is mainly expressed in pre B cells and brain, Previously we cloned the full-length cDNA of human LT R, but no ligands have so far been identified, and hence, very little is known about the physiological role of LTK. To analyze the function of the LTK kinase, we constructed chimeric receptors composed of the e xtracellular domain of epidermal growth factor receptor and the transm embrane and the cytoplasmic domains of LTK and established cell lines that stably express these chimeric molecules. When cultured in medium containing EGF, growth of these cell lines was stimulated, and these f usion proteins became autophosphorylated and associated with She in ui uo in a ligand-dependent manner, By treatment with EGF, She was associ ated with the Grb2/Ash-Sos complex, Our analyses demonstrate that LTK associates with Grb2/Ash through an internal adaptor, Shc, depending o n a ligand stimulation, The LTK binding site for She was tyrosine 862 at the carboxyl-terminal domain and to a lesser extent tyrosine 485 at the juxtamembrane domain, Both of them are located in NP/AXY motif wh ich is consistent with binding sites for She. These findings demonstra te that LTR can activate the Ras pathway in a ligand dependent manner and that at least one of the functions of this kinase is involved in t he cell growth.