Substrate phosphorylation specificity of the human c-kit receptor tyrosine kinase

The chimeric EK-receptor (EK-R), consisting of the epidermal growth factor receptor (EGF-R) extracellular binding domain and p145c-kit cytoplasmic signal-generating sequences, was fully functional in forming high and low affinity EGF binding sites and in ligand-regulated receptor and substrate phosphorylation activities. Relative to EGF-R, EK-R activation stimulated kit-characteristic phosphorylation of human 293 fibroblast substrate polypeptides. Transient coexpression of EK-R with candidate substrates resulted in ligand-induced phosphorylation of phospholipase C gamma and guanosine triphosphatase-activating polypeptide. The RAF-1 serine/threonine kinase was shown to be associated with activated EK-R, but no tyrosine phosphorylation could be detected. The faithfulness of EK-R substrate phosphorylation specificity was confirmed with stem cell factor-stimulated p145c-kit.