Abstract
The feline sarcoma virus oncogene v-fms has significantly contributed to the dissection of peptide growth factor action since it encodes the transmembrane tyrosine kinase gp140v-fms, a transforming version of colony-stimulating factor 1 receptor, a member of the growth factor receptor tyrosine kinase family. In this study, the functional significance of structural differences between distinct tyrosine kinase types, in particular between cellular receptors and viral transforming proteins of distinct structural types, has been further investigated, and their functional compatibility has been addressed. For this purpose, major functional domains of three structurally distinct tyrosine kinases were combined into two chimeric receptors. The cytoplasmic gp140v-fms kinase domain and the kinase domain of Rous sarcoma virus pp60v-src were each fused to the extracellular ligand-binding domain of the epidermal growth factor (EGF) receptor to create chimeras EFR and ESR, respectively, which were studied upon stable expression in NIH 3T3 fibroblasts. Both chimeras were faithfully synthesized and routed to the cell surface, where they displayed EGF-specific, low-affinity ligand-binding domains in contrast to the high- and low-affinity EGF-binding sites of normal EGF receptors. While the EFR kinase was EGF controlled for autophosphorylation and substrate phosphorylation in vitro, in vivo, and in digitonin-treated cells, the ESR kinase was not responsive to EGF. While ESR appeared to recycle to the cell surface upon endocytosis, EGF induced efficient EFR internalization and degradation, and phorbol esters stimulated protein kinase C-mediated downmodulation of EFR. Despite its ligand-inducible kinase activity, EFR was partly EGF independent in mediating mitogenesis and cell transformation, while ESR appeared biologically inactive.