BIOLOGICAL AND BIOCHEMICAL ACTIVITIES OF A CHIMERIC EPIDERMAL GROWTH FACTOR-ELK RECEPTOR TYROSINE KINASE

Eph, Elk, and Eck are prototypes of a large family of transmembrane pr otein-tyrosine kinases, which are characterized by a highly conserved cysteine-rich domain and two fibronectin type III repeats in their ext racellular regions. Despite the extent of the Eph family, no extracell ular ligands for any family member have been identified, and hence, li ttle is known about the biological and biochemical properties of these receptor-like tyrosine kinases. In the absence of a physiological lig and for the Elk receptor, we constructed chimeric receptor molecules, in which the extracellular region of the Elk receptor is replaced by t he extracellular, ligand-binding domain of the epidermal growth factor (EGF) receptor. These chimeric receptors were expressed in NIH 3T3 ce lls that lack endogenous EGF receptors to analyze their signaling prop erties. The chimeric EGF-Elk receptors became glycosylated, were corre ctly localized to the plasma membrane, and bound EGF with high affinit y. The chimeric receptors underwent autophosphorylation and induced th e tyrosine phosphorylation of a specific set of cellular proteins in r esponse to EGF. EGF stimulation also induced DNA synthesis in fibrobla sts stably expressing the EGF-Elk receptors. In contrast, EGF stimulat ion of these cells did not lead to visible changes in cellular morphol ogy, nor did it induce loss of contact inhibition in confluent monolay ers or growth in semisolid media. The Elk cytoplasmic domain is theref ore able to induce tyrosine phosphorylation and DNA synthesis in respo nse to an extracellular ligand, suggesting that Elk and related polype ptides function as ligand-dependent receptor tyrosine kinases.