SIGNAL-TRANSDUCTION THROUGH A BIMOLECULAR RECEPTOR TYROSINE PROTEIN-KINASE COMPOSED OF A PLATELET-DERIVED GROWTH-FACTOR RECEPTOR-CD4 CHIMERA AND THE NONRECEPTOR TYROSINE PROTEIN-KINASE LCK

We have generated a novel ''receptor tyrosine kinase'' by fusing the e xtracellular and transmembrane domain of the mouse platelet-derived gr owth factor receptor (PDGFR) to the cytoplasmic domain of CD4 and coex pressing the construct with the murine cytoplasmic tyrosine protein ki nase p56lck. NMuMG cells, which are mouse mammary gland epithelial cel ls that lack endogenous platelet-derived growth factor (PDGF) receptor expression, were stably transfected with both PDGFR-CD4 and p56lck. T he PDGFR-CD4 chimeric protein was expressed at the cell surface and fo rmed a complex with p56lck. Addition of PDGF to these cells led to inc reased tyrosine phosphorylation of a 56-kDa protein likely to be p56lc k and several unidentified cellular proteins. The enzymatic activity o f p56lck was increased after treatment with PDGF, indicating that dime rization (or oligomerization) mediated by ligand binding at the cell s urface is capable of inducing the activation not only of receptor tyro sine kinases but nonreceptor tyrosine kinases as well. However, the PD GFR-CD4.p56lck complex was, in contrast to the wild type PDGF receptor , not able to induce a PDGF-dependent mitogenic response or DNA synthe sis in NMuMG cells. Analysis of several known substrates of the PDGFR- signaling pathway indicates an early block in the transduction of the signal generated by p56lck.