Identification of a domain within the carboxyl-terminal region of the βplatelet-derived growth factor (PDGF) receptor that mediates the high transforming activity of PDGF

We have reported previously that a chimeric platelet-derived growth factor receptor (PDGFR) possessing the ligand binding domain of the αPDGFR and the intracellular domain of the βPDGFR (α³⁴⁰β³⁴²R) was markedly more efficient than the wild type αPDGFR (αRWT) in its ability to enhance PDGF-A transforming activity in NIH/ 3T3 fibroblasts. To determine the region within the cytoplasmic domain of βPDGFR that confers this higher transforming activity, we generated several additional α/βPDGFR chimerae. When a chimeric PDGFR possessing the first 933 amino-terminal amino acids from the αPDGFR and the final 165 amino acids from the carboxyl-terminal of the βPDGFR (α⁹³³β⁹⁴²R) was cotransfected with the PDGF-A gene into NIH/3T3 cells, it showed a similar high efficiency to enhance PDGF-A chain transforming activity as α³⁴⁰β³⁴²R. However, when chimeric PDGFRs in which either the kinase insert domain (αβRKI) or the last 79 amino acids from the carboxyl-terminal end of the βPDGFR (α¹⁰²⁴β¹⁰²⁸R) were substituted into αPDGFR sequences were cotransfected with PDGF-A, they showed similar low efficiencies in enhancing transforming activity as the αRWT. These results predicted that the 86 amino acids following the tyrosine kinase 2 domain of βPDGFR (amino acid residues 942-1027) were responsible for the higher transforming activity of βPDGFR. To confirm this finding, we next constructed a chimera in which amino acid residues 942-1028 of the βPDGFR (αβ^942-1028R) were substituted for those in the αPDGFR. Cotransfection experiments indicated that αβ^942-1028R increased transforming activity of PDGF-A to similar extent as the α⁹³³β⁹⁴²R or α³⁴⁰β³⁴²R. Therefore, our findings define a critical do- main within the noncatalytic region of βPDGFR intracellular domain that confers the higher focus forming activity mediated by the βPDGFR.