Platelet-derived growth factor (PDGF) is a potent mitogen for cells of mesenchymal origin. It consists of homo-E heterodimers of PDGF A and PDGF B chains. All of the isoforms of PDGF have been identified (AA, BB, AB) and they have been shown to bind to two different but related receptor molecules, designated alpha PDGFR and beta PDGFR. The alpha PDGFR binds to all three isoforms of PDGF, while the beta PDGFR exhibits high affinity binding only for PDGF BB. We utilized these different ligand-binding specificities to investigate the PDGF AA binding site in the human alpha PDGFR by constructing chimeric molecules between the human alpha PDGFR and beta PDGFR. Our results demonstrate that amino acids 1 to 340 of the alpha PDGFR comprise the region of the extracellular domain at the alpha PDGFR that confers PDGF AA binding specificities. Dissection within the first three domains of the alpha PDGFR suggests that amino acids 150-190 are required for high affinity binding of PDGF AA but not PDGF BB. Thus, our findings suggest that binding sites for PDGF AA and PDGF BB in the alpha PDGFR extracellular domain are not coincident. We have also investigated the role of alpha PDGFR's kinase insert. Our results indicate that deletions of the kinase insert region alters the gross structure of kinase domain in a manner that affects multiple functions of the receptor molecules and that its primary importance may be in maintaining the structural features of the kinase domain. Mutagenesis of putative autophosphorylation sites within the alpha PDGFR kinase insert suggests that PDGF-induced autophosphorylation of these residues is required for the ability of the mutant receptor to associate with the PI-3 kinase. However, the ligand- dependent association of alpha PDGFR with the PI-3 kinase is not required for its major biological responses.
