Platelet-derived growth factor (PDGF) is a disulfide-linked dimer consisting of two related polypeptide chains, designated PDGF A and PDGF B. that are the products of distinct genes. The gene encoding the human PDGF B chain is the normal counterpart of the v-sis oncogene. Site-directed mutagenesis has defined an internal 84-codon region responsible for transforming activity. Since PDGF A and PDGF B have distinct transforming phenotypes and cellular locations. we constructed twelve chimeric PDGF molecules to investigate structural regions of PDGF A or PDGF B associated with PDGF functions. Characterization of the chimeric constructs identified a 40-amino acid region within PDGF B responsible for its particular receptor binding properties and potent transforming phenotype. PDGF B's potent transformation segregated with activation of both PDGF receptor types. Moreover, we dissociated PDGF B's potent transforming phenotype from its membrane-associative property. Further studies of this region identified a 13-amino acid subdomain sensitive to deletions, all of which abrogated PDGF receptor functional activation. Continuing studies will be directed at further characterization of the receptor binding domain and the identification of competitive antagonists to be used for blocking PDGF-mediated responses. To facilitate functional characterization of PDGF proteins, a baculovirus vector system was employed for the overexpression of the alpha and beta PDGF receptors. The biological properties of the recombinant PDGF receptors were shown to be very similar to those of the PDGF receptor expressed in mammalian cells. Also, the recombinant PDGF receptors were purified to near homogeneity using immunoaffinity chromatography.
