Platelet-derived growth factor (PDGF) exerts its effects by interaction with two specific receptor molecules, alphaPDGFR and betaPDGFR. In an effort to better characterize this interaction and produce sufficient quantities of native and mutant receptors for structural, functional and biochemical studies, I have utilized the baculovirus expression system to produce recombinant alphaPDGFR and betaPDGFR at levels 50- to 100-fold greater than found in NIH/3T3 cells. The expression of recombinant receptor proteins was characterized by western blot analysis and ligand binding studies, and revealed that both receptors were produced as transmembrane proteins with molecular weights of approximately 160 kilodaltons. Ligand-binding analysis with radiolabelled PDGF-AA or PDGF-BB revealed saturable, high affinity binding to alphaPDGF in intact. virally infected Sf9 cells. In contrast, betaPDGFR expressed at similar levels in intact Sf9 cells showed high affinity binding only for PDGF-BB. Time course experiments demonstrated that early in the course of infection, both receptors were present in low numbers, in a nonphosphorylated state, and exhibited ligand-stimulated tyrosine protein kinase activity. However, with increasing time postinfection (and increasing receptor number). both receptors became constitutively tyrosine phosphorylated as demonstrated by antiphosphotyrosine signal on western blotting. This property has been utilized to purify each receptor to near homogeneity by antiphosphotyrosine immunoaffinity chromatography. Future studies will concentrate on the production of monoclonal antibodies to receptor and on localization of the ligand binding domain(s) for each receptor.
