Predimerization of Recombinant Platelet-Derived Growth Factor Receptor Extracellular Domains Increases Antagonistic Potency

Platelet-derived growth factor (PDGF) is a dimeric growth factor acting through tyrosine kinase α- and β-receptors. In both receptors, the extracellular parts are composed of five Ig-like domains. Functional mapping of the extracellular part of the receptors have shown that ligand-binding occurs to Ig-like domains 2 and 3 and that Ig-like domain 4 is involved in receptor−receptor interactions. Recombinant GST-fusion proteins of PDGF α-receptor Ig-like domains 1−4 and β-receptor Ig-like domains 1−3 (αRD1−4-GST and βRD1−3-GST) were generated and compared with their cleaved counterparts (αRD1−4 and βRD1−3) with regard to their ability to block PDGF binding to cell surface receptors. In the case of both the α- and the β-receptors, 100−1000-fold lower concentrations of the GST-fusion proteins were required, as compared to the cleaved forms, for inhibition of PDGF binding to cell surface receptors. αRD1−4-GST and βRD1−3-GST, in contrast to αRD1−4 and βRD1−3, were shown to occur as ligand independent dimers. Covalently cross-linked αRD1−4 dimers displayed a 50-fold increased potency as compared to αRD1−4. We thus conclude that the dimeric nature of αRD1−4-GST and βRD1−3-GST is responsible for the high antagonistic potency of the fusion proteins.