Role of immunoglobulin-like domains 2-4 of the platelet-derived growth factor α-receptor in ligand-receptor complex assembly

Abstract

Platelet-derived growth factor (PDGF) is a dimeric protein that exerts its effects through tyrosine kinase α- and β-receptors. The extracellular part of each receptor is composed of five Ig-like domains. Recombinant forms of α-receptor domains 1-4 (αRD1-4), 1-3 (αRD1-3), and 1 and 2 (αRD1-2) were prepared after expression in Chinese hamster ovary cells and were used to study the assembly of soluble ligand-receptor complexes. When incubated with micromolar concentrations of PDGF, both αRD1-3 and αRD1-4 formed complexes of 1:2 molar composition, i.e. one dimeric PDGF molecule bound two soluble receptors. αRD1-3, in contrast to αRD1-4, formed detectable 1:1 complexes under conditions of ligand excess. αRD1-4 displayed an increased ability to form 1:2 complexes as compared with αRD1-3 under conditions of limiting concentrations of ligand. We thus conclude that Ig-like domain 4- mediated receptor-receptor interactions contribute to 1:2 PDGF·αRD1-4 complex formation. Since αRD1-4 and αRD1-3 were equipotent in blocking binding of subnanomolar concentrations of PDGF to cell-surface receptors, we also conclude that this effect is predominantly achieved through formation of Ig-like domain 4-independent 1:1 ligand-receptor complexes. Finally, since aRD1-2 bound PDGF-BB with high affinity, whereas PDGF-AA was bound only with low affinity, we conclude that Ig-like domain 3 of the PDGF α-receptor contains epitopes of particular importance for PDGF-AA binding and that most of the PDGF-BB-binding epitopes reside in Ig-like domains 1 and 2.