ROLE OF IMMUNOGLOBULIN-LIKE DOMAINS 2-4 OF THE PLATELET-DERIVED GROWTH-FACTOR ALPHA-RECEPTOR IN LIGAND-RECEPTOR COMPLEX ASSEMBLY

Platelet-derived growth factor (PDGF) is a dimeric protein that exerts its effects through tyrosine Kinase alpha- and beta-receptors. The ex tracellular part of each receptor is composed of five Ig-like domains. Recombinant forms of a-receptor domains 1-4 (alpha RD1-4), 1-3 (alpha RD1-3), and 1 and 2 (alpha RD1-2) were prepared after expression in C hinese hamster ovary cells and were used to study the assembly of solu ble ligand-receptor complexes. When incubated with micromolar concentr ations of PDGF, both alpha RD1-3 and alpha RD1-4 formed complexes of 1 :2 molar composition, i.e. one dimeric PDGF molecule bound two soluble receptors, alpha RD1-3, in contrast to alpha RD1-4, formed detectable 1:1 complexes under conditions of ligand excess. alpha RD1-4 displaye d an increased ability to form 1:2 complexes as compared with alpha RD 1-3 under conditions of limiting concentrations of ligand. We thus con clude that Ig-like domain 4-mediated receptor-receptor interactions co ntribute to 1:2 PDGF.alpha RD1-4 complex formation. Since alpha RD1-4 and alpha RD1-3 were equipotent in blocking binding of subnanomolar co ncentrations of PDGF to cell-surface receptors, we also conclude that this effect is predominantly achieved through formation of Ig-like dom ain 4-independent 1:1 ligand-receptor complexes. Finally, since alpha RD1-2 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 alph a-receptor contains epitopes of particular importance for PDGF-AA bind ing and that most of the PDGF-BB-binding epitopes reside in Ig-like do mains 1 and 2.