β PDGFR‐IgG chimera demonstrates that human β PDGFR Ig‐like domains 1 to 3 are sufficient for high affinity PDGF BB binding

To localize human β PDGFR binding determinants, we constructed a fusion protein comprising β PDGFR Ig‐like domains 1 to 3 and an IgG 1 Fc domain (β PDGFR‐HFc). β PDGFR‐HFc was expressed as a 200 kDa dimeric molecule and contained Fc epitopes as demonstrated by anti‐mouse Fc antibody recognition. Scatchard analysis revealed that PDGF BB possessed a dissociation constant of 1.5 nM for β PDGFR‐HFc. Thus, β PDGFR Ig‐like domains 1 to 3 are sufficient for high affinity PDGF BB binding. We exploited this fusion protein technology to identify and characterize β PDGFR antagonists using a sensitive β PDGFR immunosorbent assay. In this assay, β PDGFR‐HFc half‐maximally bound to PDGF BB with an affinity of around 150 pM. Suramin, as well as bacterially expressed and refolded human α PDGFR domains 1‐3, inhibited β PDGFR‐HFc binding to PDGF BB half‐maximally at 25 μM and 10 nM respectively. Therefore, α PDGFR Dl‐3, like β PDGFR D1‐3, are sufficient for high affinity PDGF BB binding. Furthermore, the β PDGFR‐HFc immunosorbent assay will be useful to identify β PDGFR antagonists as well as to study α and β PDGFR substitution mutants which further map receptor binding determinants.—Heidaran, M. A., Mahadevan, D., LaRochelle, W. J. β PDGFR‐IgG chimera demonstrates that human β PDGFR Ig‐like domains 1 to 3 are sufficient for high affinity PDGF BB binding. FASEB J. 9, 140‐145 (1995)