IMMUNOGLOBULIN-LIKE DOMAIN 4-MEDIATED RECEPTOR RECEPTOR INTERACTIONS CONTRIBUTE TO PLATELET-DERIVED GROWTH FACTOR-INDUCED RECEPTOR DIMERIZATION

Platelet-derived growth factor (PDGF) is a dimeric growth factor that activates its tyrosine kinase receptor by inducing receptor dimerizati on. In this study, we investigated if receptor-receptor interactions, in addition to ligand-receptor interactions, contribute to the ligand- induced dimerization of the PDGF receptors. Analysis of two deletion m utants of the PDGF alpha-receptor indicated a role for Ig-like domain 4 in ligand-receptor or receptor-receptor interactions. When the fourt h Ig-like domain of the PDGF alpha-receptor instead was replaced with the corresponding sequence of the stem cell factor receptor, the bindi ng of PDGF-AA and -BB was not affected, nor was the ability to form ho modimeric receptor complexes. This indicates that Ig-like domain 4 doe s not participate in ligand-receptor interactions. However, the chimer as did not form heterodimers with wild-type PDGF alpha or beta-recepto rs. Together, these findings suggest that Ig-like domain 4 mediates sp ecific receptor-receptor interactions. This notion was also supported by the finding that a soluble form of Ig-like domain 4 of the PDGF alp ha-receptor acted as a PDGF alpha-receptor antagonist. We conclude tha t specific receptor-receptor interactions contribute to PDGF receptor dimerization in vivo and that complementary epitopes in Ig-like domain 4 mediate these interactions. Our experiments also identify Ig-like d omain 4 as a target for PDGF antagonists.