LIGAND-INDUCED DIMERIZATION OF THE PLATELET-DERIVED GROWTH-FACTOR RECEPTOR - MONOMER-DIMER INTERCONVERSION OCCURS INDEPENDENT OF RECEPTOR PHOSPHORYLATION

Abstract

The platelet-derived growth factor (PDGF) receptor is a single membrane-spanning polypeptide of 180,000 daltons with a ligand-stimulatable tyrosine kinase site. We have investigated changes in the structure and association state of the receptor that are induced by ligand binding, but which precede autophosphorylation. Chemical cross-linking of PDGF-bound 32P-labeled receptor and 125I-PDGF-labeled receptor resulted in the generation of a radiolabeled cross-linked complex of 370-390 kDa. This band, as well as the 180-190-kDa PDGF receptor band, were recognized by a PDGF receptor-specific antipeptide antibody. The appearance of the 370-390-kDa band was PDGF-dependent and was seen irrespective of whether the receptor was membrane-bound, solubilized, or highly (.apprx. 90%) purified. Sedimentation analysis of the 125I-PDGF cross-linked receptor showed that both 180-190- and 370-390-kDa labeled species sedimented as a single peak at about 11.5 S, a position expected of a receptor dimer, demonstrating that the liganded receptor exists essentially as a dimer. In contrast, unliganded receptors sedimented as a single species at 7 S, a position consistent with a monomeric structure. The monomer-dimer interconversion was absolutely ligand-dependent and occurred independent of autophosphorylation. These results demonstrate an intimate correlation between PDGF binding and interreceptor bond formation, and raise the possibility that the phenomenon may be causally linked to the process of kinase activation.