Synthesis, phosphorylation, and degradation of multiple forms of the platelet-derived growth factor receptor studied...

We have developed a monoclonal antibody, designated PR7212 (IgG1), which specifically recognizes the platelet-derived growth factor receptor (PDGFR) of primate cells. The antibody recognizes an extracellular epitope of the receptor, demonstrated by its ability to bind to intact cells. Using this antibody, we have detected three forms of PDGFR of approximately 180, 164, and 130 kDa. All three of the forms were detected by Western blot analysis of human dermal fibroblasts. Immunoprecipitates of 32P-labeled membrane extracts of human dermal fibroblasts demonstrate that phosphorylation of all three forms of the receptor is stimulated by PDGF. In addition, several smaller molecules were detected, ranging in size from 113 to 49 kDa, which are also phosphorylated in response to PDGF addition. These smaller molecules may be either PDGFR kinase substrates or partially degraded PDGFR. Only the 180- and the 164-kDa forms of the receptor are detectable from immunoprecipitates of soluble extracts of 35S-metabolically labeled cells. Pulse-chase experiments demonstrate that the 164-kDa form is a precursor of the 180-kDa molecule. After PDGF binding at 37 degrees C, the 180-kDa form disappears from the cell surface in parallel with a decrease in 125I-PDGF binding, providing evidence that occupation results in internalization of PDGFR rather than inactivation.