Processing of the platelet-derived growth factor receptor. Biosynthetic and degradation studies using anti-receptor antibodies.

Studies of platelet-derived growth factor (PDGF) receptor biosynthesis and degradation have been limited by the lack of anti-receptor antibodies. In this study, peptides based on the cDNA-predicted amino acid sequence of the PDGF receptor were used to produce antisera that specifically immunoprecipitated the receptor. PDGF receptor biosynthesis was examined by pulse-chase labeling of cultured fibroblasts with [35S]methionine followed by immunoprecipitation. In BALB/c 3T3 fibroblasts the receptor was synthesized as a 160-kDa precursor that was converted to a mature 180-kDa form within 30-45 min. Removal of high mannose oligosaccharides by endo-beta-N-acetylglucosaminidase H treatment reduced the apparent molecular weight of the 160-kDa precursor but did not affect the migration of the 180-kDa mature receptor. When mannosidase II was inhibited by swainsonine, the 160-kDa precursor failed to mature; instead a 168-kDa form of the receptor was observed. Nevertheless, swainsonine-treated cells responded mitogenically to PDGF. The mature 180-kDa form of the receptor had a half-life of approximately 3 h in the absence of ligand. Addition of PDGF reduced the receptor half-life to 45 min. These studies define and characterize a PDGF receptor precursor, show that receptor degradation is enhanced by PDGF, and demonstrate the functional integrity of incompletely processed PDGF receptors.