Ligand-induced ubiquitination of the platelet-derived growth factor beta-receptor plays a negative regulatory role in its mitogenic signaling.

We have recently reported that the platelet-derived growth factor beta-receptor undergoes polyubiquitination as a consequence of ligand binding and that a mutant beta-receptor lacking the carboxyl-terminal 98 amino acid residues of the receptor (CT98 mutant) does not undergo this posttranslational modification (Mori, S., Heldin, C.-H., and Claesson-Welsh, L. (1992) J. Biol. Chem. 267, 6429-6434). In this paper, we report that a mutant beta-receptor in which the carboxyl-terminally located Tyr-1009 and Tyr-1021, which are potential autophosphorylation sites, were simultaneously changed to phenylalanine residues by site-directed mutagenesis (Y1009F/Y1021F mutant) showed only 15% efficiency in ligand-induced ubiquitination compared with the wild-type receptor. Ligand-stimulated degradation of the receptor-bound ligand, as well as of the receptor itself, was partially impaired in the cells expressing the ubiquitination-deficient receptor mutants (CT98 and Y1009F/Y1021F). Furthermore, the ubiquitination-deficient receptors possessed an amplified mitogenic activity, as judged by a [3H]thymidine incorporation assay. These data suggest that ligand-induced ubiquitination plays a negative regulatory role in mitogenic signaling of the platelet-derived growth factor beta-receptor, possibly by promoting the efficient degradation of the ligand-activated receptor.