c-Cbl acts as a ubiquitin E3 ligase in the Tpo-stimulated ubiquitination of c-Mpl. (A) BaF-Mpl cells overexpressing WT-Cbl or the E3 ligase dead CblC₃₇₉A. (B) WT-Cbl– and CblC₃₇₉A-expressing cells were pretreated with MG-132, then with or without rhTpo for 60 minutes, and c-Mpl ubiquitination was analyzed by IP and Western blot. The data shown are representative of 3 independent experiments. (C) These cells were analyzed for proliferation using an MTT assay at increasing concentrations of rhTpo. The data shown represent mean (± SE) of 3 independent experiments (*P < .05, ***P < .001).