Cytoplasmic Y₈ is important in Tpo-induced c-Mpl degradation. (A) Wild-type c-Mpl–expressing cells were treated for increasing periods of time with 1 μg/mL cycloheximide to inhibit the synthesis of new protein. The bottom panel represents densitometry of the 2 different forms of c-Mpl compared with actin. The data are representative of 2 independent experiments. (B) Wild-type and c-Mpl Y₈F–expressing cells were pretreated with 1 μg/mL cycloheximide for 60 minutes before stimulation with Tpo (50 ng/mL) for increasing periods of time. Degradation of the mature 85-kDa form of c-Mpl was determined by Western blot and density compared with unstimulated cells. The data are representative of 4 independent experiments.