UBE2O remodels the proteome during terminal erythroid differentiation

Terminal differentiation is characterized by a respecification of the global protein complement, as epitomized by erythrocytes, whose cytosol is ~98% globin. Remodeling of the proteome involves programmed elimination of generic cellular constituents in parallel with synthesis of cell-type-specific proteins. The erythroid proteome undergoes a rapid transition at the reticulocyte stage; however, the mechanisms driving elimination of preexisting cytosolic proteins are unidentified. UBE2O is a ubiquitin-conjugating enzyme expressed at elevated levels during late stage erythropoiesis. A Ube2o null mutation in mice results in anemia. Proteomic analysis of this mutant suggests that UBE2O is a broad-spectrum ubiquitinating enzyme that remodels the erythroid proteome. In particular, a hallmark of the reticulocyte to mature erythrocyte transition—ribosome elimination—is defective in Ube2o-/- mutants. UBE2O recognizes ribosomal proteins and other substrates directly, targeting them to proteasomes for degradation. Thus, in reticulocytes, and perhaps other highly differentiated cells, the induction of ubiquitinating factors may drive the transition from a complex to a simple proteome.