INDUCTION OF UBIQUITIN-CONJUGATING ENZYMES DURING TERMINAL ERYTHROID-DIFFERENTIATION

A global cellular reorganization occurs during the reticulocyte stage of erythroid differentiation. This reorganization is accomplished part ly through programmed protein degradation. The selection of proteins f or degradation can be mediated by covalent attachment of ubiquitin. We have cloned cDNAs encoding two ubiquitin-conjugating (E2) enzymes, E2 -20K and E2-230K, and found their genes to be strongly induced during the differentiation of erythroblasts into reticulocytes. Induction of the E2-20K and E2-230K genes is specific, as transcript levels for at least two other ubiquitinating enzymes fall during erythroblast differ entiation. In contrast to most proteins induced in reticulocytes, E2-2 0K and E2-230K enzymes are present at strongly reduced levels in eryth rocytes and thus decline in abundance as reticulocyte maturation is co mpleted. This result suggests that both enzymes function during the re ticulocyte stage, when enhanced protein degradation has been observed. These data implicate regulated components of the ubiquitin conjugatio n machinery in erythroid differentiation.