Abstract
Previous studies have shown that the activity of the ubiquitin-mediated proteolytic system declines markedly following reticulocyte maturation, but the specific alterations responsible for this phenomenon have not been defined. We find that the rate of ATP-dependent degradation of 125I-albumin is reduced 20-fold in lysates of rabbit erythrocytes, as compared to reticulocyte lysates. The activity of the proteolytic system in erythrocyte extracts can be restored by supplementation with components of the ubiquitin-protein ligase system purified from reticulocytes by affinity chromatography. These components are the ubiquitin-carrier protein E 2, the activity of which is nearly completely absent, and the ligase E 3, the activity of which is partially reduced in erythrocytes. Erythrocyte extracts contain other ligases which attach a single, or a few ubiquitin molecules to proteins; these products are different from the multi-ubiquitin derivatives which are formed by the ligase system of protein breakdown. Mature red cells may thus serve to distinguish between different ubiquitin-protein ligase systems with presumably different functions.
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