Regulation of proteolysis in erythroid cells.

Reticulocytes contain non-lysosomal ATP dependent proteolytic activity which appears important in degrading abnormal proteins as well as certain organelles. ATP acts to repress an endogenous protease inhibitor activity by a process requiring the polypeptide ubiquitin. Free epsilon amino groups on substrates are important for recognition by the full ATP-dependent system but do not appear necessary for hydrolysis per se suggesting that ubiquitin-substrate conjugates repress the inhibitor. ATP-dependent proteolysis declines markedly with reticulocyte maturation and decreases further to negligible levels with aging of erythrocytes. With cell maturation the inhibitor and protease(s) remain but the ubiquitin-containing fraction is less effective in repressing the inhibitor. Finally, additional control mechanisms selective for certain proteins may exist.