ATP-dependent proteolysis in erythroid and muscle cells

Reticulocytes contain a soluble, non-lysosomal proteolytic activity which appears responsible for the energy-dependent degradation of several proteins within these cells. This activity requires Mg++-ATP, has a pH-optimum of 7.8, and is inhibited by hemin and sulfhydryl reagents such as iodoacetamide and N-ethylmaleimide. With reticulocyte maturation ATP-dependent activity disappears although protease not requiring ATP remains and even increases. Poly-L-lysine was found to specifically inhibit ATP-dependent activity although it is a poor substrate for this system. Various protease fractions with reduced ATP-dependence and erythrocyte activity not requiring ATP showed decreased sensitivity to polylysine. The effect of polylysine was found to increase with polymer length and may relate to the apparent selectivity of ATP-dependent degradation toward large substrates. In addition, reductive methylation of amino groups on alpha-casein selectively inhibited ATP-dependent breakdown of this protein. Such results suggest that ATP may act at a step which proceeds proteolysis and that substrate amino groups play a role in recognition by the ATP-dependent system. However, such experiments do not yet exclude other mechanisms of action for polylysine's effect. Other experiments have confirmed that reticulocyte activity may involve a non-proteolytic factor which stimulates a protease-containing fraction from these cells. An analogous ATP-dependent activity has been described in muscle suggesting a general function for this degradative activity. Future studies should lead to more insight into the role of ATP and the importance of this system in protein catabolism.