Abstract
Limited proteolysis of the plasma membrane calcium transport ATPase (Ca 2+-ATPase) from human erythrocytes by trypsin produces a calmodulin-like activation of its ATP hydrolytic activity and abolishes its calmodulin sensitivity. We now demonstrate a similar kind of activation of the human erythrocyte membrane Ca 2+-ATPase by calpain (calcium-dependent neutral protease) isolated from the human red cell cytosol. Upon incubation of red blood cell membranes with purified calpain in the presence of Ca 2+ the membrane-bound Ca 2+-ATPase activity was increased and its sensitivity to calmodulin was lost. In contrast to the action of other proteases tested, proteolysis by calpain favors activation over inactivation of the Ca 2+-ATPase activity, except at calpain concentrations more than 2 orders of magnitude higher. Exogenous calmodulin protects the Ca 2+-ATPase against calpain-mediated activation at concentrations which also activate the Ca 2+-ATPase activity. Calcium-dependent proteolytic modification of the Ca 2+-ATPase could provide a mechanism for the irreversible activation of the membrane-bound enzyme.
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