Thapsigargin protects human erythrocyte Ca(2+)-ATPase from proteolysis

The effect of thapsigargin on the activation by partial proteolysis of the plasma membrane Ca(2+)-ATPase was studied in intact human erythrocyte membranes and in the purified enzyme. The enzyme was maximally activated in the absence of thapsigargin within 1 min of exposure to trypsin. However, in the presence of thapsigargin maximal activation was achieved only after 5 min trypsin digestion. Thapsigargin did not alter the pattern of proteolysis as revealed by SDS-PAGE of the tryptic fragments, although it slowed down the rate of appearance of the fragments. Thapsigargin also enhanced the activation of the enzyme by calmodulin. These findings suggest that, although thapsigargin at low concentrations has no effect on the catalytic activity of the Ca(2+)-ATPase in vitro in the absence of calmodulin, it could interfere with its regulation in vivo.