Ca(2+)-activated neutral protease is active in the erythrocyte membrane in its nonautolyzed 80-kDa form

The aim of this study was to investigate the process leading to Ca(2+)-activated neutral protease (CANP) activation in vivo. The unautolyzed form of CANP has been targeted to the erythrocyte membrane by increasing, in a controlled way, the Ca2+ concentration in the cells; this was achieved by incubating erythrocytes with the Ca2+ ionophore A23187 and fixed Ca2+ concentrations. After isolation of the CANP-bearing erythrocyte membrane, we could observe that CANP remained bound to the membrane in the 80-kDa unautolyzed form in the presence of low Ca2+ concentrations (1.75 microM); under these conditions, the preferred CANP substrates (the Ca(2+)-ATPase and Band 3) were cleaved. That the cleavage was due to CANP was shown by the finding that the two substrates were not degraded in the presence of a membrane-permeable irreversible CANP inhibitor, Cbz-Leu-Leu-Tyr-CHN2, nor when the free Ca2+ concentration was decreased to sub microM levels with EDTA. The findings suggest an activation mechanism of CANP based on its translocation to the membrane rather than on its autolysis. In this mechanism, CANP would become reversibly activated on the membrane and would return to the quiescent state after dissociating from it when the cell Ca2+ concentration has returned to the physiological, submicromolar level.