The finding that
phospholipid micelles lowered the Ca2+ concentration required for
autolysis of the calpains led to a hypothesis suggesting that the calpains are translocated to the plasma membrane where they interact with
phospholipids to initiate their
autolysis. However, the effect of plasma membranes themselves on the Ca2+ concentration required for
calpain autolysis has never been reported. Also, if interaction with a membrane lowers the Ca2+ required for
autolysis, the membrane-bound-
calpain must autolyze itself, because it would be the only
calpain having the reduced Ca2+ requirement. This implies that the
autolysis is an intramolecular process, although several studies have shown that
autolysis of the calpains in an in vitro assay and in the absence of
phospholipid is an intermolecular process. Inside-out vesicles prepared from erythrocytes had no effect on the Ca2+ concentration required for
autolysis of either mu- or
m-calpain, although
phosphatidylinositol (PI) decreased the Ca2+ concentration required for
autolysis of the same calpains. The presence of a substrate for the calpains,
beta-casein, reduced the rate of
autolysis of both mu- and
m-calpain both in the presence and in the absence of PI, suggesting that mu- and
m-calpain autolysis is an intermolecular process in the presence of PI just as it is in its absence. Because IOV have no effect on the Ca2+ concentration required for
calpain autolysis, association with the plasma membrane, at least with erythrocyte plasma membranes, does not initiate
calpain autolysis by reducing the Ca2+ concentration required for
autolysis as suggested by the membrane-activation hypothesis. Interaction with a membrane may serve to bind calpains to their substrates rather than promoting
autolysis.