Activation of membrane-associated procaspase-3 is regulated by Bcl-2

The mechanism by which membrane-bound Bcl-2 inhibits the activation of cyto plasmic procaspases is unknown. Here we characterize an intracellular, memb rane-associated form of procaspase-3 whose activation is controlled by Bcl- 2. Heavy membranes isolated from control cells contained a spontaneously ac tivatable caspase-3 zymogen. In contrast, in Bcl-2 overexpressing cells, al though the caspase-3 zymogen was still associated with heavy membranes, its spontaneous activation was blocked. However, Bcl-2 expression had lit tie effect on the levels of cytoplasmic caspase activity in unstimulated cells. Furthermore, the membrane-associated caspase-3 differed from cytosolic cas pase-3 in its responsiveness to activation by exogenous cytochrome c. Our r esults demonstrate that intracellular membranes can generate active caspase -3 by a Bcl-2-inhibitable mechanism, and that control of caspase activation in membranes is distinct from that observed in the cytoplasm. These data s uggest that Bcl-2 may control cytoplasmic events in part by blocking the ac tivation of membrane-associated procaspases.