摘要：

The inhibitor of () family of proteins are highly conserved through evolution. However, the mechanisms by which these proteins interfere with apoptotic have been enigmatic. Recently, we showed that one of the family proteins, , can bind to and potently inhibit specific (caspases) that function in the distal portions of the proteolytic cascades involved in . In this study, we investigated three of the other known members of the family, c-, c-and . Similarly to , in vitro experiments indicated that c-and c-bound specifically to the terminal effector , caspases-3 and -7, but not to the proximal caspase-8, caspases-1 or -6. In contrast, failed to bind tightly to any of these . Recombinant c-and c-also inhibited the activity of caspases-3 and -7 in vitro, with estimated of <=0.1 microM, whereas did not. The domain-containing region of c-and c-was sufficient for inhibition of these caspases, though proteins that retained the RING domain were somewhat more potent. Utilizing a cell-free system in which caspases were activated in cytosolic extracts by addition of , c-and c-inhibited both the generation of and proteolytic processing of pro-caspase-3. Similar results were obtained in intact cells when c-and c-were overexpressed by gene transfection, and was induced by the anticancer drug, . Cleavage of c-or c-was not observed when interacting with the caspases, implying a different mechanism from the , the broad spectrum inactivator of caspases. Taken together, these findings suggest that c-and c-function similarly to by inhibiting the distal , caspases-3 and -7, whereas presumably inhibits via other targets.

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