Caspases are an extended family of cysteine proteases that play critical roles in apoptosis. Animals deficient in caspases-2 or -3, which share very similar tetrapeptide cleavage specificities, exhibit very different phenotypes, suggesting that the unique features of individual caspases may account for distinct regulation and specialized functions. Recent studies demonstrate that unique apoptotic stimuli are transduced by distinct proteolytic pathways, with multiple components of the proteolytic machinery clustering at distinct subcellular sites. We demonstrate here that, in addition to its nuclear distribution, caspase-2 is localized to the Golgi complex, where it cleaves golgin-160 at a unique site not susceptible to cleavage by other caspases with very similar tetrapeptide specificities. Early cleavage at this site precedes cleavage at distal sites by other caspases. Prevention of cleavage at the unique caspase-2 site delays disintegration of the Golgi complex after delivery of a pro-apoptotic signal. We propose that the Golgi complex, like mitochondria, senses and integrates unique local conditions, and transduces pro-apoptotic signals through local caspases, which regulate local effectors.

Caspase-3–mediated proteolysis is a critical element of the apoptotic process. Recent studies have demonstrated a central role for mitochondrial proteins (e.g., Bcl-2 and cytochrome c) in the activation of caspase-3, by a process that involves interaction of several protein molecules. Using antibodies that specifically recognize the precursor form of caspase-3, we demonstrate that the caspase-3 proenzyme has a mitochondrial and cytosolic distribution in nonapoptotic cells. The mitochondrial caspase-3 precursor is contained in the intermembrane space. Delivery of a variety of apoptotic stimuli is accompanied by loss of mitochondrial caspase-3 precursor staining and appearance of caspase-3 proteolytic activity. We propose that the mitochondrial subpopulation of caspase-3 precursor molecules is coupled to a distinct subset of apoptotic signaling pathways that are Bcl-2 sensitive and that are transduced through multiple mitochondrion-specific protein interactions.