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BAP31 is a 28-kDa integral membrane protein of the endoplasmic reticulum whose cytosolic domain contains two identical caspase recognition sites (AAVD.G) that are preferentially cleaved by initiator caspases, including caspase 8. Cleavage of BAP31 during apoptosis generates a p20 fragment that remains integrated in the membrane and, when expressed ectopically, is a potent inducer of cell death. To examine the consequences of maintaining the structural integrity of BAP31 during apoptosis, the caspase recognition aspartate residues were mutated to alanine residues, and Fas-mediated activation of caspase 8 and cell death were examined in human KB epithelial cells stably expressing the caspase-resistant mutant crBAP31. crBAP31 only modestly slowed the time course for activation of caspases, as assayed by the processing of procaspases 8 and 3 and the measurement of total DEVDase activity. As a result, cleavage of the caspase targets poly(ADP-ribosyl) polymerase and endogenous BAP31, as well as the redistribution of phosphatidylserine and fragmentation of DNA, was observed. In contrast, cytoplasmic membrane blebbing and fragmentation and apoptotic redistribution of actin were strongly inhibited, cell morphology was retained near normal, and the irreversible loss of cell growth potential following removal of the Fas stimulus was delayed. Of note, crBAP31-expressing cells also resisted Fas-mediated release of cytochrome c from mitochondria, and the mitochondrial electrochemical potential was only partly reduced. These results argue that BAP31 cleavage is important for manifesting cytoplasmic apoptotic events associated with membrane fragmentation and reveal an unexpected cross talk between mitochondria and the endoplasmic reticulum during Fas-mediated apoptosis in vivo.

E1A and c-myc are oncogenes that can deregulate the cell cycle and promote transformation under conditions where normal cell-cycle checkpoints are inactivated. In situations where cell-cycle checkpoints are intact, the E1A and c-Myc proteins potently induce apoptosis, a property that is believed to be the end result of a cellular response to uncontrolled growth-promoting signals. p53 is a key regulator of E1A and c-myc-induced apoptosis and, together with the oncoproteins, may transcriptionally activate numerous genes whose products influence, or are themselves, members of the core apoptotic machinery. The upstream signaling events and the ultimate apoptotic pathways activated by E1A and c-Myc are discussed in this review.