Mitochondrial activation of apoptosis

The History 1995 was an exciting year for the field of apoptosis. In that year, Nicholson et alo purified a protease that cleaves poly (ADP-ribose) polymerase (PARP), a protein that was known to be cleaved during apoptosis (Nicholson et al., 1995; Kaufmann et al., 1993). The cDNA encoding this protease, Apopaln, turned out to be the same as CPP32, a cDNA that was cloned in the previous year by Emad Alnemri's group through EST database mining based the conserved sequences between the C. elegans apoptotic protease Ced3 and the mammalian interleukin-l(~ converting enzyme (Femandes-Alnemri et al., 1994). The purification of Apopain unified the biochemistry of mammalian apoptosis with the genetics that had been established in C. elegans. But the crucial question remained: "What activates Apopain?" Wang remembers Alnemri's paper vividly because it caused quite a stir in the laboratory of Joe Goldstein and Mike Brown. As a postdoctoral fellow in their lab, Wang was searching for a sterol-regulated protease that cleaves a pair of membrane bound transcriptional factors, SREBP-1 and -2. He used in vitro translated, 3~Slabeled SREBP-2 as a substrate and incubated it with cell extracts from large-scale cultures of HeLa cells. After several months of fruitless searching, he became rather desperate and somewhat sloppy. One Friday, he left some cytosolic extracts in a 4°C refrigerator for the entire weekend instead of freezing them at -80°C. On the following Monday, to his surprise he found an SREBP-2 cleavage activity (Sca) that was not there the previous Friday. After a brainstorming session with Brown and Goldstein, they postulated that there must be a latent activity that was slowly activated in the refrigerator. By raising the incubation temperature to 37°C, Wang reproduced the activation in only two hours. In a few months, Wang and a graduate student, Jihtung Pai, purified this SREBP cleavage activity using classical biochemical fractionation methods. The sequence of the purified enzyme revealed a novel protein. As they were in the process of cloning this protein, the paper describing the cloning of CPP32 appeared in JBC and to their astonishment the protein sequence of the SREBP cleavage enzyme was the same as CPP32. Unknowingly, they had been following the same path as