The inhibitor of apoptosis protein-binding domain of Smac is not essential for its proapoptotic activity.
Smac/DIABLO, a recently identified inhibitor of apoptosis protein (IAP)-binding protein, is released from the mitochondria during apoptosis and reportedly potentiates apoptosis by relieving the inhibition of IAPs on caspases. We now describe the molecular characterization of Smac beta, an alternatively spliced form of Smac, which lacks the mitochondrial-targeting sequence found in Smac and has a cortical distribution in both human embryonic kidney 293 and breast epithelial tumor MCF-7 cells. Smac beta, which binds IAPs in vitro, does not bind IAPs in intact cells due to cellular processing and removal of its NH(2)-terminal IAP-binding domain. Despite its inability to interact with IAPs in cells, processed Smac beta is proapoptotic, as demonstrated by its ability to potentiate apoptosis induced by both death receptor and chemical stimuli. Furthermore, expression of a NH(2)-terminally truncated Smac mutant (Delta75), which lacks the entire IAP-interacting domain, potentiates apoptosis to the same extent as Smac and Smac beta. Our data support the hypothesis that the main proapoptotic function of Smac and Smac beta is due to a mechanism other than IAP binding.
Roberts
D L
DL
Medical Research Council Toxicology Unit, University of Leicester, Leicester LE1 9HN, United Kingdom.
Merrison
W
W
MacFarlane
M
M
Cohen
G M
GM
D000595:Amino Acid Sequence; D017209:Apoptosis; D051017:Apoptosis Regulatory Proteins; D001426:Bacterial Proteins / Q000378:metabolism*; D001665:Binding Sites; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D006801:Humans; D051034:Inhibitor of Apoptosis Proteins; D019476:Insect Proteins; D047908:Intracellular Signaling Peptides and Proteins; D024101:Mitochondrial Proteins; D008969:Molecular Sequence Data; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D011506:Proteins / Q000378:metabolism; D051636:X-Linked Inhibitor of Apoptosis Protein
D051017:Apoptosis Regulatory Proteins; D001426:Bacterial Proteins; D002352:Carrier Proteins; C412403:DIABLO protein, human; D051034:Inhibitor of Apoptosis Proteins; D019476:Insect Proteins; D047908:Intracellular Signaling Peptides and Proteins; D024101:Mitochondrial Proteins; D020033:Protein Isoforms; D011506:Proteins; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human