title: Baculovirus inhibitor of apoptosis functions at or upstream of the apoptotic suppressor P35 to prevent programmed cell death.
abstract: Members of the inhibitor of apoptosis (iap) gene family prevent programmed cell death induced by multiple signals in diverse organisms, suggesting that they act at a conserved step in the apoptotic pathway. To investigate the molecular mechanism of iap function, we expressed epitope-tagged Op-iap, the prototype viral iap from Orgyia pseudotsugata nuclear polyhedrosis virus, by using novel baculovirus recombinants and stably transfected insect cell lines. Epitope-tagged Op-iap blocked both virus- and UV radiation-induced apoptosis. With or without apoptotic stimuli, Op-IAP protein (31 kDa) cofractionated with cellular membranes and the cytosol, suggesting a cytoplasmic site of action. To identify the step(s) at which Op-iap blocks apoptosis, we monitored the effect of Op-iap expression on in vivo activation of the insect CED-3/ICE death proteases (caspases). Op-iap prevented in vivo caspase-mediated cleavage of the baculovirus substrate inhibitor P35 and blocked caspase activity upon viral infection or UV irradiation. However, unlike the stoichiometric inhibitor P35, Op-IAP failed to affect activated caspase as determined by in vitro protease assays. These findings provide the first biochemical evidence that Op-iap blocks activation of the host caspase or inhibits its activity by a mechanism distinct from P35. Moreover, as suggested by the capacity of Op-iap to block apoptosis induced by diverse signals, including virus infection and UV radiation, iap functions at a central point at or upstream from steps involving the death proteases.
authors.affiliation: Department of Biochemistry, Graduate School and College of Agricultural and Life Sciences, University of Wisconsin-Madison, 53706, USA.
authors.forename: G A
authors.initials: GA
authors.lastname: Manji
authors.forename: R R
authors.initials: RR
authors.lastname: Hozak
authors.forename: D J
authors.initials: DJ
authors.lastname: LaCount
authors.forename: P D
authors.initials: PD
authors.lastname: Friesen
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000528:radiation effects; D002451:Cell Compartmentation; D002462:Cell Membrane / Q000378:metabolism; D002478:Cells, Cultured; D003546:Cysteine Endopeptidases / Q000378:metabolism; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D051034:Inhibitor of Apoptosis Proteins; D017924:Nucleopolyhedroviruses / Q000235:genetics*; D011480:Protease Inhibitors / Q000494:pharmacology; D015398:Signal Transduction; D018411:Spodoptera / Q000166:cytology; D014466:Ultraviolet Rays; D014764:Viral Proteins / Q000378:metabolism* / Q000502:physiology*
chemical_list: D015853:Cysteine Proteinase Inhibitors; D051034:Inhibitor of Apoptosis Proteins; D011480:Protease Inhibitors; D014764:Viral Proteins; C086430:inhibitor of apoptosis, Nucleopolyhedrovirus; D003546:Cysteine Endopeptidases