Baculovirus Inhibitor-of-Apoptosis Op-IAP3 Blocks Apoptosis by Interaction with and Stabilization of a Host Insect Cellular IAP

Abstract

Baculovirus-encoded inhibitor of apoptosis (IAP) proteins likely evolved from their host cell IAP homologs, which function as critical regulators of cell death. Despite their striking relatedness to cellular IAPs, including the conservation of two baculovirus IAP repeat (BIR) domains and a C-terminal RING, viral IAPs use an unresolved mechanism to suppress apoptosis in insects. To define this mechanism, we investigated Op-IAP3, the prototypical IAP from baculovirus OpMNPV. We found that Op-IAP3 forms a stable complex with SfIAP, the native, short-lived IAP of host insect Spodoptera frugiperda. Long-lived Op-IAP3 prevented virus-induced SfIAP degradation, which normally causes caspase activation and apoptosis. In uninfected cells, Op-IAP3 also increased SfIAP steady-state levels and extended SfIAP's half-life. Conversely, SfIAP stabilization was lost or reversed in the presence of mutated Op-IAP3 that was engineered for reduced stability. Thus, Op-IAP3 stabilizes SfIAP and preserves its antiapoptotic function. In contrast to SfIAP, Op-IAP3 failed to bind or inhibit native Spodoptera caspases. Furthermore, BIR mutations that abrogate binding of well-conserved IAP antagonists did not affect Op-IAP3's capacity to prevent virus-induced apoptosis. Remarkably, Op-IAP3 also failed to prevent apoptosis when endogenous SfIAP was ablated by RNA silencing. Thus, Op-IAP3 requires SfIAP as a cofactor. Our findings suggest a new model wherein Op-IAP3 interacts directly with SfIAP to maintain its intracellular level, thereby suppressing virus-induced apoptosis indirectly. Consistent with this model, Op-IAP3 has evolved an intrinsic stability that may serve to repress signal-induced turnover and autoubiquitination when bound to its targeted cellular IAP. IMPORTANCEThe IAPs were first discovered in baculoviruses because of their potency for preventing apoptosis. However, the antiapoptotic mechanism of viral IAPs in host insects has been elusive. We show here that the prototypical viral IAP, Op-IAP3, blocks apoptosis indirectly by associating with unstable, autoubiquitinating host IAP in such a way that cellular IAP levels and antiapoptotic activities are maintained. This mechanism explains Op-IAP3's requirement for native cellular IAP as a cofactor and the dispensability of caspase inhibition. Viral IAP-mediated preservation of the host IAP homolog capitalizes on normal IAP-IAP interactions and is likely the result of viral IAP evolution in which degron-mediated destabilization and ubiquitination potential have been reduced. This mechanism illustrates another novel means by which DNA viruses incorporate host death regulators that are modified for resistance to host regulatory controls for the purpose of suppressing host cell apoptosis and acquiring replication advantages.A poptosis is an important defense against virus infection. This highly conserved self-destruct process is regulated by the inhibitor of apoptosis (IAP) proteins, a family of critical survival factors that are also involved in development, mitosis, inflam...

The Complete Genome Sequence of a Second Distinct Betabaculovirus from the True Armyworm, Mythimna unipuncta

“…Such scenarios resemble the mechanism by which Orgyia pseudotsugata multiple nucleopolyhedrovirus (OpMNPV) inhibitor-of-apoptosis protein 3 (IAP-3) inhibits baculovirus replication-induced apoptosis in host cells. Rather than binding to and inhibiting activator caspases directly, OpMNPV IAP-3 binds and stabilizes host cell IAP, which in turn continues to prevent the onset of apoptosis [63]. …”

Function analysis and application of IAP1/2 of Autographa californica multiple nucleopolyhedrovirus

“…its anti-apoptosis by binding to its targeted cellular IAP. 26,27 In this study, the amino acid sequence before the BIR1 domain in Ac-IAP2 was extremely short, that is, Ac-IAP2 didn't have the N-terminal "leader", which suggested that Ac-IAP2 was likely to have the same anti-apoptotic mechanism as Op-IAP3. Of course, its RING domain also needed to have E3 ubiquitin ligase activity.…”

“…27 According to this presumption, if the RING domain of Ac-IAP2 also had E3 ubiquitin ligase activity, the 8 lysine residues in its BIR2 domain (Fig. 5A), which were not existed in the BIR2 domain of Ac-IAP1, were completely unable to enhance its anti-apoptotic function.…”

The diversity of insect antiviral immunity: insights from viruses

“…For example, in Drosophila cells infected with FHV activation of p53 leads to induction of the pro-apoptotic gene reaper that blocks the activity of IAPs [19]. In baculovirus infected Lepidopteran cells, inhibition of host translation leads to depletion of cellular IAPs, whose stability is tightly regulated [18]. Indeed, Lepidopteran and Drosophila IAPs contain N-terminal instability motifs that are targeted by different signaling pathways in response to virus infection [47].…”

“…Unlike their cellular counterparts, vIAPs lack an N-terminal instability motif [47]. As a result, they act by stabilizing cellular IAPs, thus preventing apoptosis in infected cells as recently shown for the prototypical baculovirus Op-IAP3 from Orgya pseudotsugata multiple nucleopolyhedrovirus [18]. The p35 suppressor from Autographa californica multiple nucleopolyhedrovirus functions as a substrate for effector caspases such as DRICE, while the related p49 suppressor from Spodoptera littoralis nucleopolyhedrovirus has the potential to block both initiator and effector caspases [48].…”