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The Rubella virus capsid is an anti-apoptotic protein that attenuates the pore-forming ability of Bax.

Ilkow CS, Goping IS, Hobman TC - PLoS Pathog. (2011)

Bottom Line: The main mechanism of action was specific for Bax as capsid bound Bax and prevented Bax-induced apoptosis but did not bind Bak nor inhibit Bak-induced apoptosis.Intriguingly, interaction with capsid protein resulted in activation of Bax in the absence of apoptotic stimuli, however, release of cytochrome c from mitochondria and concomitant activation of caspase 3 did not occur.Accordingly, we propose that binding of capsid to Bax induces the formation of hetero-oligomers that are incompetent for pore formation.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology, University of Alberta, Edmonton, Canada.

ABSTRACT
Apoptosis is an important mechanism by which virus-infected cells are eliminated from the host. Accordingly, many viruses have evolved strategies to prevent or delay apoptosis in order to provide a window of opportunity in which virus replication, assembly and egress can take place. Interfering with apoptosis may also be important for establishment and/or maintenance of persistent infections. Whereas large DNA viruses have the luxury of encoding accessory proteins whose primary function is to undermine programmed cell death pathways, it is generally thought that most RNA viruses do not encode these types of proteins. Here we report that the multifunctional capsid protein of Rubella virus is a potent inhibitor of apoptosis. The main mechanism of action was specific for Bax as capsid bound Bax and prevented Bax-induced apoptosis but did not bind Bak nor inhibit Bak-induced apoptosis. Intriguingly, interaction with capsid protein resulted in activation of Bax in the absence of apoptotic stimuli, however, release of cytochrome c from mitochondria and concomitant activation of caspase 3 did not occur. Accordingly, we propose that binding of capsid to Bax induces the formation of hetero-oligomers that are incompetent for pore formation. Importantly, data from reverse genetic studies are consistent with a scenario in which the anti-apoptotic activity of capsid protein is important for virus replication. If so, this would be among the first demonstrations showing that blocking apoptosis is important for replication of an RNA virus. Finally, it is tempting to speculate that other slowly replicating RNA viruses employ similar mechanisms to avoid killing infected cells.

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Related in: MedlinePlus

The anti-apoptotic function of capsid protein maps to the carboxyl-terminal region.A549 cells were transiently transfected with various capsid constructs or Bcl-XL and at 42 hours post-transfection, cells were treated with staurosporine or anti-Fas for 6 hours after which the numbers of transfectants that were positive for active caspase 3 (double positive) were determined by indirect immunofluorescence. Error bars indicate standard deviations calculated from three independent experiments. *p≤0.0001, **p≤0.001.
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ppat-1001291-g009: The anti-apoptotic function of capsid protein maps to the carboxyl-terminal region.A549 cells were transiently transfected with various capsid constructs or Bcl-XL and at 42 hours post-transfection, cells were treated with staurosporine or anti-Fas for 6 hours after which the numbers of transfectants that were positive for active caspase 3 (double positive) were determined by indirect immunofluorescence. Error bars indicate standard deviations calculated from three independent experiments. *p≤0.0001, **p≤0.001.

Mentions: We next determined whether association of capsid with mitochondria correlated with its ability to block apoptosis. Transfected cells expressing the various capsid constructs were challenged with staurosporine or anti-Fas, and then apoptosis induction was assessed using the activated caspase 3 assay. The amino-terminal capsid construct (CapNT) neither associates with mitochondria nor protects against apoptosis (Figures 8, 9). Conversely, CapCT, a pool of which is targeted to mitochondria, protects as well as full-length capsid protein against staurosporine and anti-Fas challenge. CapΔRSP, which lacks the hydrophobic E2 signal peptide and a membrane proximal arginine-rich (R) motif, is not targeted to mitochondria and does not block staurosporine or anti-Fas-mediated induced activation of caspase 3. Interestingly, although CapΔSP does not localize to mitochondria, it did confer resistance to both Fas- and staurosporine-induced apoptosis (Figure 9). This observation suggests that the membrane-proximal R motif is important for the anti-apoptotic function of capsid. Table 1 summarizes the localization and anti-apoptotic properties of the capsid deletion mutants.


The Rubella virus capsid is an anti-apoptotic protein that attenuates the pore-forming ability of Bax.

Ilkow CS, Goping IS, Hobman TC - PLoS Pathog. (2011)

The anti-apoptotic function of capsid protein maps to the carboxyl-terminal region.A549 cells were transiently transfected with various capsid constructs or Bcl-XL and at 42 hours post-transfection, cells were treated with staurosporine or anti-Fas for 6 hours after which the numbers of transfectants that were positive for active caspase 3 (double positive) were determined by indirect immunofluorescence. Error bars indicate standard deviations calculated from three independent experiments. *p≤0.0001, **p≤0.001.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3040668&req=5

ppat-1001291-g009: The anti-apoptotic function of capsid protein maps to the carboxyl-terminal region.A549 cells were transiently transfected with various capsid constructs or Bcl-XL and at 42 hours post-transfection, cells were treated with staurosporine or anti-Fas for 6 hours after which the numbers of transfectants that were positive for active caspase 3 (double positive) were determined by indirect immunofluorescence. Error bars indicate standard deviations calculated from three independent experiments. *p≤0.0001, **p≤0.001.
Mentions: We next determined whether association of capsid with mitochondria correlated with its ability to block apoptosis. Transfected cells expressing the various capsid constructs were challenged with staurosporine or anti-Fas, and then apoptosis induction was assessed using the activated caspase 3 assay. The amino-terminal capsid construct (CapNT) neither associates with mitochondria nor protects against apoptosis (Figures 8, 9). Conversely, CapCT, a pool of which is targeted to mitochondria, protects as well as full-length capsid protein against staurosporine and anti-Fas challenge. CapΔRSP, which lacks the hydrophobic E2 signal peptide and a membrane proximal arginine-rich (R) motif, is not targeted to mitochondria and does not block staurosporine or anti-Fas-mediated induced activation of caspase 3. Interestingly, although CapΔSP does not localize to mitochondria, it did confer resistance to both Fas- and staurosporine-induced apoptosis (Figure 9). This observation suggests that the membrane-proximal R motif is important for the anti-apoptotic function of capsid. Table 1 summarizes the localization and anti-apoptotic properties of the capsid deletion mutants.

Bottom Line: The main mechanism of action was specific for Bax as capsid bound Bax and prevented Bax-induced apoptosis but did not bind Bak nor inhibit Bak-induced apoptosis.Intriguingly, interaction with capsid protein resulted in activation of Bax in the absence of apoptotic stimuli, however, release of cytochrome c from mitochondria and concomitant activation of caspase 3 did not occur.Accordingly, we propose that binding of capsid to Bax induces the formation of hetero-oligomers that are incompetent for pore formation.

View Article: PubMed Central - PubMed

Affiliation: Department of Cell Biology, University of Alberta, Edmonton, Canada.

ABSTRACT
Apoptosis is an important mechanism by which virus-infected cells are eliminated from the host. Accordingly, many viruses have evolved strategies to prevent or delay apoptosis in order to provide a window of opportunity in which virus replication, assembly and egress can take place. Interfering with apoptosis may also be important for establishment and/or maintenance of persistent infections. Whereas large DNA viruses have the luxury of encoding accessory proteins whose primary function is to undermine programmed cell death pathways, it is generally thought that most RNA viruses do not encode these types of proteins. Here we report that the multifunctional capsid protein of Rubella virus is a potent inhibitor of apoptosis. The main mechanism of action was specific for Bax as capsid bound Bax and prevented Bax-induced apoptosis but did not bind Bak nor inhibit Bak-induced apoptosis. Intriguingly, interaction with capsid protein resulted in activation of Bax in the absence of apoptotic stimuli, however, release of cytochrome c from mitochondria and concomitant activation of caspase 3 did not occur. Accordingly, we propose that binding of capsid to Bax induces the formation of hetero-oligomers that are incompetent for pore formation. Importantly, data from reverse genetic studies are consistent with a scenario in which the anti-apoptotic activity of capsid protein is important for virus replication. If so, this would be among the first demonstrations showing that blocking apoptosis is important for replication of an RNA virus. Finally, it is tempting to speculate that other slowly replicating RNA viruses employ similar mechanisms to avoid killing infected cells.

Show MeSH
Related in: MedlinePlus