Novel viral strategy for evading RNA silencing

Viral protein blocks RNA silencing in Arabidopsis by inhibiting Argonaute1

Dec 1, 2006
Jeffrey M. Perkel
Researchers have uncovered a novel viral strategy for evading RNA silencing in plants, according to a report in Genes & Development. Nam-Hai Chua, the Andrew W. Mellon Professor and head of the plant molecular biology laboratory at Rockefeller University, and colleagues demonstrate both in Arabidopsis and in vitro that the cucumber mosaic virus (CMV) protein 2b binds to and inhibits Argonaute1, thereby circumventing the plant's antiviral response. Though other viral suppressors of RNA silencing had previously been identified, this is the first to be shown to clearly bind an effector protein in the silencing pathway. "I think it's a beautiful lesson in how you conceptualize this 'arms race' between pathogen and host," said James Carrington, director of the Center for Genome Research and Biocomputing at Oregon State University, who also studies viral suppressor proteins. "Imagine this as a battlefield where on one side you have the host and on the other the virus," said Carrington, who was not involved in the study. "The host has a number of defensive mechanisms, such as silencing, to inactivate the invading virus, but the virus has ways to neutralize or counter the defense mechanisms. This is another very nice illustration, with some biochemical detail, of how one suppressor does it."In RNA interference, enzymes called Dicers process double-stranded RNAs into short, double-stranded short-interfering RNAs (siRNAs), which are then loaded into a second protein machine, the RNA-induced silencing complex (RISC), programming it to target and cleave homologous RNAs. Central to this complex is an Argonaute protein dubbed "Slicer." Virus-induced gene silencing was thought to operate via a similar mechanism to target viral RNA in plants, but that was largely speculative, said Olivier Voinnet of the Institut de Biologie Moléculaire des Plantes du CNRS, Strasbourg, France, who authored a 2005 review on the topic. "We had no clue what would mediate the cleavage of viral transcripts, and what would cleave the viral RNAs." Also unclear, he added, was how viral suppressors interfered with the process. "The nice thing about the Chua paper is it basically addresses both issues," said Voinnet, who was not part of the study. "On one hand, it suggests very strongly what might be the protein involved in cleavage, and on the other hand, it also shows that the 2b protein from CMV is very likely to inhibit this component, Argonaute1."Chua's team used a combination of genetics and biochemistry to show that CMV 2b suppresses both endogenous microRNA processing and virus-induced gene silencing, via physical interaction with Argonaute1, though precisely how inhibition occurs is unknown. The group also isolated from infected plants Argonaute1 protein loaded with virus-derived siRNAs, demonstrating that this protein, at least, can function as an antiviral Slicer. "For a long time, we didn't know what was the [antiviral] Slicer or the Dicer," said Voinnet. Voinnet's lab identified an antiviral Dicer called DCL4 earlier this year; now, he said, "Chua shows that at least Argonaute1 can play the role of Slicer. That's a key aspect of the work."That's not to say Argonaute1 is the sole antiviral Slicer; with 10 Argonaute family members in Arabidopsis, functional redundancy is likely, said Voinnet. Nor is Argonaute1 likely to be the 2b protein's only target, Chua said. "This is a gene family, and the proteins share homology. So I think it's reasonable to say that if there are other Argonautes involved in viral defense, 2b probably interacts with them as well." 2b also probably plays other roles, Voinnet added. "I think it quite likely that silencing suppressor proteins will have multiple targets." Indeed, Carrington noted that when 2b was first identified, it was thought to play a role in systemic silencing, in which virally induced silencing in one tissue triggers a similar response in distant tissues. "It's a bit difficult to reconcile how 2b would be interfering with this systemic response by its mode of action here." 2b is not the first viral suppressor protein to be identified. Others include the tombusviral p19 protein, which sequesters siRNAs, and the p38 protein from turnip crinkle virus, which inhibits Dicer. But it does have a unique mode of action. "So the big-picture story," said Carrington, "is there's 20 ways to skin this cat, and viruses are using all those ways."Jeffrey M. Perkel Links within this article:X. Zhang et al., "Cucumber mosaic virus-encoded 2b suppressor inhibits ArabidopsisArgonaute1 cleavage activity to counter plant defense," Genes Dev, Dec. 1, 2006. http://www.genesdev.orgNam-Hai Chua Carrington Weitzman, "Virus-induced RNA silencing," The Scientist, May 20, 2002 Voinnet Voinnet, "Induction and suppression of RNA silencing: Insights from viral infections," Nat Rev Genet, 6:206-21, 2005. Deleris et al., "Hierarchical action and inhibition of plant Dicer-like proteins in antiviral defense," Science, 313:68 = 71, July 7, 2006.