Refusing to Be Silenced

Molecular Biology![Figure][1] CREDIT: AZEVEDO ET AL., GENES DEV. 24 , 904 (2010) Pathogens are in a continual evolutionary struggle with their hosts—each side exploits new avenues of attack while simultaneously patching breaches in its defenses. A major defense mechanism used by plants against viral infection involves the RNA interference (RNAi) pathway, whereby small RNAs cleaved from the virus are subsequently used to target for destruction any virally produced RNAs. At the same time, viruses have developed proteins (or to be more accurate, modified existing ones) that subvert this antiviral RNAi system. Azevedo et al. find that the capsid protein P38 of the turnip crinkle RNA virus (TCV) has acquired two glycine-tryptophan (GW) repeat motifs. These motifs are commonly found in proteins that bind to and promote the function of Argonaute (Ago) proteins in the RNAi pathway. P38 binds to the plant Ago1 though its GW motifs, probably preventing Ago1 from interacting with other RNAi pathway proteins; this probably involves the ability of P38 to multimerize (hexamers are shown at left)—intrinsic to its role as a capsid protein. The suppression of Ago1 also represses the action of microRNAs, which bind to and act though Ago1. One such miRNA, miR162, normally acts to suppress the enzyme Dicer-like 1 (DCL1), which suppresses two other Dicer family members, DCL2 and DCL3, both of which are critical for cleaving small RNAs from TCV for targeting by the RNAi pathway. Thus, suppressing Ago1 miRNA function allows DCL1 activity to increase, which reduces DCL2 and DCL3 levels, which in turn blunts the ability of the plant to capture the targeting RNAs from the virus. Genes Dev. 24 , 904 (2010). [1]: pending:yes