RNA silencing constitutes the primary plant immune system against viruses besides its essential roles in development, but plant viruses encode RNA silencing suppressors (VSRs) to counteract the antiviral RNA silencing response.

The 2b protein encoded by cucumber mosaic virus (CMV) was one of the first two identified VSRs. The 2b protein is predominantly localized to the nucleus, and found to directly interacts with both the siRNA and AGO protein, components of the RNA silencing machinery. However, it is not clear what domains of the 2b protein determine these activities and how each of these activities contributes to the in vivo silencing suppression.

To address this issue, supported by grants from the National Science Foundation of China, GUO Huishan and her colleagues in the State Key Laboratory of Plant Genomics, Institute of Microbiology, Chinese Academy of Sciences, detailed characterization of the 2b protein in subcellular localization, RNA binding, AGO interactions and suppression of post-transcriptional gene silencing (PTGS) and RNA-directed DNA methylation (RdDM).

Their analysis found that the N-terminal half of the 2b protein is required for dsRNA-binding and a nucleolar localization signal (NoLS) encoded within the dsRNA-binding domain. Whereas, the physical interactions with AGO proteins required the C-terminal half domain of the 2b protein. Functional characterization of the SD2b mutants indicates that suppression of the in vitro activity of AGO1 by 2b requires the physical interaction with AGO1, but not the dsRNA-binding and nucleolar targeting activities. In contrast, in vivo suppression of PTGS and RdDM by the 2b protein depends on its dsRNA-binding activity. Interestingly, they found that a functional NoLS is required for the direct in vivo interactions of the 2b-AGO proteins, which causes redistribution of both 2b and AGO proteins in the nucleus, but is not essential for 2b suppression of either PTGS or RdDM.

The related paper has been published in The Plant Cell (Duan et al.,2012). http://www.plantcell.org/content/early/2012/01/11/tpc.111.092718.

--Images by GUO Huishan--