The RNA silencing pathway in plants acts in part as an antiviral defense mechanism. Plant viruses have evolved to encode suppressors of RNA silencing. Plant viral RNA silencing suppressor proteins (RSS) show little sequence conservation, but act on similar key steps in the RNA silencing pathway. Viral RSS are typically multifunctional proteins that are involved in other distinct roles such as encapsidation, cell-to-cell movement, vector transmission, replication and transcription initiation.
The majority of known plant viral RSS function by sequestering primary or secondary small interfering RNAs (siRNAs) and preventing their incorporation into the RNA induced silencing complex (RISC). However, recent insights indicate that viral RSS may compromise multiple rather than single components of the RNA silencing pathway. This study shows that the lettuce necrotic yellows virus (LNYV) P protein inhibits the activity of multiple proteins of the RNA silencing pathway, in particular, those involved in RISC and double-stranded RNA amplification.
Cytorhabdovirus P protein suppresses RISC-mediated cleavage and RNA silencing amplification in planta. (2016) Virology 22(490): 27-40. doi: 10.1016/j.virol.2016.01.003
Plant viruses have evolved to undermine the RNA silencing pathway by expressing suppressor protein(s) that interfere with one or more key components of this antiviral defense. Here we show that the recently identified RNA silencing suppressor (RSS) of lettuce necrotic yellows virus (LNYV), phosphoprotein P, binds to RNA silencing machinery proteins AGO1, AGO2, AGO4, RDR6 and SGS3 in protein-protein interaction assays when transiently expressed. In planta, we demonstrate that LNYV P inhibits miRNA-guided AGO1 cleavage and translational repression, and RDR6/SGS3-dependent amplification of silencing. Analysis of LNYV P deletion mutants identified a C-terminal protein domain essential for both local RNA silencing suppression and interaction with AGO1, AGO2, AGO4, RDR6 and SGS3. In contrast to other viral RSS known to disrupt AGO activity, LNYV P sequence does not contain any recognizable GW/WG or F-box motifs. This suggests that LNYV P may represent a new class of AGO binding proteins.