RNA silencing is a major antiviral defense mechanism in plants and invertebrates. Plant ARGONAUTE1 (AGO1) is pivotal in RNA silencing, and hence is a major target for counteracting viral suppressors of RNA-silencing proteins (VSRs). P0 from Turnip yellows virus (TuYV) is a VSR that was previously shown to trigger AGO1 degradation via an autophagy-like process. However, the identity of host proteins involved and the cellular site at which AGO1 and P0 interact were unknown. Here we report that P0 and AGO1 associate on the endoplasmic reticulum (ER), resulting in their loading into ER-associated vesicles that are mobilized to the vacuole in an ATG5- and ATG7-dependent manner. We further identified ATG8-Interacting proteins 1 and 2 (ATI1 and ATI2) as proteins that associate with P0 and interact with AGO1 on the ER up to the vacuole. Notably, ATI1 and ATI2 belong to an endogenous degradation pathway of ER-associated AGO1 that is significantly induced following P0 expression. Accordingly, ATI1 and ATI2 deficiency causes a significant increase in posttranscriptional gene silencing (PTGS) activity. Collectively, we identify ATI1 and ATI2 as components of an ER-associated AGO1 turnover and proper PTGS maintenance and further show how the VSR P0 manipulates this pathway.

In , ARGONAUTE1 (AGO1) plays a central role in microRNA (miRNA) and small interfering RNA (siRNA)-mediated silencing and is a key component in antiviral responses. The polerovirus F-box P0 protein triggers AGO1 degradation as a viral counterdefense. Here, we identified a motif in AGO1 that is required for its interaction with the S phase kinase-associated protein1-cullin 1-F-box protein (SCF) P0 (SCF) complex and subsequent degradation. The AGO1 P0 degron is conserved and confers P0-mediated degradation to other AGO proteins. Interestingly, the degron motif is localized in the DUF1785 domain of AGO1, in which a single point mutation (, obtained by forward genetic screening) compromises recognition by SCF Recapitulating formation of the RNA-induced silencing complex in a cell-free system revealed that this mutation impairs RNA unwinding, leading to stalled forms of AGO1 still bound to double-stranded RNAs. In vivo, the DUF1785 is required for unwinding perfectly matched siRNA duplexes, but is mostly dispensable for unwinding imperfectly matched miRNA duplexes. Consequently, its mutation nearly abolishes phased siRNA production and sense transgene posttranscriptional gene silencing. Overall, our work sheds new light on the mode of AGO1 recognition by P0 and the in vivo function of DUF1785 in RNA silencing.

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