The Arabidopsis F-box protein FBW2 degrades AGO1 to avoid spurious loading of illegitimate small RNA [ncRNA-Seq]

RNA silencing is a conserved mechanism in eukaryotes and is involved in development, heterochromatin maintenance and defense against viruses. In plants, ARGONAUTE1 (AGO1) protein plays a central role in both microRNA (miRNA) and small interfering RNA (siRNA)-directed silencing and its expression is regulated at multiple levels. Here we report that the F-box protein FBW2 targets proteolysis of AGO1 by a CDC48-mediated autophagy mechanism. We found that FBW2 assembles an SCF complex that recognizes the MID-PIWI domain of AGO1 and requires its C-terminal domain containing a GW motif for AGO1 turnover. We show that FBW2 has a preference for unloaded AGO1 and also for some mutated forms of the protein. FBW2 loss of function increases RNA silencing activity, but does not lead to strong growth or developmental defects. However, under conditions in which small RNA production or accumulation is affected, the failure to degrade AGO1 in fbw2 mutants becomes more deleterious for the plant. Hence, we show that the non-degradable AGO1 protein assembles high molecular complexes and binds illegitimate small RNAs. small RNA seq profiling of five different Arabidopsis mutants and the corresponding wildtype (Col0) with 3 biological replicates each and two conditions, Total RNA and AGO1-IP RNA.