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Molecular characterization of the F-box protein FBW2 in the RNA silencing in Arabidopsis thaliana

Abstract : RNA silencing is a conserved molecular mechanism in eukaryotes, of which the main effectors are the ARGONAUTE (AGO) proteins. In plants, AGO1 is a protein that is essential for growth and antiviral defence. It uses small RNAs as probe to recognize and regulate messenger RNAs. Viruses have developed suppressors of RNA silencing to overcome this defence. One of these, P0 from the Turnip Yellows Virus, acts as an F-box protein to hijack the SCF complex, an E3 ubiquitin ligase, and guide AGO1 to the ubiquitin-dependent proteolysis. This degradation uses the vacuole instead of the 26S proteasome, generally associated with ubiquitin-dependant proteolysis. This proteolysis mechanism is not understood and is also apparent when AGO1 is endogenously destabilized, suggesting that P0 uses an already existing pathway. An Arabidopsis F-box protein, FBW2, has been shown to impact AGO1 homeostasis independently from the proteasome. My PhD project aimed at characterizing FBW2 F-box activity and understanding the relationship between AGO1 and FBW2, as well as its consequences on the RNA silencing. The results obtained in this manuscript show that the SCFFBW2 interacts with AGO1 and triggers its degradation through an autophagy- and proteasome- independent process, while only weakly affecting the RNA silencing. FBW2 would actually target a subset of AGO1 proteins, which appears not to contain small RNAs. This regulation would play a surveillance role in order to prevent a deleterious activity of AGO1 in absence of small RNAs.
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Thibaut Hacquard. Molecular characterization of the F-box protein FBW2 in the RNA silencing in Arabidopsis thaliana. Cellular Biology. Université de Strasbourg, 2018. English. ⟨NNT : 2018STRAJ069⟩. ⟨tel-02917982⟩

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