An endogenous F-box protein regulates ARGONAUTE1 in Arabidopsis thaliana

Argonaute proteins are core components of the RNA-induced silencing complex (RISC) [1-3]. These proteins use microRNAs (miRNAs) and/or small interfering RNAs (siRNAs) as guides to direct RISC to a specific site in target mRNAs, resulting in the cleavage or translational repression of these target mRNAs. Some Argonaute proteins also promote transcriptional repression through their effect on chromatin structure [2-4].ARGONAUTE1 (AGO1) is one of 10 Argonaute proteins in Arabidopsis thaliana [2,5]. Genetic analyses [6-9], as well as the identification of the small RNAs that co-purify with AGO1 [10,11], indicate that AGO1 plays a central role in both miRNA and siRNA-mediated RNA silencing. Arabidopsis is exquisitely sensitive to the level of AGO1 activity, as evident from the broad range of phenotypes displayed by hypomorphic mutations of this gene [5,6,8,12]. In wild-type plants, the expression of AGO1 is maintained at a constant level by a negative feedback loop involving miR168. AGO1 is a target of miR168 and negatively regulates its own activity by promoting the activity and stability of miR168 [9,13] and by promoting the activity of siRNAs derived from the AGO1 transcript [14]. AGO1 activity is negatively regulated by PNH/ZLL/AGO10 [15] and positively regulated by SQUINT (SQN), the Arabidopsis orthologue of the protein chaperone, Cyclophilin-40 [12].Null alleles of SQN have a morphological phenotype that is nearly identical to the phenotype of weak loss-of-function alleles of AGO1 [12]. In order to identify genes involved in AGO1-mediated processes, we screened for mutations that suppress the phenotype of sqn-1. This screen yielded several alleles of the F-box gene FBW2. Here we show FBW2 is a negative regulator of AGO1 and controls the sensitivity of plants to the hormone abscisic acid.Previously we found that SQN directly or indirectly promotes AGO1 activity [12]. In particular, we showed that the phenotype of loss-of-function alleles of SQN can be largely, if not