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. 2009 May 1;137(3):498-508.
doi: 10.1016/j.cell.2009.04.028.

An effector of RNA-directed DNA methylation in arabidopsis is an ARGONAUTE 4- and RNA-binding protein

Xin-Jian He  1 , Yi-Feng Hsu, Shihua Zhu, Andrzej T Wierzbicki, Olga Pontes, Craig S Pikaard, Hai-Liang Liu, Co-Shine Wang, Hailing Jin, Jian-Kang Zhu
Affiliations

An effector of RNA-directed DNA methylation in arabidopsis is an ARGONAUTE 4- and RNA-binding protein

Xin-Jian He et al. Cell. .

Abstract

DNA methylation is a conserved epigenetic mark in plants and mammals. In Arabidopsis, DNA methylation can be triggered by small interfering RNAs (siRNAs) through an RNA-directed DNA methylation (RdDM) pathway. Here, we report the identification of an RdDM effector, KTF1. Loss-of-function mutations in KTF1 reduce DNA methylation and release the silencing of RdDM target loci without abolishing the siRNA triggers. KTF1 has similarity to the transcription elongation factor SPT5 and contains a C-terminal extension rich in GW/WG repeats. KTF1 colocalizes with ARGONAUTE 4 (AGO4) in punctate nuclear foci and binds AGO4 and RNA transcripts. Our results suggest KTF1 as an adaptor protein that binds scaffold transcripts generated by Pol V and recruits AGO4 and AGO4-bound siRNAs to form an RdDM effector complex. The dual interaction of an effector protein with AGO and small RNA target transcripts may be a general feature of RNA-silencing effector complexes.

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Figures

Figure 1
Figure 1. Transcriptional gene silencing of RD29A-LUC is suppressed by the rdm3-1 and rdm3-2 mutations
(A) Effect of rdm3-1 and rdm3-2 on luminescence and kanamycin-resistance phenotypes in the ros1 background. Plants were grown on MS plates and subjected to luminescence imaging after cold treatment (4°C, 24 h). The plants were also grown on MS plates with kanamycin (50 μg/ml) and photographed after 10 days. (B) Northern blot analysis of RNA levels of endogenous RD29A, RD29A-LUC, and 35S-NPTII in wild type, ros1, and ros1rdm3-1. The constitutively expressed 18S rRNA was used as an RNA loading control while COR15A was used as a cold treatment control.
Figure 2
Figure 2. The rdm3 mutations reduce DNA methylation at RdDM target loci
The percentage of cytosine methylation was determined by bisulfite sequencing at transgenic (A) and endogenous (B) RD29A promoters, AtSN1 (C) and MEA-ISR (D). The percentage of cytosine methylation on CG, CHG, and CHH sites is shown. H represents A, T, or C. (E) The rdm3-1 mutation suppressed DNA methylation in AtSN1, AtGP1, and AtMU1. After the indicated genomic DNA was digested with the methylation-sensitive restriction enzyme HaeIII, it was used for amplification of AtSN1. After the genomic DNA was digested with the methylated DNA-specific restriction enzyme McrBC, it was used for amplification of AtGP1 and AtMU1. The amplifications of non-digested genomic DNA were used as controls. (F) The rdm3-3 mutation reduced AtMU1 methylation at CHH sites. Genomic DNA from the indicated genotypes was digested with HaeIII, followed by Southern blot analysis. The three undigested bands (arrows) that are present in the Col-0 wild type were mostly digested in rdm3-3, nrpd1-3 and nrpe1-11.
Figure 3
Figure 3. Effect of the rdm3 mutations on RNA and siRNA levels from the RdDM target loci
(A) The rdm3 mutations increase the RNA expression levels of AtSN1, AtGP1, and AtMU1. Semi-quantitative RT-PCR was used to detect the transcript levels of AtSN1 (interval A, see diagram in panel C), AtGP1, and AtMU1 in the indicated genotypes. TUB8 was amplified as an internal control. (B) Small RNA blot analysis of 24-nt siRNAs, 21-nt ta-siRNAs, and microRNAs in the various genotypes. The positions of size markers (21 nt and 24 nt) are indicated. The ethidium bromide-stained small RNA gel is shown as a loading control. (C) Strand-specific RT-PCR analysis of IGN5, IGN6, AtSN1 and solo LTR transcripts in the Col-0 wild type, nrpe1-11 and rdm3-3. Actin PCR products and total RNA resolved by agarose gel electrophoresis serve as loading controls. Reactions without reverse transcriptase (no RT) were performed to control for background DNA contamination. The positions of the different AtSN1 intervals tested by RT-PCR are indicated in the diagram on the left. (D) RT-PCR analysis of RD29A promoter transcript. TUB8 and ethidium bromide-stained gel are shown as controls. (E) RT-PCR detection of RD29A promoter transcript in KTF1 immunoprecipitates. The background signal from TUB8 was used as an internal control, which indicated no difference between the RNA amounts from ros1 and ros1rdm3-1. No AB, controls without using anti-KTF1 antibody.
Figure 4
Figure 4. The WG/GW repeats in KTF1 C-terminal domain interact with AGO4
(A) KTF1, NRPE1, HsGW182 and proteins are shown schematically. All three proteins are characterized by the reiterated WG/GW repeats-containing domains (in yellow). The red stripes represent each of the WG/GW repeat. WG-1 and WG-2 represent two highly conserved WG repeat regions in KTF1. (B) Diagram of the bacterially expressed NRPE1-CTD and truncated KTF1 proteins. (C) The purified proteins were subjected to SDS-PAGE and gels were stained with Coomassie. Arrows point to the proteins of interest. (D) Western blot analysis showing that the GST-fused truncated KTF1 and NRPE1-CTD interact with Myc-AGO4 from plant extracts. Ten percent of the input was used in the “Input” lane. (E) Anti-Myc antibody-conjugated beads captured truncated KTF1 proteins and NRPE1-CTD from a mixture of the proteins with extract from Myc-AGO4 plants. Arrows point to the proteins of interest. (F) and (G) Western blot analysis showing coimmunoprecipitation of KTF1 and Myc-AGO4. Ler wild-type plants without the Myc-AGO4 transgene were used as controls. No AB, control precipitation without using antibodies.
Figure 5
Figure 5. Sub-nuclear localization of KTF1 in interphase Arabidopsis nuclei
A. Detection of KTF1 (in red) in wild type (WT) and rdm3-1 mutant nuclei by immunofluorescence using anti-KTF1. B. Simultaneous localization of KTF1 and AGO4 or NRPE1. KTF1 (red) was localized using its specific antibody in cells expressing cMyc- and Flag-tagged AGO4 and NRPE1 (in green), respectively. The bright yellow signals due to the overlap of red and green channels in merged images indicate colocalization of two labeled proteins. In all panels DNA was stained with DAPI (blue). Size bar corresponds to 5μm.
Figure 6
Figure 6. The KTF1 C-terminal domain binds RNAs
(A) KTF1-P3 but not KTF1-P2 binds to a 40-nt RNA (corresponding to the RD29A promoter) in electrophoretic mobility shift assays. (B) KTF1-P3 but not KTF1-P2 binds to a 500-nt RNA corresponding to the RD29A promoter. (C) KTF1-P3 binds to both the forward (F) and reverse (R) strands of the 40-nt RNA. (D) Protein concentration-dependence of the RNA-binding and competition by unlabeled RNA. The protein-RNA complex increased when an increasing amount of KTF1-P3 protein (0.2, 0.4, 0.8, 1.6 μg) was added to the binding reaction. The protein-RNA complex decreased when an increasing amount of unlabeled 40-nt RNA (1x, 5x, 25x, 125x of labeled RNA) was added to the binding reaction. (E) KTF1-P3 binds to the 40-nt RNA but does not bind to DNA of the same sequence. (F) KTF1-P3 binds to the single-stranded but not double-stranded 40-nt RNA.
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