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. 2012 Aug 21;22(16):1536-42.
doi: 10.1016/j.cub.2012.06.040. Epub 2012 Jul 12.

Slicing-independent RISC activation requires the argonaute PAZ domain

Shuo Gu  1 , Lan Jin, Yong Huang, Feijie Zhang, Mark A Kay
Affiliations

Slicing-independent RISC activation requires the argonaute PAZ domain

Shuo Gu et al. Curr Biol. .

Abstract

Small RNAs regulate genetic networks through a ribonucleoprotein complex called the RNA-induced silencing complex (RISC), which, in mammals, contains at its center one of four Argonaute proteins (Ago1-Ago4). A key regulatory event in the RNA interference (RNAi) and microRNA (miRNA) pathways is Ago loading, wherein double-stranded small-RNA duplexes are incorporated into RISC (pre-RISC) and then become single-stranded (mature RISC), a process that is not well understood. The Agos contain an evolutionarily conserved PAZ (Piwi/Argonaute/Zwille) domain whose primary function is to bind the 3' end of small RNAs. We created multiple PAZ-domain-disrupted mutant Ago proteins and studied their biochemical properties and biological functionality in cells. We found that the PAZ domain is dispensable for Ago loading of slicing-competent RISC. In contrast, in the absence of slicer activity or slicer-substrate duplex RNAs, PAZ-disrupted Agos bound duplex small interfering RNAs, but were unable to unwind or eject the passenger strand and form functional RISC complexes. We have discovered that the highly conserved PAZ domain plays an important role in RISC activation, providing new mechanistic insights into how miRNAs regulate genes, as well as new insights for future design of miRNA- and RNAi-based therapeutics.

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Figures

Figure 1
Figure 1. Truncated Ago2 without the PAZ Domain Can Associate Small RNAs, Elicit Noncleavage Gene Repression, and Slice Target RNA
(A) Schematic representation of human Ago2 and its Flag-tagged truncated version lacking the PAZ domain. (B) Perfect-stem sh-miR30 or bulged-stem shRNA-miR30-B coexpressed with Flag-GFP (negative control), Flag-Ago2, or its mutants in Ago2 KO MEF cells. 24 hr posttransfection, IP experiments with anti-Flag antibody were performed on cell lysates. RNA extracted from either 20% of input or IP were run on 15% polyacrylamide 7 M urea denaturing gels. Guide- and passenger-strand RNAs from each sample were identified by sequential northern blotting. The intensity of the bands was determined by phosphoimaging. The pull-down efficiency listed in the figure was calculated by dividing the IP signal with total input. (C) Cell lysates were also subjected to a western blot using Ago2 antibody. Wild-type Ago2, Ago2-ΔPAZ, and Ago2-D5 were expressed at similar levels. The signal from endogenous GAPDH served as the loading control. (D) PsiCHECK vectors with four tandem target sites in the 3′ UTR, which were mismatched to the guide strand of sh-miR30 and sh-miR30-B, were cotransfected with various shRNAs in Ago2 KO MEF cells. Dual-luciferase assays were performed 24 hr posttransfection. RL-luciferase activities were normalized with Firefly (FF)-luciferase, and the percentage of relative enzyme activity compared to the negative control (treated with sh-scramble) was plotted. Error bars represent the SD from two independent experiments, each performed in triplicate transfections. (E) The cleavage activities of the wild-type human Ago2 and the mutant derivatives were measured with an in vitro cleavage assay. Target RNA was incubated with 32P-labeled sh-miR30 guide-strand RNA and IP-derived Argonaute proteins at 26°C for 90 min. Products of the cleavage reaction were purified, separated, and detected by autoradiography. The expected size of 5′ cleavage products is 27 nucleotides (nt) as indicated in the figure. The intensity of the bands was determined by phosphoimaging as shown in the figure (the Ago2-treated sample was assigned a value of 100). A nonspecific band is labeled with # and was not observed in a repeat experiment. See also Figure S1.
Figure 2
Figure 2. The PAZ Domain Is Required for HsAgo1, HsAgo3, and HsAgo4 to Form Mature RISC
(A) Sh-miR30 or shRNA-miR30-B coexpressed with Flag-GFP or various Flag-Agos and their mutants in Ago2 KO MEF cells. IP and northern blotting experiments were performed as described in Figure 1. The expression levels of the proteins used in the assays were assessed by western blotting with anti-Flag antibody and are shown in the lower panel. (B) A psiCHECK vector with four tandem target sites in the 3′ UTR, which were mismatched to the guide strand of sh-miR30 and sh-miR30-B, was co-transfected with various shRNAs and Argonaute-expressing plasmids in Ago2 KO MEF cells. Dual-luciferase assays were performed and analyzed as described in Figure 1. Error bars represent the SD from two independent experiments, each performed in triplicate transfections. (C) IP samples in (A) were separated on 15% polyacrylamide native gels. 20 fmol synthetic si-miR30 duplex was also loaded on the gel as a control. Guide- and passenger-strand RNAs were identified by sequential northern blot. Notably, the si-miR30 control ran as a mixture of dsRNA and a trace amount of ssRNA. See also Figure S2.
Figure 3
Figure 3. Ago2-ΔPAZ-Forming Mature RISC through a Slicer-Dependent Pathway
(A) Sh-miR30 or shRNA-miR30-B coexpressed with Flag-GFP or various Flag-Ago2 mutants in Ago2 KO MEF cells. IP and northern blotting experiments were performed as described in Figure 1. The expression levels of the proteins used in the assays were assessed by western blotting with Ago2 antibody. Signals from GAPDH served as transfection and loading controls. (B) A psiCHECK vector with four tandem target sites in the 3′ UTR, which were mismatched to the guide strand of sh-miR30 and sh-miR30-B, was cotransfected with various shRNAs and Argonaute-expressing plasmids in Ago2 KO MEF cells. Dual-luciferase assays were performed and analyzed as described in Figure 1. Error bars represent the SD from two independent experiments, each performed in triplicate transfections. (C) IP samples in (A) were separated on 15% polyacrylamide native gels. 20 fmol synthetic RNAs (si-miR30 duplex and single-stranded guide-strand RNA of miR-30) were also loaded on the gel as controls. Guide- and passenger-strand RNAs were identified by sequential northern blot. Notably, the si-miR30 control ran as a mixture of dsRNA and a trace amount of ssRNA. See also Figure S3.
Figure 4
Figure 4. Ago2 with Point Mutations in PAZ Is Defective in Slicer-Independent RISC Unwinding
(A) Sh-miR30 or shRNA-miR30-B coexpressed with Flag-GFP or various Flag-Ago2 mutants in Ago2 KO MEF cells. IP and northern blotting experiments were performed as described in Figure 1. The expression levels of the proteins used in the assays were assessed by western blotting with Ago2 antibody. The signal from GAPDH served as the transfection and loading controls. (B) A psiCHECK vector with four tandem target sites in the 3′ UTR, which were mismatched to the guide strand of sh-miR30 and sh-miR30-B, was cotransfected with various shRNAs and Argonaute-expressing plasmids in Ago2 KO MEF cells. Dual-luciferase assays were performed and analyzed as described in Figure 1. Error bars represent the SD from two independent experiments, each performed in triplicate transfections. (C) IP samples in (A) were separated on 15% polyacrylamide native gels. 20 fmol synthetic RNAs (si-miR30 duplex and single-stranded guide-strand RNA of miR-30) were also loaded on the gel as controls. Guide- and passenger-strand RNAs were identified by sequential northern blot. Notably, the si-miR30 control ran as a mixture of dsRNA and a trace amount of ssRNA. (D) The amount of miRNAs and their corresponding star strands were measured in various Ago-IP samples by deep sequencing. For each miRNA, the percentage of its star strand to total reads was determined. The weighted average and SD were calculated by using the total reads of each miRNA as the weight. Number sign (#) indicates statistical significance (p < 0.05, unpaired t test, using Ago2-IP as a control group). See also Figure S4 and Table S1.
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References

    1. Carthew RW, Sontheimer EJ. Origins and Mechanisms of miRNAs and siRNAs. Cell. 2009;136:642–655. - PMC - PubMed
    1. Bartel DP. MicroRNAs: genomics, biogenesis, mechanism, and function. Cell. 2004;116:281–297. - PubMed
    1. Malone CD, Hannon GJ. Small RNAs as guardians of the genome. Cell. 2009;136:656–668. - PMC - PubMed
    1. Siomi H, Siomi MC. On the road to reading the RNA-interference code. Nature. 2009;457:396–404. - PubMed
    1. Czech B, Hannon GJ. Small RNA sorting: matchmaking for Argonautes. Nat. Rev. Genet. 2011;12:19–31. - PMC - PubMed

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