a) The overall structure of a full-length Argonaute protein from Pyrococcus furiosus (PfAgo). Loading into new RITS complexes for additional targeting is coupled to the Rik1 complex, which contains a histone methyltransferase resulting in H3K9me2 and spreading of silencing. RDRC converts the cleaved transcript into a dsRNA substrate for Dicer which produces secondary siRNAs. An siRNA guided RITS complex slices nascent transcripts synthesized by Pol II. Silencing is achieved through the formation of heterochromatin induced by H3 Lys9 dimethylation (H3K9me2, black diamonds). pombe is dependent on the RITS complex which contains Argonaute, Tas3 and Chp1 proteins. c) Transcriptional silencing and spreading in S. miRNPs are believed to not slice, but instead inhibit translation or sequester transcripts to cytoplasmic foci called P-bodies. Argonaute is loaded with miRNAs in miRNP complexes and are guided to the 3’ UTR of mature mRNA transcripts. Drosha containing complexes process the pri-miRNA into pre-miRNA which is transported to the cytoplasm where mature miRNAs are made by Dicer. b) MicroRNAs (miRNAs) are derived from endogenous hairpin precursors transcribed in the nucleus (pri-miRNA). Argonaute is the enzymatic component of RISC and is therefore called Slicer. The siRNA charged RISC seeks out complementary mRNA and catalyzes endonucleolytic cleavage of the transcript, a process called slicing. The siRNAs are loaded into Argonaute proteins which are at the center of the RNA-induced silencing complex (RISC). a) In “classical RNAi”, or the siRNA pathway, exogenous dsRNAs are processed into siRNAs by Dicer complexes. RNA interference pathways involving the Argonaute protein subfamily. Interestingly, SAGOs lack conserved Slicer amino acid residues and probably act in a Slicer-independent fashion. Secondary siRNAs are 5' triphosphorylated that may allow specific loading into SAGO complexes that are rate limiting for RNAi in C. elegans is mediated by secondary siRNAs selectively bound to secondary Argonautes (SAGOs) that belong to a worm-specific Argonaute subfamily (WAGO). Piwi/piRNA complexes in mammals and flies are directly linked to the control of transposable elements during germline development. Piwi-interacting RNAs (piRNAs) carry a 2'-O-methylation on their 3' end and appear to be synthesized by a Piwi Slicer dependent mechanism. The Piwi subfamily functions in the germline through a novel class of small RNAs that are longer than Argonaute-specific siRNAs and miRNAs. Two recently described Argonaute protein subfamilies mediate distinct functions in RNAi. In the classical RNAi pathway Argonaute functions as the Slicer enzyme that cleaves an mRNA target directed by a complementary siRNA. Argonaute is at the heart of all effector complexes in RNA interference.
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