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Argonaute (Ago) proteins are found in all three domains of life. The best-characterized group is eukaryotic Argonautes (eAgos). Being the structural core of RNA interference machinery, they use guide RNA molecules for RNA targeting. Prokaryotic Argonautes (pAgos) are more diverse, both in terms of structure (there are eAgo-like ‘long’ and truncated ‘short’ pAgos) and mechanism, as many pAgos are specific for DNA, not RNA guide and/or target strands. Some long pAgos act as antiviral defence systems. Their defensive role was recently demonstrated for short pAgo-encoding systems SPARTA and GsSir2/Ago, but the function and action mechanisms of all other short pAgos remain unknown. In this work, we focus on the guide and target strand preferences of AfAgo, a short Argonaute protein encoded by an archaeon Archaeoglobus fulgidus. We demonstrate that AfAgo associates with small RNA molecules carrying 5′-terminal AUU nucleotides in vivo, and characterize its affinity to various RNA and DNA guide/target strands in vitro. We also present X-ray structures of AfAgo bound to oligoduplex DNAs that provide atomic details for base-specific AfAgo interactions with both guide and target strands. Our findings broaden the range of currently known Argonaute-nucleic acid recognition mechanisms.

Argonaute (Ago) proteins are found in all three domains of life. The so-called long Agos are composed of four major domains (N, PAZ, MID, and PIWI) and contribute to RNA silencing in eukaryotes (eAgos) or defence against invading mobile genetic elements in prokaryotes (pAgos). Intriguingly, the majority (~60%) of prokaryotic Agos (pAgos) identified bioinformatically are shorter (comprised of only MID and PIWI domains) and are typically associated with Sir2, Mrr or TIR domain-containing proteins. The cellular function and mechanism of short pAgos remain enigmatic. Here, we show that short pAgos from Geobacter sulfurreducens, Caballeronia cordobensis and Paraburkholderia graminis, together with the NAD+-bound Sir2-proteins form a stable heterodimeric Sir2/Ago complex that recognizes invading plasmid or phage DNA through the pAgos subunit and activates Sir2 subunit triggering the endogenous NAD+ depletion and cell death thus preventing the propagation of invading DNA. This is the first demonstration that short Sir2-associated pAgos provide defence against phages and plasmids and underscores the diversity of mechanisms of prokaryotic Agos.

Nature microbiology 7(11), 1857 - 1869 (2022). doi:10.1038/s41564-022-01239-0, Argonaute (Ago) proteins are found in all three domains of life. The so-called long Agos are composed of four major domains (N, PAZ, MID and PIWI) and contribute to RNA silencing in eukaryotes (eAgos) or defence against invading mobile genetic elements in prokaryotes (pAgos). The majority (~60%) of pAgos identified bioinformatically are shorter (comprising only MID and PIWI domains) and are typically associated with Sir2, Mrr or TIR domain-containing proteins. The cellular function and mechanism of short pAgos remain enigmatic. Here we show that Geobacter sulfurreducens short pAgo and the NAD$^+$-bound Sir2 protein form a stable heterodimeric complex. The GsSir2/Ago complex presumably recognizes invading plasmid or phage DNA and activates the Sir2 subunit, which triggers endogenous NAD$^+$ depletion and cell death, and prevents the propagation of invading DNA. We reconstituted NAD$^+$ depletion activity in vitro and showed that activated GsSir2/Ago complex functions as a NADase that hydrolyses NAD$^+$ to ADPR. Thus, short Sir2-associated pAgos provide defence against phages and plasmids, underscoring the diversity of mechanisms of prokaryotic Agos., Published by Nature Publishing Group, London

Argonaute (Ago) proteins are found in all three domains of life. The so-called long Agos are composed of four major domains (N, PAZ, MID, and PIWI) and contribute to RNA silencing in eukaryotes (eAgos) or defence against invading mobile genetic elements in prokaryotes (pAgos). The majority (~60%) of pAgos identified bioinformatically are shorter (comprised of only MID and PIWI domains) and are typically associated with Sir2, Mrr or TIR domain-containing proteins. The cellular function and mechanism of short pAgos remain enigmatic. Here, we show that Geobacter sulfurreducens short pAgo and the NAD+-bound Sir2-protein form a stable heterodimeric complex. The GsSir2/Ago complex presumably recognizes invading plasmid or phage DNA and activates the Sir2 subunit, which triggers endogenous NAD+ depletion and cell death, and prevents the propagation of invading DNA. We reconstituted NAD+ depletion activity in vitro and showed that activated GsSir2/Ago complex functions as a NADase that hydrolyses NAD+ to ADPR. Thus, short Sir2-associated pAgos provide defence against phages and plasmids and underscores the diversity of mechanisms of prokaryotic Agos.

Scientific reports 11(1), 4518 (2021). doi:10.1038/s41598-021-83889-4, Argonaute (Ago) proteins are found in all three domains of life. The best-characterized group is eukaryotic Argonautes (eAgos), which are the core of RNA interference. The best understood prokaryotic Ago (pAgo) proteins are full-length pAgos. They are composed of four major structural/functional domains (N, PAZ, MID, and PIWI) and thereby closely resemble eAgos. It was demonstrated that full-length pAgos function as prokaryotic antiviral systems, with the PIWI domain performing cleavage of invading nucleic acids. However, the majority of identified pAgos are shorter and catalytically inactive (encode just MID and inactive PIWI domains), thus their action mechanism and function remain unknown. In this work we focus on AfAgo, a short pAgo protein encoded by an archaeon Archaeoglobus fulgidus. We find that in all previously solved AfAgo structures, its two monomers form substantial dimerization interfaces involving the C-terminal ��-sheets. Led by this finding, we have employed various biochemical and biophysical assays, including SEC-MALS, SAXS, single-molecule FRET, and AFM, to show that AfAgo is indeed a homodimer in solution, which is capable of simultaneous interaction with two DNA molecules. This finding underscores the diversity of prokaryotic Agos and broadens the range of currently known Argonaute-nucleic acid interaction mechanisms., Published by Macmillan Publishers Limited, part of Springer Nature, [London]

Argonaute (Ago) proteins are found in all three domains of life. The best characterized group is eukaryotic Argonautes (eAgos), which are the core of RNA interference. The best understood prokaryotic Ago (pAgo) proteins are full-length pAgos. They are composed of four major structural/functional domains (N, PAZ, MID and PIWI) and thereby closely resemble eAgos. It was demonstrated that full-length pAgos function as prokaryotic antiviral systems, with the PIWI domain performing cleavage of invading nucleic acids. However, the majority of identified pAgos are shorter and catalytically inactive (encode just MID and inactive PIWI domains), thus their action mechanism and function remain unknown. In this work we focus on AfAgo, a short pAgo protein encoded by an archaeon Archaeoglobus fulgidus. We find that in all previously solved AfAgo structures, its two monomers form substantial dimerization interfaces involving the C-terminal β-sheets. Led by this finding, we have employed various biochemical and biophysical assays, including SEC-MALS, SAXS, single-molecule FRET and AFM, to show that AfAgo is indeed a homodimer in solution, which is capable of simultaneous interaction with two DNA molecules. This finding underscores the diversity of prokaryotic Agos and broadens the range of currently known Argonaute-nucleic acid interaction mechanisms.