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Short answer
Argonaute (Ago, AGO) proteins are the guide-binding core of RNA-silencing complexes. An Argonaute protein holds a short guide nucleic acid and uses it to find a matching target. Having a guide and cutting a target are separate things: of the four human Argonautes, only hAgo2 acts as a nuclease.
Definition and scope
“Argonaute protein”, “Argonaute proteins”, “Ago protein” and “Ago proteins” all name the same protein family; Ago and AGO are interchangeable abbreviations in the literature. Eukaryotic Argonautes (eAgos) share a common architecture of N-terminal, PAZ (PIWI-Argonaute-Zwille), MID (middle) and PIWI domains (Swarts et al., 2014; Willkomm et al., 2015).
The family is not limited to eukaryotes. Prokaryotic Argonautes (pAgos) exist and behave differently from the human proteins in what kind of guide they prefer, which is covered below. This page is about the protein itself; for the experimental use of the silencing pathway see our guide to shRNA knockdown design and validation.
How an Argonaute protein uses a guide
The guide is anchored at both ends. The 5′ phosphorylated end of the guide is inserted into a nucleotide-binding pocket in the MID domain, and the PAZ domain binds the 3′ end of the guide through contacts with the backbone of nucleotides 20 and 21 in the structures reviewed by Swarts et al. (2014). In the structures, the electron density of the guide can be detected easily for nucleotides 2 to 7 or 8, the seed region (Willkomm et al., 2015).
What happens after target binding depends on the protein. For miRNA-guided hAgo2 targeting, the result is either mRNA degradation by hAgo2 or translational inhibition (Willkomm et al., 2015). So the outcome you measure, lost transcript or lost protein with an intact transcript, partly reflects which Argonaute carries the guide.
Domains and what each one does
| Domain | Role reported in the sources |
|---|---|
| N (N-terminal) | Not involved in guide loading, but plays a critical role in target cleavage (Swarts et al., 2014) |
| PAZ | Binds the 3′ end of the guide through backbone interactions with nucleotides 20 and 21 (Swarts et al., 2014) |
| MID | Its nucleotide-binding pocket receives the 5′ phosphorylated end of the guide (Swarts et al., 2014) |
| PIWI | Adopts an RNase H fold; the catalytic site binds two divalent cations and is completed by a glutamate residue on a mobile PIWI loop (Swarts et al., 2014) |
Human AGO1 to AGO4: binding a guide is not cutting a target
Humans have four Argonautes. Among them, only hAgo2 acts as a nuclease (Willkomm et al., 2015). The catalytic residues alone do not settle the question: hAGO3 harbors all four catalytic residues but cannot cleave targets in vitro, and catalytically inactive hAGO1 can be activated by minimal changes in the active site (Swarts et al., 2014).
Checklist when an assay reports “Ago”
- Name the paralog. “Ago” in a methods section could be any of the four, and only hAgo2 is the nuclease.
- Do not infer slicing from sequence. Having every catalytic residue, as hAGO3 does, did not give cleavage activity in vitro in the source.
- Separate the two outcomes. mRNA degradation points to hAgo2; translational inhibition does not require cleavage.
- Note the seed. Guide nucleotides 2 to 7 or 8 are the seed region, whose electron density is easily detected.
When the outcome is a change in transcript level, methods such as RT-qPCR or a northern blot measure it, and the RNA integrity of the input affects how far a cleavage result can be trusted. A protein-level readout is needed to see translational inhibition without transcript loss.
Prokaryotic Argonautes: the other meaning of “Ago protein”
Some pAgos have higher affinity for DNA guides than for RNA guides (Swarts et al., 2014). Two examples from that review show how different their guide and target combinations can be:
| Protein | Guide and target behavior (Swarts et al., 2014) |
|---|---|
| Mj_Ago | Uses single-stranded DNA guides to cleave single-stranded DNA strands, but cannot cleave RNA targets |
| Aa_Ago | Uses single-stranded DNA guides to cleave single-stranded RNA strands |
Willkomm et al. (2015) review the prokaryotic proteins in more depth. A paper that says “Ago” without naming the species may therefore describe a DNA-guided enzyme, not the RNA-guided human protein.
Where it matters for labs
For a lab running knockdown work, the Argonaute in the system is part of the experimental design. Delivering a silencing construct is covered in the transfection guide; what the protein does with the guide afterwards is the content of this page. Because only hAgo2 is reported as a human nuclease, a cleavage-based explanation for a knockdown points to that paralog, while repression without transcript loss points to translational inhibition.
Common misunderstandings
- “All Argonautes slice.” Among the four human Agos, only hAgo2 acts as a nuclease.
- “The catalytic residues tell you it cuts.” hAGO3 has all four and cannot cleave targets in vitro.
- “Argonautes are RNA-only.” Some prokaryotic Argonautes prefer DNA guides, and Mj_Ago cannot cleave RNA targets at all.
- “Silencing always means degraded mRNA.” For miRNA-guided hAgo2 it can instead mean translational inhibition.
Frequently asked questions
What is an Argonaute protein?
Argonaute (Ago, AGO) proteins are the guide-binding core of RNA-silencing complexes. They consist of N-terminal, PAZ, MID and PIWI domains (Willkomm et al., 2015).
Which human Argonaute cuts RNA?
Among the four human Agos, only hAgo2 acts as a nuclease (Willkomm et al., 2015).
Does having the catalytic residues mean an Argonaute can cleave?
No. hAGO3 harbors all four catalytic residues but cannot cleave targets in vitro (Swarts et al., 2014).
Do all Argonautes use RNA guides?
No. Some prokaryotic Argonautes have higher affinity for DNA guides than for RNA guides; Mj_Ago uses single-stranded DNA guides to cleave single-stranded DNA but cannot cleave RNA targets (Swarts et al., 2014).
What is the seed region?
Guide nucleotides 2 to 7 or 8, whose electron density is easily detected in the structures (Willkomm et al., 2015).
Sources
- Swarts DC, et al. The evolutionary journey of Argonaute proteins. Nat Struct Mol Biol. 2014;21(9):743-753. pmc.ncbi.nlm.nih.gov/articles/PMC4691850 (checked 2026-10-29)
- Willkomm S, et al. A Prokaryotic Twist on Argonaute Function. Life (Basel). 2015;5(1):538-553. doi:10.3390/life5010538. pmc.ncbi.nlm.nih.gov/articles/PMC4390867 (checked 2026-10-29)
