Argonaute

The Argonaute protein family plays a central role in RNA silencing processes, as essential components of the RNA-induced silencing complex (RISC). RISC is responsible for the gene silencing phenomenon known as RNA interference (RNAi). Argonaute proteins bind different classes of small non-coding RNAs, including microRNAs (miRNAs), small interfering RNAs (siRNAs) and Piwi-interacting RNAs (piRNAs). Small RNAs guide Argonaute proteins to their specific targets through sequence complementarity (base pairing), which then leads to mRNA cleavage or translation inhibition. The Argonaute protein family plays a central role in RNA silencing processes, as essential components of the RNA-induced silencing complex (RISC). RISC is responsible for the gene silencing phenomenon known as RNA interference (RNAi). Argonaute proteins bind different classes of small non-coding RNAs, including microRNAs (miRNAs), small interfering RNAs (siRNAs) and Piwi-interacting RNAs (piRNAs). Small RNAs guide Argonaute proteins to their specific targets through sequence complementarity (base pairing), which then leads to mRNA cleavage or translation inhibition. The name of this protein family is derived from a mutant phenotype resulting from mutation of AGO1 in Arabidopsis thaliana, which was likened by Bohmert et al. to the appearance of the pelagic octopus Argonauta argo. RNA interference (RNAi) is a biological process in which the RNA molecules inhibit gene expression. The method of inhibition is via the destruction of specific mRNA molecules or by simply suppressing the protein translation. The RNA interference has a significant role in defending cells against parasitic nucleotide sequences. In many eukaryotes, including animals, the RNA interference pathway is found, and it is initiated by the enzyme Dicer. Dicer cleaves long double-stranded RNA (dsRNA) molecules into short double stranded fragments of around 20 nucleotide siRNAs. The dsRNA is then separated into two single-stranded RNAs (ssRNA) – the passenger strand and the guide strand. Subsequently, the passenger strand is degraded, while the guide strand is incorporated into the RNA-induced silencing complex (RISC). The most well-studied outcome of the RNAi is post-transcriptional gene silencing, which occurs when the guide strand pairs with a complementary sequence in a messenger RNA molecule and induces cleavage by Argonaute, that lies in the core of RNA-induced silencing complex. Argonaute proteins are the active part of RNA-induced silencing complex, cleaving the target mRNA strand complementary to their bound siRNA. Theoretically the dicer produces short double-stranded fragments so there should be also two functional single-stranded siRNA produced. But only one of the two single-stranded RNA here will be utilized to base pair with target mRNA. It is known as the guide strand, incorporated into the Argonaute protein and leads gene silencing. The other single-stranded named passenger strand is degraded during the RNA-induced silencing complex process. Once the Argonaute is associated with the small RNA, the enzymatic activity conferred by the PIWI domain cleaves only the passenger strand of the small interfering RNA. RNA strand separation and incorporation into the Argonaute protein are guided by the strength of the hydrogen bond interaction at the 5'-ends of the RNA duplex, known as the asymmetry rule. Also the degree of complementarity between the two strands of the intermediate RNA duplex defines how the miRNA are sorted into different types of Argonaute proteins. In animals, Argonaute associated with miRNA binds to the 3'-untranslated region of mRNA and prevents the production of proteins in various ways. The recruitment of Argonaute proteins to targeted mRNA can induce mRNA degradation. The Argonaute-miRNA complex can also affect the formation of functional ribosomes at the 5'-end of the mRNA. The complex here competes with the translation initiation factors and/or abrogate ribosome assembly. Also, the Argonaute-miRNA complex can adjust protein production by recruiting cellular factors such as peptides or post translational modifying enzymes, which degrade the growing of polypeptides. In plants, once de novo double-stranded (ds) RNA duplexes are generated with the target mRNA, an unknown RNase-III-like enzyme produces new siRNAs, which are then loaded onto the Argonaute proteins containing PIWI domains, lacking the catalytic amino acid residues, which might induce another level of specific gene silencing. The argonaute (AGO) gene family encodes for four characteristic domains: N- terminal, PAZ, Mid and a C-terminal PIWI domain. The PAZ domain is named after proteins PIWI, AGO, and Zwille, whereby it is found to be conserved. The PAZ domain is an RNA binding module that recognizes the 3' end of both siRNA and miRNA, in a sequence independent manner. Consequently, it targets the mRNA for cleavage or translation inhibition by base-pairing interaction.