Trends in Plant Science
ReviewSpecial Issue: Noncoding and small RNAsPlant ARGONAUTES
Section snippets
History of ARGONAUTE nomenclature
Proteins of the ARGONAUTE (AGO) family were referred to first as PAZ proteins because of a unique domain in the central part of the three founding members of the family: Drosophila P ELEMENT-INDUCED WIMPY TESTIS (PIWI), Arabidopsis ARGONAUTE1 (AGO1) and Arabidopsis ZWILLE (ZLL). To distinguish AGO proteins from DICER proteins, which later were also found to harbour a PAZ domain, AGO proteins were renamed PPD (PAZ PIWI DOMAINS) proteins, owing to the presence of a PIWI domain, which is not found
Diversity of ARGONAUTE proteins among kingdoms
Although all AGO proteins harbour PAZ, MID (middle) and PIWI domains (see below), they are divided into three groups on the basis of both their phylogenetic relationships and their capacity to bind to small RNAs (see Box 1 for a description of the different classes of eukaryotic small RNAs, and Box 2 for a thorough description of plant small RNAs) 1, 2. Group 1 members bind to microRNAs (miRNAs) and small interfering RNAs (siRNAs) and are referred to as AGO proteins. Group 2 members bind to
Domains and activities of ARGONAUTE proteins
AGOs are large proteins (ca 90–100 kDa) consisting of one variable N-terminal domain and conserved C-terminal PAZ, MID and PIWI domains 1, 2. Experiments with bacterial and animal AGO proteins have elucidated the roles of these three domains in small RNA pathways 6, 7. The MID domain binds to the 5′ phosphate of small RNAs, whereas the PAZ domain recognizes the 3′ end of small RNAs 1, 2. The PIWI domain adopts a folded structure similar to that of RNaseH enzymes and exhibits endonuclease
Functions of plant AGO proteins
Phylogenic analyses group plant AGO proteins in three clades (Figure 1). Arabidopsis exhibits an equal distribution of its ten members within the three clades: AtAGO1, AtAGO5 and AtAGO10 within the first clade; AtAGO2, AtAGO3 and AtAGO7 within the second clade; and AtAGO4, AtAGO6, AtAGO8 and AtAGO9 within the third clade [4]. By contrast, rice exhibits an expanded first clade (12 members), an equivalent second clade (three members), and a reduced third clade (two members) [5].
Conclusions
Despite the fact that AGO proteins were first discovered in plants ten years ago, only some members of the Arabidopsis AGO family have been analyzed in depth, and the functions of half of the family remain unexplored. Therefore, many fundamental questions about the functions of this large conserved gene family remain unanswered in plants:
- (i)
What is the reason for the absence of PIWI or group 3 proteins in plants? Do plant AGO members perform functions similar to those of PIWI proteins during
Acknowledgements
I thank Allison Mallory for critically reading the manuscript, and I apologize to colleagues whose work could not be fully cited owing to space constraints.
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