Trends in Cell Biology
ReviewThe Functions of MicroRNAs: mRNA Decay and Translational Repression
Section snippets
miRNAs Mediate Two Modes of Silencing
miRNAs are endogenous, small noncoding RNAs approximately 20–22 nucleotides (nt) long that regulate gene expression by binding to their complementary target mRNAs. To date, over 2000 miRNAs have been identified in the human genome, while the model plant Arabidopsis thaliana has ∼300 miRNAs [1]. They control a broad array of biological processes, including development, differentiation, proliferation, and stress responses 2, 3, 4, 5, 6.
miRNAs cannot work alone. To silence target mRNAs, they need
Mechanism of miRNA-Mediated mRNA Decay in Animals
Although animal miRNAs were initially thought to repress translation of target mRNAs with little or no decrease in mRNA abundance 13, 14, later studies revealed that miRNAs can also promote mRNA destabilization by recruiting deadenylases onto target mRNAs through GW182 protein (TNRC6A–C in mammals and GW182 or Gawky in Drosophila) 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27. GW182 protein plays key roles in the animal miRNA pathway through interaction with Ago. Biochemical and structural
Mechanism of miRNA-Mediated mRNA Decay in Plants
In contrast to animal miRNAs, plant miRNAs cannot promote deadenylation [64]; instead, they can direct target RNA cleavage (Figure 2) 8, 65. In the model plant A. thaliana, there are ten Ago proteins (AGO1–10) 66, 67, of which miRNAs are mainly sorted into AGO1. AGO1 has a catalytic tetrad in its PIWI domain and cleaves target mRNAs with fully or nearly fully complementary sequence to the miRNA 65, 68, 69, 70. Although it is unknown how quickly miRNAs cleave their target mRNAs in vivo, in vitro
miRNA-Mediated Translational Repression in Animals
To date, many studies using different organisms and methods have suggested that miRNAs inhibit the initiation step of translation (Box 2) 76, 77, 78, 79. Translational repression at the initiation step is also supported by recent genome-wide analyses of endogenous miRNA targets 80, 81, 82. Although it is still unclear exactly how miRNAs repress translation, three major mechanisms have been proposed in the past few years, including (i) GW182-mediated PABP displacement 48, 49, (ii) recruitment of
Evidence for Translational Silencing and Introduction of Factors Involved
Initially, plant miRNAs were thought to silence target genes only through endonucleolytic activity of Argonaute proteins. Contrary to animal miRNAs, however, which silence many partially complementary target mRNAs without cleavage, each plant miRNA has a few specific target mRNAs with fully or nearly fully complementary sequence [11]. Indeed, plant miRNA-mediated target cleavage has been validated with Northern blotting, 5’-Rapid Amplification of cDNA Ends (RACE) analyses and other assays in
In Animals
Although genetic, biochemical, and structural analyses have uncovered the intricate mechanisms of miRNA-mediated silencing, the relative contributions of mRNA decay and translational repression to the overall silencing remain under debate. Of course the relative contributions of the two pathways should vary depending on the concentration of RISC components, the sequence of target mRNAs (e.g., number or position of miRNA-binding sites), or cell types, but recent genome-wide analyses have
Concluding Remarks
Over the past years, much progress has been achieved in understanding the effector step of animal and plant miRNA pathways. Accordingly, our understanding of the mechanism of miRNA-mediated mRNA decay in animals and target cleavage in plants has significantly matured. In contrast to this, there are many unanswered questions regarding the mechanism of miRNA-mediated translational repression (see Outstanding Questions). In animals, it remains unclear at which step(s) of translation is blocked by
Acknowledgments
We thank Shintaro Iwasaki, Takashi Fukaya, and members of the Tomari laboratory for discussions and critical reading of the manuscript. This work was supported in part by Grant-in-Aids for Scientific Research on Innovative Areas to Y.T. (‘Non-coding RNA neo-taxonomy’) and H-o.I. (‘Nascent-chain biology’).
References (145)
MicroRNAs and developmental timing
Curr. Opin. Genet. Dev.
(2011)Functions of microRNAs in plant stress responses
Trends Plant Sci.
(2012)- et al.
Making RISC
Trends Biochem. Sci.
(2010) MicroRNAs: target recognition and regulatory functions
Cell
(2009)- et al.
The lin-4 regulatory RNA controls developmental timing in Caenorhabditis elegans by blocking LIN-14 protein synthesis after the initiation of translation
Dev. Biol.
(1999) Posttranscriptional regulation of the heterochronic gene lin-14 by lin-4 mediates temporal pattern formation in C. elegans
Cell
(1993)Differential regulation of germline mRNAs in soma and germ cells by zebrafish miR-430
Curr. Biol.
(2006)Regulation by let-7 and lin-4 miRNAs results in target mRNA degradation
Cell
(2005)GW182 proteins directly recruit cytoplasmic deadenylase complexes to miRNA targets
Mol. Cell
(2011)Mammalian miRNA RISC recruits CAF1 and PABP to affect PABP-dependent deadenylation
Mol. Cell
(2009)