Trip to ER: MicroRNA-mediated translational repression in plants

Abstract

miRNAs elicit gene silencing at the post-transcriptional level by several modes of action: translational repression, mRNA decay, and mRNA cleavage. Studies in animals have suggested that translational repression occurs at early steps of translation initiation, which can be followed by deadenylation and mRNA decay. Plant miRNAs were originally thought to solely participate in mRNA cleavage, but increasing evidence has indicated that they are also commonly involved in translational inhibition. Here we discuss recent findings on miRNA-mediated translational repression in plants. The identification of AMP1 in Arabidopsis as a protein required for the translational repression but not the mRNA cleavage activity of miRNAs links miRNA-based translational repression to the endoplasmic reticulum (ER). Future work is required to further elucidate the miRNA machinery on the ER.

Keywords: argonaute; endoplasmic reticulum (ER); mRNA cleavage; mRNA decay; miRNA; translational repression.

Figure 1. Subcellular locations where miRNA biogenesis or activity takes place in plants. A pri-miRNA is processed by a DCL1 complex into a pre-miRNA (arrow 1) and, subsequently, a miRNA:miRNA* duplex (arrow 2). The mature miRNA is loaded into AGO1 and represses target mRNAs through both mRNA cleavage (not diagramed) and translational repression. It is not known whether miRISC is capable of translational repression in the cytosol, but it is likely transported to the rough ER (arrow 3) or P-bodies (arrow 4), both of which are subcellular compartments where translational repression by miRNAs has been implicated to take place. The integral ER membrane protein AMP1 interacts with AGO1 and inhibits the loading of miRNA target mRNAs onto membrane-bound polysomes. How the P-body component and decapping factor VCS mediates the translational repression activity of plant miRNAs is currently unknown. Lipid metabolism (e.g., sterol synthesis), which influences membrane composition or states, also affects miRNA activity via an unknown mechanism. In addition, cytoskeleton (e.g., microtubules) dynamics may influence miRNA activities by affecting the subcellular trafficking of miRISCs.