Function and Localization of MicroRNAs in Mammalian Cells

microRNAs (miRNAs) represent a large set of master regulators of gene expression. They constitute 1-4% of human genes and probably regulate 30% of protein-encoding genes. These small regulatory RNAs act at a posttranscriptional level-mediating translational repression and/or mRNA degradation-through their association with Argonaute protein and target mRNAs. In this paper, we discuss various mechanisms by which miRNAs regulate posttranscriptionally, including their subcellular localization. Recent results indicate that the majority of miRNA-targeted and thus translationally repressed mRNA is probably distributed in the diffuse cytoplasm, even though a small fraction is concentrated in subcellular compartments, such as processing bodies or stress granules; notably, the stress granule localization of Argonaute depends on the presence of miRNAs. Here we discuss the structural requirement of these subcellular compartments in light of their potential miRNA functions.