tRNA, new aspects in intracellular dynamics

The nuclear envelope divides a eukaryotic cell into two compartments, the nucleus and the cytoplasm. Transcription and maturation of RNAs encoded on nuclear chromosomes are carried out in the nucleus, while the proteins coded by these RNAs are translated and processed in the cytoplasm. Cytosolic tRNAs, essential factors for translation, are transcribed in the nucleus and undergo extensive processing before reaching functionality. It had previously been believed that tRNAs have only one-way tickets to pass through the nuclear envelope after maturation in the nucleus, and that the small amounts of mature tRNAs found in the nucleus are biosynthetic intermediates. However, two reports from our lab and Anita Hopper's group recently demonstrated that tRNAs have multi-round commuter tickets to shuttle between the nucleus and the cytoplasm in the yeast Saccharomyces cerevisiae. In fact, various tRNA species, including aminoacylated full-length tRNAs and 3' end-shortened tRNAs, are actively imported into the nucleus under various conditions. These findings force a reconsideration of our view of intracellular dynamics of tRNAs, and re-evaluations of the physiological meanings of the nuclear mature tRNAs.