Nuclear export of tRNA

Exit of tRNA from the nucleus was shown, long time ago, to be a saturable and carrier-mediated process. Nevertheless, only recently, progress in the held of nucleocytoplasmic transport gave first insight into the mechanism of tRNA nuclear export. A nuclear export receptor for tRNA (Los1p/Xpo-t), belonging to the importin beta (karyopherin) family, has been characterized in yeast and mammalian cells. Mature tRNA molecules can associate with Los1p/Xpo-t and the GTP-bound form of the small GTPase Ran to form an export complex in the nucleus. This complex translocates through the nuclear-pore complexes and dissociates upon GTP hydrolysis in the cytoplasm. Genetic studies in yeast have, however, shown that LOS1 is not essential, unless additional steps in the tRNA biogenesis pathway are impaired, suggesting the existence of additional tRNA nuclear export routes. Furthermore, modification and amino-acylation of tRNA may also be important for efficient transport of tRNA into the cytoplasm.