Nuclear export of tRNA

Exit of tRNA from the nucleus was shown, long time ago, to be a saturable a nd carrier-mediated process. Nevertheless, only recently, progress in the h eld of nucleocytoplasmic transport gave first insight into the mechanism of tRNA nuclear export. A nuclear export receptor for tRNA (Los1p/Xpo-t), bel onging 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 comp lex in the nucleus. This complex translocates through the nuclear-pore comp lexes and dissociates upon GTP hydrolysis in the cytoplasm. Genetic studies in yeast have, however, shown that LOS1 is not essential, unless additiona l steps in the tRNA biogenesis pathway are impaired, suggesting the existen ce of additional tRNA nuclear export routes. Furthermore, modification and amino-acylation of tRNA may also be important for efficient transport of tR NA into the cytoplasm.