Transport of tRNA out of the Nucleus - Direct channeling to the ribosome?

Although tRNA was the first substrate whose export from the nuclei of eukar yotic cells had been shown to be carrier-mediated and active, it has only b een in the last 2 years that the first mechanistic details of this nucleocy toplasmic transport pathway have begun to emerge. A member of the importin/ karyopherin beta superfamily, Los1p in yeast and Xpo-t in vertebrates, has been shown to export tRNA in cooperation with the small GTPase Ran (Gsp1p) from the nucleus into the cytoplasm, where tRNA becomes available for tran slation. However, Los1p is not essential for viability in yeast cells, sugg esting that alternative tRNA export pathways exist. Recent results show tha t aminoacylation and a translation factor are also required for efficient n uclear tRNA export. Thus, protein translation and nuclear export of tRNA ap pear to be coupled processes. (C) 2000 Academic Press.