tRNAExport

tRNAExport is the process by which mature transfer RNAs (tRNAs) are transported from the nucleus to the cytoplasm in eukaryotic cells, enabling them to participate in translation.

In most eukaryotes, tRNA genes are transcribed by RNA polymerase III inside the nucleus. Pre-tRNAs contain extra sequences such as 5' leaders, 3' trailers, and often introns. Nuclear maturation steps trim these precursors, and intron-containing tRNAs are spliced in some organisms. The 3' end is commonly extended by the CCA terminus, added by tRNA nucleotidyltransferase when not genomically encoded. Once tRNAs are properly processed and structurally mature, they are exported through the nuclear pore complex to the cytoplasm where they participate in protein synthesis.

The export receptor most commonly responsible for tRNA export is exportin-t (XPO-t; XPOT in humans). In the yeast Saccharomyces cerevisiae, the functional homolog is Los1p. tRNA export proceeds in a Ran GTPase–dependent manner: in the nucleus, RanGTP promotes assembly of the export complex with the cargo; after translocation to the cytoplasm, GTP hydrolysis leads to complex disassembly and release of the tRNA. Some organisms or tRNA species can use alternative export pathways or receptors, such as Crm1/XPO1 or Msn5, under certain conditions.

Regulation of tRNAExport can be influenced by the aminoacylation status of tRNAs, post-transcriptional modifications, and cellular stress. Defects in export result in nuclear retention of tRNAs, depletion of cytoplasmic tRNA pools, and translational stress or growth defects. Overall, tRNA export is a conserved and essential step in eukaryotic gene expression.