Characterization of the A. thaliana cytoplasmic tRNA export protein (CTEXP) and investigation of retrograde tRNA movement to the nucleus in plant cells

Nuclear export of mature and aminoacylated tRNAs for delivery to the protein translation machinery in the cytoplasm is essential to all life, but the process remains largely uncharacterized and poorly understood in plant cells. PSI-BLAST searches resulted in the identification of the uncharacterized A. thaliana open reading frame AT2G40730 as a putative orthologue of Cex1p and SCYL1, known tRNA export factors in Saccharomyces cerevisiae and mammals, respectively. Characterization of the At2g40730 protein, which is named here cytoplasmic tRNA export protein (CTEXP), suggests that it participates in nuclear tRNA export in plant cells.Recent reports that tRNA moves retrograde from the cytoplasm to the nucleus in S. cerevisiae and rat hepatoma H4IIE in response to nutrient stress led us to investigate whether plant cells have the capacity to translocate cytoplasmic tRNA to the nucleus in response to nutrient availability. In contrast to reported findings in other organisms, the distribution of tRNA remains chiefly cytoplasmic upon sucrose or nitrogen deprivation. This unexpected finding may be due to the nuclear localization of the tRNA intron splicing apparatus in plant cells, which, unlike S. cerevisiae, do not require a tRNA import pathway for intron splicing.