Retrograde nuclear transport from the cytoplasm is required for tRNA^Tyrmaturation inT. brucei

Abstract

Retrograde transport of tRNAs from the cytoplasm to the nucleus was first described in Saccharomyces cerevisiae and most recently in mammalian systems. Although the function of retrograde transport is not completely clear, it plays a role in the cellular response to changes in nutrient availability. Under low nutrient conditions tRNAs are sent from the cytoplasm to nucleus and presumably remain in storage there until nutrient levels improve. However, in S. cerevisiae tRNA retrograde transport is constitutive and occurs even when nutrient levels are adequate. Constitutive transport is important, at least, for the proper maturation of tRNA, which undergoes cytoplasmic splicing, but requires the action of a nuclear modification enzyme that only acts on a spliced tRNA. A lingering question in retrograde tRNA transport is whether it is relegated to S. cerevisiae and multicellular eukaryotes or alternatively, is a pathway with deeper evolutionary roots. In the early branching eukaryote Trypanosoma brucei, tRNA splicing, like in yeast, occurs in the cytoplasm. In the present report, we have used a combination of cell fractionation and molecular approaches that show the presence of significant amounts of spliced tRNA in the nucleus of T. brucei. Notably, the modification enzyme tRNA-guanine transglycosylase (TGT) localizes to the nucleus and, as shown here, is not able to add queuosine (Q) to an intron-containing tRNA. We suggest that retrograde transport is partly the result of the differential intracellular localization of the splicing machinery (cytoplasmic) and a modification enzyme, TGT (nuclear). These findings expand the evolutionary distribution of retrograde transport mechanisms to include early diverging eukaryotes, while highlighting its importance for queuosine biosynthesis.

A link between mitochondrial gene expression and life stage morphologies in Trypanosoma cruzi

“…We are currently unable to speculate on whether an mRNA would actually be transported out of the mitochondrion, or if its increasing presence would generate another signal that would be transmitted out of the mitochondrion. However, the presence of retrograde tRNA from cytoplasm to nucleus has been already been established in T. brucei (Kessler et al, ). The main scenario presented in Figure is a preliminary idea that requires additional experimental support.…”

Formation of tRNA wobble inosine in humans is perturbed by a millennia-old mutation linked to intellectual disability

“…Thus, the disruption of the nucleocytoplasmic ratio by the V144M mutation combined with the activity defect of ADAT2/3-V144M complexes could lead to the reduction in wobble inosine modification levels observed in the tRNAs of individuals homozygous for the ADAT3-V144M mutation. In addition, newly-exported tRNAs lacking inosine could undergo retrograde transport back into the nucleus to be modified by nuclear ADAT2/3 (59,60). Since ADAT2/3-V144M complexes in extract appear to be defective in modifying pre- but not mature tRNA, retrograde transport could play a critical role in providing at least enough tRNA wobble inosine modification for sufficient translation to sustain cell viability.…”

tRNA dynamics between the nucleus, cytoplasm and mitochondrial surface: Location, location, location

“…A key function of retrograde tRNA import into the nucleus is to allow for proper modification and maturation of several tRNAs [95,97]; however, there are additional functions for this process.…”