Proofreading and Aminoacylation of tRNAs Before Export from the Nucleus

Abstract

After synthesis and processing in the nucleus, mature transfer RNAs (tRNAs) are exported to the cytoplasm in a Ran.guanosine triphosphate-dependent manner. Export of defective or immature tRNAs is avoided by monitoring both structure and function of tRNAs in the nucleus, and only tRNAs with mature 5' and 3' ends are exported. All tRNAs examined can be aminoacylated in nuclei of Xenopus oocytes, thereby providing a possible mechanism for functional proofreading of newly made tRNAs. Inhibition of aminoacylation of a specific tRNA retards its appearance in the cytoplasm, indicating that nuclear aminoacylation promotes efficient export.

Have tRNA, will travel

“…Similarly, disruption of a nuclear retention element of Schizosaccharomyces pombe La protein, a protein required for tRNA 3Ј end processing (18), led to the accumulation of spliced tRNA precursors that had not been processed at their 5Ј and 3Ј ends (19). Indeed, in Xenopus oocytes, a reexamination of tRNA processing established that splicing naturally occurs before end processing under physiological conditions (20).…”

“…The demonstration that nuclear aminoacylation of tRNA was important for tRNA export added a new dimension to tRNA processing (20), giving birth to the idea that tRNA proofreading was effected by aminoacylation of mature tRNA before export. Aminoacylation also proved to be important for tRNA export in yeast, as demonstrated by analysis of tRNA synthetase mutants, amino acid starvation, and addition of inhibitors of synthetases (21,22).…”

Healing for destruction: tRNA intron degradation in yeast is a two-step cytoplasmic process catalyzed by tRNA ligase Rlg1 and 5′-to-3′ exonuclease Xrn1

“…In vertebrates, tRNA splicing occurs in the nucleus (Melton et al 1980;Lund and Dahlberg 1998;Paushkin et al 2004), whereas it is a cytoplasmic process in yeast, since the SEN complex is located on the outer surface of mitochondria (Yoshihisa et al 2003). Despite the difference in localization, the structure and function of the SEN complex are conserved from yeast to humans (Trotta et al 1997;Paushkin et al 2004).…”

P-body components, Dhh1 and Pat1, are involved in tRNA nuclear-cytoplasmic dynamics

“…This increase is specific to the initiation step of translation as disruption of elongation fails to induce increased P-body formation in yeast (Sheth and Parker 2003;Teixeira et al 2005). Addition of the translation elongation inhibitor, cyclohexamide, also fails to induce tRNA nuclear accumulation in Xenopus oocytes (Lund and Dahlberg 1998). As inhibition of translation initiation causes P-body formation, it may also lead to tRNA nuclear accumulation.…”