Nuclear tRNA export in Saccharomyces cerevisiae has been proposed to involve three pathways, designated Los1p-dependent, Los1p-independent nuclear aminoacylation-dependent, and Los1p- and nuclear aminoacylation-independent. Here, a comprehensive biochemical analysis was performed to identify tRNAs exported by the aminoacylation-dependent and -independent pathways of S. cerevisiae. Interestingly, the major tRNA species of at least 19 families were found in the aminoacylated form in the nucleus. tRNAs known to be exported by the export receptor Los1p were also aminoacylated in the nucleus of both wild-type and mutant Los1p strains. FISH (fluorescence in situ hybridization) analyses showed that tRNATyr co-localizes with the U18 small nucleolar RNA in the nucleolus of a tyrosyl-tRNA synthetase mutant strain defective in nuclear tRNATyr export because of a block in nuclear tRNATyr aminoacylation. tRNATyr was also found in the nucleolus of a utp8 mutant strain defective in nuclear tRNA export but not nuclear tRNA aminoacylation. These results strongly suggest that the nuclear aminoacylation-dependent pathway is principally responsible for tRNA export in S. cerevisiae and that Los1p is an export receptor of this pathway. It is also likely that in mammalian cells tRNAs are mainly exported from the nucleus by the nuclear aminoacylation-dependent pathway. In addition, the data are consistent with the idea that nuclear aminoacylation is used as a quality control mechanism for ensuring nuclear export of only mature and functional tRNAs, and that this quality assurance step occurs in the nucleolus.
Abbreviations used: Cca1p, ATP(CTP):nucleotidyltransferase; eEF, eukaryotic elongation factor; TyrRS, tyrosyl-tRNA synthetase; U18 snoRNA, U18 small nucleolar RNA; FISH, fluorescence in situ hybridization.