Abstract
In the current concept, tRNA maturation in vertebrate cells, including splicing of introns, trimming of 5’ leader and 3’ trailer, and adding of CCA, is thought to occur exclusively in the nucleus. Here we provide evidence to challenge this concept. Unspliced intron-containing precursor tRNAIle was identified in Human Immunodeficiency Virus type 1 (HIV-1) virions, which are synthesized in the cytoplasm. Northern blot, confocal microscopy and quantitative RT-PCR further verified enrichment of this unspliced tRNAIle within the cytoplasm in human cells. In addition to containing an intron, the cytoplasmic precursor tRNAIle also contains a short incompletely processed 5´ leader and a 3´ trailer, which abundance is around 1000 fold higher than the nuclear precursor tRNAIle with long 5’ leader and long 3’ trailer. In vitro data also suggest that the cytoplasmic unspliced end-immature precursor tRNAIle could be processed by short isoform of RNase Z, but not long isoform of RNase Z. These data suggest that precursor tRNAs could export from the nucleus to the cytoplasm in human cells, instead of be processed only in the nucleus.
Highlights
Mature transfer RNA plays an important role in protein translation
Extracellular Human Immunodeficiency Virus type 1 (HIV-1) virions released from human HEK293T cells were purified by ultracentrifugation through sucrose gradients
Intron-containing tRNAIle is thought to be processed in the nucleus in human cells, and it is totally unanticipated to be found in HIV-1 virions
Summary
Mature transfer RNA (tRNA) plays an important role in protein translation. Mature tRNAs are thought to solely function in the cytoplasm for translation. The discovery of accumulation of mature tRNAs in the nucleus in eukaryotes changes this concept [2]. Two groups have independently shown that mature tRNAs can actively shuttle between the nucleus and the cytoplasm in yeast [2, 3]. The significance of this shuttling is unknown, but implies tRNA quality control, cellular response to nutrient shortage, or cell cycle check-point control in response to DNA damage [4,5,6]
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