FBS Colloquia No.365Laboratory of Mitochondrial Dynamics

Autophagic degradation of tRNA and extracellular export of modified nucleosides – from phenomenology to molecular understanding

RNAs are intracellular molecules responsible for gene expression that become functional with diverse post-transcriptional modifications. RNA synthesis and degradation are also critical for proper gene expression. In particular, degradation of dysfunctional or surplus RNA is thought to control its quality and quantity, thereby contributing to normal gene expression. Recently, it has been reported that autophagy, an intracellular membrane transport system, is involved in RNA degradation. Autophagy mediates encapsulation of intracellular components by double-membrane bound autophagosomes, and transport of those cargoes to hydrolases-enriched lysosomes (vacuoles in yeast) for degradation. In starved yeast, rRNA and mRNA are degraded by autophagy, and their degradation products such as nucleosides and nucleobases are exported to the extracellular space. However, the mechanisms of RNA degradation and its resulting nucleoside/nucleobase export remain unclear. For example, whether tRNA is degraded by autophagy has not been clarified. RNA degradation generates nucleosides with various residual post-transcriptional modifications. Modified nucleosides are thought to be unnecessary and unrecyclable for nucleic acid and nucleotide synthesis, and thus actively exported to the extracellular space. However, the molecular mechanisms underlying extracellular modified nucleoside export are largely unknown.

To analyze tRNA degradation and tRNA-derived nucleoside export, we focused on modified nucleosides in yeast culture medium. Furthermore, we performed a genome-wide screen with approximately 5,000 non-essential gene deletion yeast library and identified a number of nex (nucleoside export) mutants exhibiting a reduction in extracellular modified nucleosides. In this colloquium, we will present our recent findings on a mode of autophagic tRNA degradation and a function of the plasma membrane ABC transporter Nex1 in modified nucleoside export.