Glucose-Regulated Phosphorylation of the PUF Protein Puf3 Regulates the Translational Fate of Its Bound mRNAs and Association with RNA Granules

PUF proteins are post-transcriptional regulators that bind to the 3' UTRs of mRNA transcripts. Herein, we show how a yeast PUF protein, Puf3p, responds to glucose availability to switch the fate of its bound transcripts that encode proteins required for mitochondrial biogenesis. Upon glucose depletion, Puf3p becomes heavily phosphorylated within its N-terminal region of low complexity, associates with polysomes, and promotes translation of its target mRNAs. Such nutrient-responsive phosphorylation toggles the activity of Puf3p to promote either degradation or translation of these mRNAs according to the needs of the cell. Moreover, activation of translation of pre-existing mRNAs might enable rapid adjustment to environmental changes without the need for de novo transcription. Strikingly, a Puf3p phosphomutant no longer promotes translation but becomes trapped in intracellular foci in an mRNA-dependent manner. Our findings suggest that the inability to properly resolve Puf3p-containing RNA-protein granules via a phosphorylation-based mechanism might be toxic to a cell. Lee and Tu report that the PUF protein Puf3 becomes phosphorylated in its low-complexity region upon glucose depletion to promote the translation of its bound mRNAs that are important for mitochondrial biogenesis. A Puf3 phosphomutant becomes trapped in punctate foci, providing insights into the dynamic nature of RNA granules.