γ-Glutamyl kinase is involved in selective autophagy of ribosomes in Saccharomyces cerevisiae

gamma-Glutamyl kinase (GK; the PRO1 gene product) is a key enzyme in the Saccharomyces cerevisiae proline biosynthesis pathway. Delta pro1 cells are more sensitive to various stresses than wild-type cells, suggesting that GK has an alternative function independent of proline biosynthesis. We show that PRO1 genetically interacts with UBP3, which encodes ubiquitin-specific protease, and is required for selective autophagy of ribosomes (ribophagy). Interestingly, yeast cells with PRO1 deletion or expressing inactive GK display a defect for ribophagy but not for nonselective autophagy, indicating that GK activity is indispensable for ribophagy. Gene disruption analysis suggests that ribophagy is important for cell survival during nitrogen starvation.