Significance of amino acid recycling from vacuoles for viability and sporulation of yeast cells under starvation conditions

Amino acid recycling is one of the fundamental roles of autophagy. To fulfill this role, amino acid export from vacuoles must operate. Its machinery, however, has not been well elucidated so far, making it difficult to accurately evaluate its physiological significance. Avt6 is a transporter that exports acidic amino acids, such as aspartate and glutamate. We found that disrupting AVT6 in cells lacking other vacuolar amino acid exporters, including Avt3, Avt4, and Avt7, increased vacuolar neutral amino acid content. Conversely, Avt6 overexpression decreased these amino acid levels. These results suggest that Avt6 is also involved in the vacuolar export of neutral amino acids. Although avt3∆avt4∆avt6∆avt7∆ cells maintained viability under nitrogen starvation as wild-type cells, protein synthesis was reduced. By further deleting LEU2, a gene involved in leucine biosynthesis, viability was significantly reduced. Moreover, avt3∆avt4∆avt6∆avt7∆ diploid cells formed fewer spores in a sporulation medium. These phenotypes overlapped with those of cells defective in macroautophagy. This study reveals an extensive redundancy of transporters in vacuolar amino acid recycling and experimentally demonstrates that the vacuolar amino acids generated by macroautophagy are utilized to manage nutrient stress.