Yeast and other organisms have evolved to survive extended periods of starvation by digesting their own proteins and other cell materials and thereby recycle them into new proteins and structures. One way in which these cell materials can be destroyed is by a process called autophagy. A membrane forms around the cell material to isolate it from the rest of the cell. In yeast, the resulting structure fuses with a cell compartment called the vacuole, which contains enzymes that break down the cargo into smaller molecules that can be re-used by the cell.
When cells experience starvation, autophagy is not very selective in what it destroys and so it is tightly controlled to avoid damaging important structures in healthy cells. Alongside autophagy, specific proteins in the membrane surrounding a yeast cell can be targeted for destruction by another process called the MVB pathway. Certain membrane proteins are tagged with a small protein called ubiquitin, which leads them to being selectively incorporated into cell compartments called MVBs that then go on to fuse with the vacuole. However, it is not clear how the MVB pathway and autophagy may cooperate to enable the cell to survive periods of starvation.
Here, Müller et al. monitored the changes in the proteins present in yeast cells during a period of starvation. The experiments show that many different membrane proteins in the yeast cells were destroyed via the MVB pathway within three hours of the removal of their food source. This was essential to allow the cells to carry on producing new proteins at this early stage in starvation. These new proteins included the enzymes found in vacuoles, which increased the ability of the cells to break down the proteins and other cell materials that were transported there via autophagy.
These findings show how the MVB pathway and autophagy are co-ordinated to allow cells to survive periods of starvation. The next challenge is to work out how the MVB pathway is regulated at the molecular level in response to fluctuations in nutrient availability.