Proteasomes (red) are scattered around the nucleus of a dividing cell (top), but cluster in the cytoplasm when the cell is quiescent (bottom).

When a yeast cell hunkers down for hard times, it kicks its proteasomes out of the nucleus, as Laporte et al. show.

The cell's garbage disposal, the proteasome, digests proteins that are damaged or no longer needed. In dividing yeast, proteasomes in the nucleus help drive the cell cycle by demolishing regulatory proteins such as cyclins at crucial times. But yeast spend much of their lives in a nondividing state prompted by food scarcity. Laporte et al. tracked what happens to proteasomes during this quiescence.

Hungry yeast ejected proteasomes from the nucleus and stashed them in previously undescribed cytoplasmic structures that the researchers dubbed proteasome storage granules. Whether the exiled proteasomes were functional wasn't clear. But when the cells found a meal, the proteasomes returned to the nucleus, the team found.

Laporte et al. speculate that a cell stores its proteasomes, instead of breaking down and then resynthesizing them, so it can quickly fire them up after quiescence and clear damaged proteins that have built up in the nucleus. Whether mammalian cells relocate their proteasomes is an open question. Proteasomes do bunch up in certain diseases, but these congregations might not be comparable to the granules, the researchers note. Pinning down what happens to proteasomes in mammalian cells might provide clues about the origin and growth of cancer, since most of the cells in our bodies are in a nondividing state similar to yeast quiescence. Moreover, many cancer cells depend on proteasome activity to continue multiplying.

Laporte, D., et al.
J. Cell Biol.