Abstract
Yeast cells exposed to stress conditions such as starvation can enter a protective dormant state in which cell division, growth and metabolism are reduced or downregulated. They can remain in that state until nutrients become available again. How cells enter this dormant state and why it is protective is largely unknown. Here we use correlative light and electron microscopy and electron tomography to investigate the ultrastructural changes in the cytoplasm of starved yeast cells. We report that starved yeast cells undergo an extensive cytoplasmic reorganization that involves the formation of both membrane-bound and membraneless organelles. By determining the density of ribosomal particles, we show that the cytoplasm of starved yeast also experiences significant compaction. We further demonstrate that the key enzyme eukaryotic translation initiation factor 2B (eIF2B) polymerizes into large bundles of filaments through a reversible process. We propose that these changes allow yeast cells to store and inhibit proteins, and thus endure unfavorable environmental conditions.
