Reorganization of budding yeast cytoplasm upon energy depletion

Abstract

Yeast cells, when exposed to stress, can enter a protective state in which cell division, growth and metabolism are downregulated. They remain viable in this state until nutrients become available again. How cells enter this protective survival state and what happens at a cellular and subcellular level is largely unknown. In this study, we used electron tomography to investigate the stress-induced ultrastructural changes in the cytoplasm of yeast cells. After ATP depletion, we observed a significant cytosolic compaction and an extensive cytoplasmic reorganization, as well as the emergence of distinct membrane-bound and membrane-less organelles. By using correlative light and electron microscopy (CLEM), we further demonstrate that one of these membrane-less organelles is generated by the reversible polymerization into large bundles of filaments of the eukaryotic translation initiation factor 2B (eIF2B), an essential enzyme in the initiation of protein synthesis. The changes we observe are part of a stress-induced survival strategy, allowing yeast cells to save energy, protect proteins from degradation, and inhibit protein functionality by forming assemblies of said proteins.