Cell freezing in response to advanced glucose starvation : a novel cytoplasmic state in fission yeast
Ibeneche, Chieze Chinenye
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Critical to a cell's survival is its ability to deal with stress by making an appropriate response. This response often takes place in the cytoplasm, which is everything contained within the cell's plasma membrane that is not the nucleus. The cytoplasm is a dynamic environment and its ability to reorganize is essential to the cell's function. This dissertation presents a novel, previously undiscovered state of cytoplasm organization for the model system Schizosaccharomyces pombe, also known as fission yeast. Typically the fission yeast cytoplasm is a fluid-like environment in which endogenous lipid granules subject to thermal fluctuations, move freely as they explore their local surroundings through diffusion. When the cell is in a nutrient depleted environment it is exposed to the stress of advanced glucose starvation. As a result, we find that the cytoplasm undergoes drastic reorganization reminiscent of a phase transition; it is now a solid-like environment in which there is no visible motion. Lipid granules throughout the cell appear to be completely immobilized and are unable to move through the cytoplasm, despite the application of force through optical tweezers. We term this cytoplasmic state the cell frozen state. The cell frozen state is a physiological state, one that the cell can recover from with the addition of fresh nutrients. It is characterized by an anomalous diffusion exponent of [alpha] = 0.23 ± 0.01, which is a significant reduction from the anomalous diffusion exponent [alpha] = 0.66 ± 0.01 found for exponentially growing cells in which there is visible motion. To account for the cell wide immobilization of lipid granules, we hypothesize the formation of a polymer network all through the cytoplasm, and identify septins 1-3 as the most likely filament formers. In addition, we find there is an increase in the number of vacuoles in the cytoplasm during starvation, and propose a vacuole-septin model to describe the cytoplasm reorganization for the cell frozen state.