Friday, May 9, 2025
5/9/2025
Mechanisms of cell size determination by the mitotic cyclin Cdc13 - PROJECT SUMMARY Eukaryotic cells tightly control their size to maintain proper cell physiology. Defects in cell size control can lead to developmental infidelity and disease. One mechanism for accurate cell size control is the coupling of cell growth and division. We use the fission yeast, Schizosaccharomyces pombe, which has a reproducible rod shape and strong genetic tools to study cell size control. In fission yeast, a cell size checkpoint delays entry into mitosis (the G2/M transition) until cells have grown to a critical size threshold. This checkpoint acts through activation of the cyclin-dependent kinase Cdk1 in complex with its cyclin subunit Cdc13. Recent work has shown that the nuclear concentration of Cdc13/cyclin increases over time, providing a mechanism to promote increased Cdk1 activity as cells grow. Nutrient limitation is known to reduce cell growth rate and cell size at division, meaning that the size control system monitors and responds to nutrient levels. It is not known how the Cdc13 time-dependent nuclear accumulation for cell size control is regulated under normal and modified growth conditions. The central hypothesis is that two factors are regulated by nutrient availability which connects size and the environment: time-dependent accumulation of Cdc13 in the nucleus and the threshold concentration of Cdc13 that promotes mitotic entry. To test this hypothesis, I will first determine the regulatory mechanisms of Cdc13 nuclear accumulation by measuring rates of nuclear import, nuclear export, and degradation. I will utilize a combination of live-cell microscopy techniques including photobleaching and photoconversion methods. Second, I will define how altered nutrient limitation modulates Cdc13 nuclear accumulation and its regulation. I will quantify the nuclear accumulation of Cdc13 under different nutrient conditions, as well as explore the role of the nutrient sensing pathways, such as AMPK and Greatwall-Endosulfine, in setting a PP2A threshold for the control of Cdc13 nuclear accumulation. The proposed work will establish a systems-level understanding of cell size control and how it can be tuned by environmental conditions. In addition to the significant scientific contributions this work will make in the field of cell size control, I will also gain invaluable technical skills, knowledge, and communication experience.
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