Determining the molecular mechanisms underlying the size-scaling of biosynthesis

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The overarching goal of this project is to understand how cell growth triggers cell division. It has long been known that there is often a requirement for cell growth in order to initiate cell division, but the way this works has remained unknown. The central question is what is biochemically different about a large cell compared to a small cell that makes it divide? To address this question, this project is based on a recent breakthrough by the investigator in understanding how growth triggers division in budding yeast. Contrary to expectations, the investigator found that growth did not increase the activity of a cell division activator. Instead, cell growth triggers division by diluting a protein that inhibits cell division. This project leverages this conceptual breakthrough to rephrase the question of how cell growth triggers cell division to the question of how cell growth results in different amounts of specific proteins being made. In other words, what are the mechanisms that ensure that proteins activating cell division are made in proportion to the size of the cell so that their concentrations do not change as cells get larger? Moreover, how do big and small cells make the same number of cell division inhibitors so that their concentrations decrease as cells grow larger to trigger division? By answering these questions, this project will determine the molecular mechanisms cells use to link cell growth to cell division, which is a fundamental question in cell and developmental biology. The Broader Impact of the project includes the intrinsic nature of the research as all dividing cells need to regulate cell size. Additional activities include the training of undergraduate and graduate students along with post-doctoral researchers. Activities to broadly promote quantitative training along with outreach to high school students will also be carried out. Cell growth, division, and biosynthesis must be carefully orchestrated to maintain efficient cellular function. To achieve this, cell size and biosynthesis are inextricably linked. Total RNA and protein synthesis rates increase in proportion to size so that gene product copy numbers scale with cell volume. This ensures that macromolecule concentrations are kept constant as a cell grows to maintain efficient cellular function. However, if cells become excessively large, this scaling of biosynthesis collapses and cellular physiology and efficient growth become severely compromised. To prevent this, budding yeast cells maintain their size within an optimal range for cellular function by coupling cell growth to cell division. The researchers found that the synthesis of the cell cycle activator Cln3 increased in proportion to cell size, while the synthesis of the budding yeast cell cycle inhibitor Whi5 was independent of cell size such that a constant number of Whi5 molecules are made in each cell cycle. Thus, Whi5 concentration is more dilute in larger cells which triggers an earlier cell cycle entry in larger cells. Thus, the differential size-dependencies of cell cycle activators, which scale in proportion to cell size, and cell cycle inhibitors, which are independent of cell size, couple growth to division to maintain cells within a physiologically optimal size range. The differential size-dependencies in the synthesis of cell cycle activators and inhibitors raise two key questions will be addressed in this project. First, what ensures most protein and mRNA synthesis increases in proportion to cell size? Second, what allows proteins, such as Whi5, to break the general rule and be expressed independently of cell size? This project will employ the model organism budding yeast to address these twin questions using a combination of proteomics, genetics, and quantitative live cell imaging. Successful completion of the project will thereby resolve a fundamental question in cell biology of how cell size, cell division, and biosynthesis are intimately and mechanistically linked to promote cellular function.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.