The ability to rationally control the growth and specialization of most cells requires knowledge of how cells grow asymmetrically. Asymmetric growth, i.e., growth exclusively toward one pole of a cell, is necessary for the survival and propagation of diverse species. This project is part of a long-term effort to decipher cellular events that allow fungi, plants, and animal cells to grow exclusively at one pole. This project is to study polar growth in baker's yeast (S. cerevisiae) because of ease of experimental manipulation, its known similarity to other fungi as well as plant and animal cells, and comparatively low cost as a research tool. This project focuses on determining the function of Osh proteins during the process of asymmetric growth. Osh proteins are the fungal equivalent of mammalian oxysterol sterol binding proteins (OSBPs), a functionally enigmatic protein family that binds sterols such as cholesterol. Although it is known that Osh proteins are essential for life and important for polar cell growth, how these proteins function is not well defined. Moreover, not all Osh proteins share a clear sterol binding region. Experiments in one part of the project will answer a fundamental question about how Osh proteins function in cells: Is the binding of a sterol molecule to an Osh protein necessary for Osh protein function in general, i.e., do Osh proteins have sterol-dependent as well as sterol-independent functions? Experiments in the other part of this project use yeast strains lacking Osh proteins to distinguish between several models to explain basic aspects of polar cell growth. Specific focus is given to determining how cells keep a signal for polar cell growth (the highly conserved GTP binding protein Cdc42) at the pole of the cell designated for growth. Thus, in addition to determining what is required for Osh protein function in cells, this project will reveal part of the cellular mechanism by which cells regulate polar cell growth.
Broader Impact:
Graduate and undergraduate students will participate in the project, working with the PI. Experiments from the project will be included in an undergraduate lab course, which will expose more students to "real"research. Finally, the PI will participate in a University of Virginia K-12 outreach program to promote science and careers in science.
Innovations in engineering will depend upon the ability to assemble control circuitry from diffusible molecules. Innovations in applied biology will depend upon the ability to control precisely the rate of cell growth and the shape of cells as they grow. To create a foundation for these innovations, this research was part of a long-term effort to determine how cells use proteins and lipids, i.e., diffusible molecules, to create and maintain an internal control circuitry that allows them to grow asymetrically at a defined rate. S. cerevisiae (baker's yeast) served as the experimental platform for this research because, relative to other experimental organsims and cell types, i) it is economical to grow and manipulate in the laboratory ; ii) it is highly amenable to the complementary approaches of genetic and biochemical analyses; and iii) many of the molecules that were studied in the context of specific sub-cellular processes are identical or nearly identical to those found in plants and animals, including humans, obviating the need for animal studies. Research completed under this award, applicable to human biology, animals, plants, and fungi, answered questions about of how cells transmit signals internally, how cells transport molecules from one end of a cell to another, and how cells know it is safe and appropriate to divide. Key peer-reviewed and published findings include i) the discovery that proteins known as palmitoyltransferases have a function necessary for asymmetric cell growth that is independent of their ability to add fatty acids to proteins; ii) the discovery that a family of proteins that bind sterols (e.g., cholesterol) do not require sterol binding for their function, dismissing the prevailing view that the primary function of these proteins is to transport sterols from one location in a cell to another in support asymmetric cell growth; and iii) discovery of the first part of a communication network in cells that link messages from the cell surface to the biochemical chekcpoint that tells a cell that it is safe and appropriate to divide. Broader impacts of this award included i) the training of college undergraduates and graduate students, especially under-represented minorities in science, in the fundamentals of research in molecular cell biology; ii) the expansion of research networks domestically and internationally, especially in Canada; and iii) the establishment of novel research links between biology and engineering at the University of Virginia.
