Genetic and Molecular Analysis of Differentiation Mechanismsin Saccharomyces cerevisae

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Myers 9604247 This project seeks to characterize mechanisms that specifically regulate cellular form and function in response to environmental stimuli. The ascomycete Saccharomyces cerevisiae can switch between the yeast form (YF) and the pseudohyphal form (PH) in response to defined culture conditions. Cellular characteristics that are affected in this differentiation include cell shape, cell size, cell polarity, and cell division cycle mechanisms. This project has identified two protein kinases, Elm1p and CDC28, in which mutations cause cells to adopt pseudohyphal form characteristics in all growth conditions. Elimination of Elm1p function causes cells to adopt the pseudohyphal form constitutively, indicating that this protein inhibits development of pseudohyphal characteristics in inappropriate environments. One or more forms of the cyclin-dependent kinase complex (CDK) in which CDC28 is the catalytic subunit also regulates yeast- pseudohyphal differentiation. A specific amino acid substitution in CDC28 causes constitutive execution of most aspects of the pseudohyphal form. These data are consistent with the hypothesis that various signaling processes impinge on CDC28, which in response, is functionally modified in a specific way. Modified CDK activity then causes various aspects of the pseudohyphal growth form to occur. In this way CDK may function to integrate various input signals from the environment and control the decision of whether or not to execute a certain differentiation process. This project will address the following specific objectives: 1) Determination of whether CDK alteration is required for cells to attain the pseudohyphal form in response to either of two signaling regimes known to induce this differentiation process in wild type cells; 2) Characterization of the functional differences between CDK in yeast form- and pseudohyphal form cells; 3) Characterization of the interaction between Elm1p and the evolutionarily conserved protein kinase cascade coded for by SEL2, SWE1, and CDC28; 4) Characterization of additional proteins that function in this differentiation circuitry. Completion of this research will advance our knowledge of the basic mechanisms by which genetically identical cells convert between different functional forms, and the mechanisms by which cell division is controlled. Cells with identical genetic information can take a wide variety of shapes and execute many different functions. Because multicellular animals and plants develop from a single cell, specific mechanisms must exist that cause the various descendants of the progenitor cell to adopt the specific forms and functions that arise reproducibly in mature organisms. Fundamental understanding of how multicellular organisms arise requires an understanding of these mechanisms, however, little is understood currently about the ways in which cells are instructed to adopt a specific shape. This project uses a tractable experimental system in which cell shape changes reproducibly in response to an external signal. By discerning the molecular mechanism that control this programmed shape change, fundamental information regarding the processes by which multicellular organisms develop is likely to be obtained.