Keith G. Kozminski

Human development, immunity, wound healing, fertility, and nutrient uptake all depend upon the ability of cells to change shape. Acquisition of an asymmetric or polarized morphology by a cell is especially critical to these biological processes. Mutations that disrupt the polarity of cells will lead to diseased tissues, tumors, and, in many cases, congenital defects. Polarized cell growth, that is cellular growth biased toward one pole of a cell, is the result of dynamic developmental processes that entail an extensive reorganization of the cytoplasm in response to both intracellular and extracellular signals. Understanding how a cell integrates spatial and temporal signals to effect the organization of the cytoskeleton and secretory apparatus, in preparation for polarized cell growth, is a long-term goal of the laboratory. Current research in the lab primarily focuses on the role of small G proteins in signal transduction during polarized cell growth. In particular, we are interested in how the Rho-family GTPase Cdc42p promotes protrusion of the cell cortex. In mammalian cells, the protrusions form filopodia and are essential for cell movement during processes such as neuronal migration, wound healing, and immune responses. In some fungi, these protrusions produce a daughter cell or bud. In each of these examples, Cdc42p asymmetrically organizes the actin cytoskeleton and, in turn, the secretory apparatus, prior to polarized growth. Thus, Cdc42p is a key regulator of polarized cell growth. Interestingly, to function properly, Cdc42p itself must acquire an asymmetric distribution on the cell cortex. This observation presents an important question: How does a protein that triggers the development of cellular asymmetry become asymmetrically distributed in the first place and remain asymmetrically distributed? To address this question, the lab has turned to the budding yeast S. cerevisiae (baker's yeast) as an experimental model for Cdc42p-dependent cell polarization. Budding yeast offers many experimental advantages. Among these are the amenability of this organism to classical genetics, molecular genetics, high throughput genomic/proteomic analysis, cell biology, and biochemistry. In addition, and very importantly, polarized cell growth and Cdc42p function in yeast is very similar to that found in mammalian cells. Thus, a less complex eukaryote such as yeast is being used to decipher how more complex eukaryotic cells (i.e., human) function. Genetic analyses revealed that an asymmetric distribution of Cdc42p on the cell cortex depends upon the function of oxysterol binding proteins (OSBPs), a family of proteins conserved among eukaryotes. In mammalian cells, prototypical OSBP binds cholesterol and its derivative, oxysterol; in yeast most, but not all, OSBP homologues bind ergosterol, the "fungal cholesterol." Although it is known that OSBPs are important in intracellular sterol transport and intracellular signalling, it is not known how these proteins or the sterol composition of the plasma membrane affects the asymmetric distribution of Cdc42p or Cdc42p activity. Using OSBP mutants, the lab is answering these questions and identifying the links between small G proteins and sterols in the process of polarized cell growth. A related project in the lab is directed at deciphering how cells regulate polarized growth in coordination with the cell cycle. Yeast lacking the paralogous genes ZDS1 and ZDS2, which appear conserved among the Fungi, display hyperpolarized bud growth, apparently in response to the activation of a Swe1p kinase-dependent cell cycle checkpoint. How the Zds proteins regulate polarized growth via this checkpoint remains unknown and is currently under investigation.