Symmetry Breaking in the Life Cycle of the Budding Yeast

The budding yeast Saccharomyces cerevisiae has been an invaluable model system for the study of the establishment of cellular asymmetry and growth polarity in response to specific physiological cues. A large body of experimental observations has shown that yeast cells are able to break symmetry and establish polarity through two coupled and partially redundant intrinsic mechanisms, even in the absence of any pre-existing external asymmetry. One of these mechanisms is dependent upon interplay between the actin cytoskeleton and the Rho family GTPase Cdc42, whereas the other relies on a Cdc42 GTPase signaling network. Integral to these mechanisms appear to be positive feedback loops capable of amplifying small and stochastic asymmetries. Spatial cues, such as bud scars and phero-mone gradients, orient cell polarity bymodulating the regulation of the Cdc42GTPase cycle, thereby biasing the site of asymmetry amplification. The budding yeast Saccharomyces cerevisiaeis a gift of nature, not just for its superb ability in fermentation to provide us food for hunger and pastime, but also for its relatively simple physiology, which has illuminated ou