The ability to polarize is a fundamental property of cells. The yeast Saccharomyces cerevisiae has proven to be a fertile ground for dissecting the molecular mechanisms that regulate cell polarity during growth. Here we discuss the signaling pathways that regulate polarity. In the second installment of this two-part commentary, which appears in the next issue of Journal of Cell Science, we discuss how the actin cytoskeleton responds to these signals and guides the polarity of essentially all events in the yeast cell cycle. During the cell cycle, yeast cells assume alternative states of polarized growth, which range from tightly focused apical growth to non-focused isotropic growth. RhoGTPases, and in particular Cdc42p, are essential to guiding this polarity. The distribution of Cdc42p at the cell cortex establishes cell polarity. Cyclin-dependent protein kinase, Ras, and heterotrimeric G proteins all modulate yeast cell polarity in part by altering the distribution of Cdc42p. In turn, Cdc42p generates feedback signals to these molecules in order to establish stable polarity states and coordinate cytoskeletal organization with the cell cycle. Given that many of these signaling pathways are present in both fungi and animals, they are probably ancient and conserved mechanisms for regulating polarity.
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