Establishment of Cell Polarity in Yeast

The establishment of cell polarity is a central feature of morphogenesis in many types of cells (Schnepf 1986; Horvitz and Herskowitz 1992; Rodriguez-Boulan and Nelson 1993; Shapiro 1993; Priess 1994). Polarity estab-lishment involves selection of an axis of polarization followed by the asymmetric organization of cytoskele-tal elements, membranous organdíes, components of the plasma membrane, and components of the ex-tracellular matrix or cell wall along this axis. In bud-ding yeasts such as Saccharomyces cerevisiae, cell polarization is vividly manifested during the vegetative cell cycle by the appearance and selective growth of the bud, which depends on the highly polarized move-ment of secretory vesicles carrying new cell-surface material, and perhaps of the Golgi cisternae that gen-erate such vesicles (Preuss et al. 1992), to the bud site and into the growing bud. This movement appears to depend primarily on the actin/myosin system (Bretscher et al. 1994; Welch et al. 1994; Govindan et al. 1995), but other cytoskeletal elements such as the cytoplasmic microtubules and the septin-containing neck filaments also polarize before bud emergenc