Mechanisms of polarized growth and organelle segregation in yeast

Cell polarity, as reflected by polarized growth and organelle segregation during cell division in yeast, appears to follow a simple hierarchy. On the basis of physical cues from previous cell cycles or stochastic processes, yeast cells select a site for bud emergence that also defines the axis of cell division. Once polarity is established, rho protein-based signal pathways set up a polarized cytoskeleton by activating localized formins to nucleate and assemble polarized actin cables. These serve as tracks for the transport of secretory vesicles, the segregation of the trans Golgi network, the vacuole, peroxisomes, endoplasmic reticulum, mRNAs for cell fate determination, and microtubules that orient the nucleus in preparation for mitosis, all by myosin-Vs encoded by the MYO2 and MYO4 genes. Most of the proteins participating in these processes in yeast are conserved throughout the kingdoms of life, so the emerging models are likely to be generally applicable. Indeed, several parallels to cellular organization in animals are evident.