Key Concepts of Section 21.4

Key Concepts of Section 21.4

Mechanisms of Cell Polarity and Asymmetric Cell Division

  • Cell polarity involves the asymmetric distribution of proteins, lipids, and other macromolecules in the cell.

  • Cells have an intrinsic program that can generate polarity using feedback loops.

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    A key regulator of the polarity program in many systems is the small GTP-binding protein Cdc42.

  • When a yeast cell buds, the intrinsic polarity program exploits feedback loops to concentrate Cdc42·GTP at a single site.

  • Asymmetry requires cells to sense a cue, respond to it by assembling a polarized cytoskeleton, and then using this polarity to distribute polarity factors appropriately.

  • Mating in haploid yeast involves assembly of a mating projection (shmoo) by polarization of the cytoskeleton in the direction of highest concentration of mating pheromone and targeting of secretion of cellular components for cell expansion there.

  • Anterior/posterior asymmetry in the first division of the C. elegans embryo involves asymmetric contraction of the actin-myosin network to localize the anterior Par3-Par6-aPKC complex to the anterior cortex followed by the association of posterior factors such as Par2 with the posterior cortex. The asymmetry of the anterior and posterior complexes is maintained by mutually antagonistic pathways.

  • Apical/basal epithelial-cell polarity is also driven by an apical Par3-Par6-aPKC complex, which functions in antagonistic relationships with the apical Crumbs complex and the basal Scribble complex.

  • Planar cell polarity regulates the orientation of epithelial cells in a sheet using a different set of antagonistic relationships.

  • Asymmetric cell division requires that cells first become polarized, then divide so as to segregate fate determinants asymmetrically.

  • Asymmetric division of stem cells often involves association of the stem cell with a niche, in which case the stem cell gives rise to another stem cell and a differentiating cell.

  • Asymmetric stem-cell division also involves the asymmetric distribution of the Par complex, which is retained in the stem cell during division, whereas fate determinants are localized away from the Par complex to end up in the differentiating cell.