Symmetry breaking in the life cycle of the budding yeast

Abstract

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 pheromone gradients, orient cell polarity by modulating the regulation of the Cdc42 GTPase cycle, thereby biasing the site of asymmetry amplification.

Figure 1.

Symmetry breaking processes in the life cycle of budding yeast. Shown are the locations of actin patches, actin cables, and Cdc42 during polarized growth for both cycling cells and cells undergoing pheromone response. In G1 cells, Cdc42 is distributed symmetrically, and the actin cytoskeleton is not polarized. In response to cell cycle signals or mating pheromone stimulation, Cdc42 and the actin cytoskeleton become polarized: Cdc42 forms a “polar cap” and actin cables become oriented to allow for targeted secretion. Polarized growth further leads to formation of a bud (cell cycle signal) or formation of a mating projection (pheromone signal). Images represent GFP-Cdc42 (green), and rhodamine-phalloidin staining of filamentous actin (red).

Figure 2.

An intrinsic mechanism for symmetry breaking through an actin-dependent positive feedback loop. Initial stochastic accumulation of Cdc42-GTP triggers (directly or indirectly) nucleation of actin cables by formin family proteins. This in turn leads to transport of internal Cdc42 to the polarizing site, leading to further nucleation of actin cables.

Figure 3.

In the absence of actin, cells are able to polarize through a mechanism dependent upon the adaptor protein Bem1. (A) Binding domains and partners of Bem1, including Cdc24, Cdc42-GTP, and Cdc42 effectors. (B) A proposed signaling feedback loop that involves Bem1, where Cdc42-GTP recruits Bem1, which in turn recruits and/or activates Cdc24, leading to localized conversion of Cdc42-GDP to Cdc42-GTP.

Figure 4.

Representation of the axial budding pattern of haploid budding yeast and the bipolar budding pattern of diploid budding yeast. In haploid cells, the new bud forms next to the scar from the previous division, resulting in an axial budding pattern. In diploid cells, the first generation daughter buds in a distal position relative to the scar, whereas in mother cells, the new bud can be formed either at a proximal or distal pole relative to the bud scar.

Figure 5.

Molecular interactions in bud site selection in haploid and diploid yeast cells. Direct interactions between bud scar markers (patterned objects) and the Rsr1-GEF Bud5 is thought to link the Rsr1 GTPase cycle near the bud scar. Interactions between Rsr1-GTP and Rsr1-GDP with polarity regulators Bem1, Cdc24, and Cdc42 initiate amplification of the local cue through actin-dependent and Bem1-dependent feedback mechanisms, leading to formation of the polar cap in bud scar vicinity. T represents GTP, whereas D represents GDP.

Figure 6.

Molecular interactions during pheromone induced cell polarization. (A) Polarized growth is initiated toward the activated pheromone receptor through a complex signaling network. Cdc24 accumulates in the region of activated pheromone receptor (G_βγ) through interactions with the adaptor protein, Far1. The scaffold protein of the MAPK pathway, Ste5, binds both to G_βγ and Bem1, whereas Bem1 binds to Far1, further linking Cdc24 and Cdc42 to this site. Activation of Ste20, an upstream member of the MAPK pathway, is dependent on Cdc42, ensuring that MAPK activation and accumulation of Cdc42 occur at the same location. Activation of the kinase Fus3 plays a role in formin activation, which leads to an increase in nucleation of actin cables and possibly increased local transport of pheromone receptor and growth machinery. (B) Far1 plays a key role in determining if Cdc24 will localize to the presumptive bud site or the site of accumulated pheromone receptor. In cells undergoing pheromone response, a Far1-Cdc24 complex is exported from the nucleus. In the cytosol, Far1 plays a role in inhibition of CDK1, contributing to cell cycle arrest, whereas the Far1-Cdc24 complex localizes to the site of accumulation of pheromone receptor. In budding cells, Cdc24 is exported from the nucleus while Far1 undergoes proteolysis triggered by CDK1 phosphorylation. Cdc24 then reaches the plasma membrane likely because of interactions with Rsr1 and/or Bem1.