Sequential logic of polarity determination during the haploid-to-diploid transition in Saccharomyces cerevisiae

Roles for the canonical polarity machinery in the de novo establishment of polarity in budding yeast spores

The yeast Saccharomyces cerevisiae buds at sites predetermined by cortical landmarks deposited during prior budding. During mating between haploid cells in the lab, external pheromone cues override the cortical landmarks to drive polarization and cell fusion. By contrast, in haploid gametes (called spores) produced by meiosis, a predetermined polarity site drives initial polarized morphogenesis independent of mating partner location. Spore membranes are made de novo so existing cortical landmarks were unknown, as were the mechanisms by which the spore polarity site is made and how it works. We find that the landmark canonically required for distal budding, Bud8, stably marks the spore polarity site along with Bud5, a GEF for the GTPase Rsr1 that canonically links cortical landmarks to the conserved Cdc42 polarity machinery. Cdc42 and other GTPase regulators arrive at the site during its biogenesis, after spore membrane closure but apparently at the site where membrane synthesis began, and then these factors leave, pointing to the presence of discrete phases of maturation. Filamentous actin may be required for initial establishment of the site, but thereafter Bud8 accumulates independent of actin filaments. These results suggest a distinct polarization mechanism that may provide insights into gamete polarization in other organisms.

Roles for the canonical polarity machinery in the de novo establishment of polarity in budding yeast spores

The yeast Saccharomyces cerevisiae buds at sites pre-determined by cortical landmarks deposited during prior budding. During mating between haploid cells in the lab, external pheromone cues override the cortical landmarks to drive polarization and cell fusion. By contrast, in haploid gametes (called spores) produced by meiosis, a pre-determined polarity site drives initial polarized morphogenesis independent of mating partner location. Spore membranes are made de novo so existing cortical landmarks were unknown, as were the mechanisms by which the spore polarity site is made and how it works. We find that the landmark canonically required for distal budding, Bud8, stably marks the spore polarity site along with Bud5, a GEF for the GTPase Rsr1 that canonically links cortical landmarks to the conserved Cdc42 polarity machinery. Cdc42 and other GTPase regulators arrive at the site during its biogenesis, after spore membrane closure but apparently at the site where membrane synthesis began, and then these factors leave, pointing to the presence of discrete phases of maturation. Filamentous actin may be required for initial establishment of the site, but thereafter Bud8 accumulates independent of actin filaments. These results suggest a distinct polarization mechanism that may provide insights into gamete polarization in other organisms.

Cell cycle arrest of S. cerevisiae in conjunction with labeling of the cell wall

S. cerevisiae can be arrested in metaphase by depleting Cdc20. We describe (1) how to achieve this arrest and verify it, (2) how to label cell surface glycans covalently to distinguish mother from bud, and (3) how to detect the nucleolus and learn that it remains in the mother domain upon arrest. For complete details on the use and execution of this protocol, please refer to Tartakoff et al. (2021), Rai et al. (2017), and Zapanta Rinonos et al. (2014).

A zygote-based assay to evaluate intranuclear shuttling in S. cerevisiae

It is often necessary to learn whether macromolecules occupy a fixed place in cells. This protocol makes it possible to learn whether individual nucleolar proteins in S. cerevisiae remain in place or depart from and return to the nucleolus. The protocol uses early zygotes in which parental nucleoli are separate for at least one hour. The protocol demonstrates that the localization of many nucleolar proteins is in fact highly dynamic. Photobleaching is not required.

For complete details on the use and execution of this protocol, please refer to Tartakoff et al. (2021).

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The mobility of proteins within the yeast nucleus can be judged by constructing zygotes

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The assay requires minimal equipment other than a sensitive fluorescent microscope

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The protocol can be completed within one day once appropriate cells are available

Saccharomyces spores are born prepolarized to outgrow away from spore-spore connections and penetrate the ascus wall

How nonspore haploid Saccharomyces cells choose sites of budding and polarize towards pheromone signals in order to mate has been a subject of intense study. Unlike nonspore haploids, sibling spores produced via meiosis and sporulation by a diploid cell are physically interconnected and encased in a sac derived from the old cell wall of the diploid, called the ascus. Nonspore haploids bud adjacent to previous sites of budding, relying on stable cortical landmarks laid down during prior divisions, but because spore membranes are made de novo, it was assumed that, as is known for fission yeast, Saccharomyces spores break symmetry and polarize at random locations. Here, we show that this assumption is incorrect: Saccharomyces cerevisiae spores are born prepolarized to outgrow, prior to budding or mating, away from interspore bridges. Consequently, when spores bud within an intact ascus, their buds locally penetrate the ascus wall, and when they mate, the resulting zygotes adopt a unique morphology reflective of repolarization towards pheromone. Long-lived cortical foci containing the septin Cdc10 mark polarity sites, but the canonical bud site selection programme is dispensable for spore polarity, thus the origin and molecular composition of these landmarks remain unknown. These findings demand further investigation of previously overlooked mechanisms of polarity establishment and local cell wall digestion and highlight how a key step in the Saccharomyces life cycle has been historically neglected.

Cell biology of yeast zygotes, from genesis to budding

The zygote is the essential intermediate that allows interchange of nuclear, mitochondrial and cytosolic determinants between cells. Zygote formation in Saccharomyces cerevisiae is accomplished by mechanisms that are not characteristic of mitotic cells. These include shifting the axis of growth away from classical cortical landmarks, dramatically reorganizing the cell cortex, remodeling the cell wall in preparation for cell fusion, fusing with an adjacent partner, accomplishing nuclear fusion, orchestrating two steps of septin morphogenesis that account for a delay in fusion of mitochondria, and implementing new norms for bud site selection. This essay emphasizes the sequence of dependent relationships that account for this progression from cell encounters through zygote budding. It briefly summarizes classical studies of signal transduction and polarity specification and then focuses on downstream events.