Cell biology of yeast zygotes, from genesis to budding

Abstract

The zygote is the essential intermediate that allows interchange of nuclear, mitochondrial and cytosolic determinants between cells. Zygote formation in S. 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 to zygote budding. It briefly summarizes classical studies of signal transduction and polarity specification and then focuses on downstream events.

Phosphorylated Gβ is a directional cue during yeast gradient tracking

“…8A), indicating no deficiency in Gb 3TA/SA signaling to the nucleus. Interestingly, a significant fraction of the ste4 3TA/SA cells lysed after shmooing, consistent with a defect in cell wall remodeling at the polarized growth site (Barral et al, 2000;Huberman et al, 2014;Tartakoff, 2015). To study the spatiotemporal dynamics of Gb 3TA/SA while assessing its effects on mating, we tagged ste4 3TA/SA with RFP in situ.…”

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

“…In these mating protocols, cell encounters, fusion and karyogamy are asynchronous, due to the variable proximity of potential mating partners and the fact that cell fusion can occur only when cells have reached the beginning of the cell cycle ( Tartakoff, 2015 , Gammie and Rose, 2002 ). Cells can be presynchronized, e.g., by exposure to α-factor which is then washed away when the two cell types are mixed.…”

“…According to the time of incubation prior to being applied to the slide, a variable fraction of the zygotes will have progressed through the classic stages of 1) shmoos with a polarized nucleus (in which the spindle pole body faces the shmoo tip and the nucleolus is at the opposite extremity), 2) prezygotes (in which a flat interface has been established between the apices of both cells), 3) early "linear" zygotes (in which the nuclei make symmetric contact at their apices), 4) later "linear" zygotes (in which nuclei have fused together but there still is no sign of bud emergence), and 5) mature zygotes (in which the nucleoli have merged together and there are single buds at various stages of growth). Zygotes bud repeatedly, as do haploid and diploid cells ( Stone et al., 2000 , Zapanta Rinonos et al., 2014 , Tartakoff, 2015 , Ydenberg and Rose, 2008 ).…”

“…For example, the remodeling of the cell surface that is intrinsic to establishment of cis-trans continuity during zygote formation requires control of cell wall integrity to ensure protection against osmotic stress. A variety of yeast mutants cannot exert this control and therefore are not suitable for efficient formation of normal zygotes ( Tartakoff, 2015 , Aguilar et al., 2007 , Philips and Herskowitz, 1997 ). Indeed, even crosses of wildtype cells show occasional evidence of incomplete fusion or what appears to be osmotic rupture.…”

Saccharomyces cerevisiae: A Unicellular Model Genetic Organism of Enduring Importance

“…As summarized in Figure 1B and reviewed by Tartakoff (Tartakoff, 2015), when haploid a and α cells are mixed, each senses the pheromone secreted by the opposite mating type, which triggers cell cycle arrest in G1. As summarized in Figure 1B and reviewed by Tartakoff (Tartakoff, 2015), when haploid a and α cells are mixed, each senses the pheromone secreted by the opposite mating type, which triggers cell cycle arrest in G1.…”