Tubulin and Actin Topology during Zygote Formation of Saccharomyces cerevisiae

Abstract

The topology of tubulin and actin during mating of Saccharomyces cerevisiae was analysed by fluorescence microscopy with the monoclonal anti-tubulin antibody Tu01 and rhodamine-labelled phalloidin. Preconjugatory cells displayed an asymmetric distribution of the microtubule and actin cytoskeleton and an overall polarization of the cells preceding cell fusion. Prior to karyogamy, the haploid spindle pole bodies were associated with abundant cytoplasmic microtubules. Budding zygotes revealed the same tubulin and actin patterns as vegetative cells. Treatment of the mating mixture with the microtubule inhibitor nocodazole (10 micrograms ml-1) did not prevent polarization and fusion of haploids, zygote formation and emergence of the first zygotic bud. In marked contrast, the migration of the nucleus in preconjugatory cells as well as nuclear migration and fusion within the zygotes was unequivocally blocked by the action of the drug. It is suggested that the problem of the morphogenesis of mating should be approached by considering interactions at the cell periphery.

Pea2 protein of yeast is localized to sites of polarized growth and is required for efficient mating and bipolar budding.

“…During mating, each haploid cell polarizes towards its mating partner: the actin cytoskeleton, secretion, and insertion of new cell wall material are all organized towards a single site on the cell surface (Byers, 1981;Ford and Pringle, 1986;Hasek et al, 1987;Tkacz and MacKay, 1979). Polarized morphogenesis is also seen in a cells in a uniform field of pheromone (Lipke et al, 1976;Tkacz and MacKay, 1979;Field and Schekman, 1980), but instead of polarizing towards a mating partner, the cell polarizes adjacent to the previous bud site (Madden and Snyder, 1992;Valtz et al, 1995).…”

“…During mating, cells ignore the polarity information at the presumptive bud site and polarize instead towards their mating partner (Madden and Snyder, 1992;Valtz et al, 1995): the actin and microtubule cytoskeletons as well as secretion and new cell surface growth are oriented towards the partner (Byers, 1981;Ford and Pringle, 1986;Hasek et al, 1987;Tkacz and MacKay, 1979). In this instance, the site for polarization is determined by an external signal, a gradient of pheromone secreted by the mating partner (Jackson and Hartwell 1990a,b;Segall, 1993;Dorer et al, 1995;Valtz et al, 1995).…”

Cell polarization directed by extracellular cues in yeast.

“…Secretion and new cell-surface growth are concentrated in the direction of the mating partner (Tkacz and MacKay, 1979). The actin and microtubule cytoskeletons also polarize toward the mating partner (Byers, 1981;Ford and Pringle, 1986;Hasek et al, 1987). This response facilitates efficient cell and nuclear fusion, resulting in a diploid a/a cell (reviewed in Cross et al, 1988) (see Figure 1A).…”

“…When two yeast cells of opposite mating type (a and a) come into contact, they grow toward each other in a polarized fashion (Byers and Goetsch, 1975;Tkacz and MacKay, 1979;Field and Schekman, 1980;Hasek et al, 1987). Secretion and new cell-surface growth are concentrated in the direction of the mating partner (Tkacz and MacKay, 1979).…”

“…Many molecules with roles in mating localize to the shmoo tip, including a-agglutinin (Watzele et al, 1988), Fuslp (Trueheart et al, 1987), Spa2p (Gehrung andSnyder, 1990), Ste2p (Jackson et al, 1991), Fus2p (Elion, personal communication), and Ste6p (Kuchler et al, 1993). Most organelles, including the nucleus, accumulate on the side of the cell where the projection forms (Byers and Goetsch, 1975;Tkacz and MacKay, 1979;Hasek et al, 1987;Rose and Fink, 1987;Baba et al, 1989;Gehrung and Snyder, 1990). The cytoskeleton is oriented in a polarized manner in a shmoo (Barnes et al, 1990;Read et al, 1992) ( Figure 1B).…”