Nuclear congression

Nuclear fusion

FUS1 FUS2 FUS3 FUSS FUS6 FUS7 FUS8

KAR1 KAR3KAR4 KAR9 CIK1 BIK1 TUB2

KAR1 KAR3KAR4 KAR9 CIK1 BIK1 TUB2

KAR2 KAR5 KAR7/SEC71 KAR8 SEC63 SEC72

Figure 3.7 Mating and nuclear fusion in Saccharomyces. The pathway of events followed by the nuclei of mating cells and the zygote is depicted. The spindle pole bodies with their attached cytoplasmic microtubules orient toward each other and toward the schmoo projection, which has formed at this position in response to the higher concentration of pheromone. Fusion of the cytoplasms allows the cytoplasmic microtubules to make contact and to draw the two nuclei together. This is followed by nuclear fusion and the migration of the zygote nucleus into the newly forming bud by the cytoplasmic microtubules. Several of the genes involved in this process are listed. Taken from Rose (1996), with permission from Annual Reviews

KAR2 KAR5 KAR7/SEC71 KAR8 SEC63 SEC72

Figure 3.7 Mating and nuclear fusion in Saccharomyces. The pathway of events followed by the nuclei of mating cells and the zygote is depicted. The spindle pole bodies with their attached cytoplasmic microtubules orient toward each other and toward the schmoo projection, which has formed at this position in response to the higher concentration of pheromone. Fusion of the cytoplasms allows the cytoplasmic microtubules to make contact and to draw the two nuclei together. This is followed by nuclear fusion and the migration of the zygote nucleus into the newly forming bud by the cytoplasmic microtubules. Several of the genes involved in this process are listed. Taken from Rose (1996), with permission from Annual Reviews daughter cells is a significant factor in this differing response. Little is reported about bud site selection in unipolar budding.

Following the establishment of the position of the growth site, the general bud site selection proteins, Rsrlp, Bud2p, and Bud5p, position themselves at the site. Rsrlp is a Ras-related GTPase. These proteins allow for assembly of the polarity-

establishment proteins that include Cdc42p, a Rac/Rho-type GTPase, Cdc24p, a GTP exchange factor, and Bemlp. Several other proteins, including kinases, are in this complex and it is this complex that interacts with the actin cytoskeleton.

The actin cytoskeleton plays a major role in all aspects of polarized cell growth in Saccharomyces, whether bud growth, pseudohyphal differentiation, or formation of a mating projection (reviewed in Botstein et al., 1997; Madden & Snyder, 1998; Johnson, 1999). During budding, cortical actin patches (see below) with attached actin cables localize first to a ring surrounding the incipient site of bud emergence. Then they move to the bud tip before becoming evenly distributed throughout the bud, and finally positioning at the bud neck for cytokinesis. Secretory vesicles (see below) are directed via the actin cytoskeleton to these sites to allow the distribution of materials required for growth of the plasma membrane, elaboration of the cell wall, and chitin synthesis where appropriate. Thus, initially and in small buds growth is limited to the bud tip (apical growth) and in larger buds cell wall growth is more uniform (isotropic growth).

The mating projection is formed at the site of highest concentration of mating pheromone. Pheromone binding to the receptor stimulates the MAP kinase pheromone response pathway and also stimulates the binding of the polarity-establishment proteins (Cdc42p, Cdc24p, and Bemlp) to the site of the activated receptors. This then allows for the redistribution of the actin cytoskeleton to the region and provides the materials required for remodeling of the cell wall.

It is important to note that all of these processes of polarized cell growth must be tightly coupled with the cell cycle. Bud site selection occurs in Gl. Prior to the formation of a mating projection, cells are arrested in Gl and bud site selection is repressed elsewhere.

SPORE FORMATION

The end product of Saccharomyces meiosis is an ascus containing four haploid ascospores, the four meiotic products. Meiosis results in a four-lobed nucleus, each lobe having a haploid set of chromosomes and a spindle pole body. When sporulation is complete, the four lobes are separated into four distinct haploid spore nuclei. Spore wall synthesis initiates as a thickening of the outer 'cytoplasmic' plaque of the spindle pole body (see below) and grows out from this site. The mature spore wall completely surrounds the contents of the nucleus and a small amount of cytoplasm and protects the contents from desiccation and other stresses (reviewed in Kupiec et al., 1997). The ascus is approximately the size of the diploid cell from which it developed and the four ascospores are so tightly packed that they appear to form a pyramid.

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