Cell Wall Cell Surface Morphology And Morphological Variation

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The Saccharomyces cell wall is about 200 nm thick and completely surrounds the cell. Its function is to preserve the osmotic integrity of the cell and define morphology but it has several other roles. Proteins involved in cell-cell recognition and adhesion, such as occurs during mating, are found in the cell wall. Other proteins are immobilized or retained in the periplasm, the space between the plasma membrane and the cell wall, by the cell wall. These are secreted proteins and localize specifically to this region. Finally, the cell wall may also be a partial permeability barrier. The cell wall is a very dynamic structure that is synthesized during bud growth but must also undergo remodeling during cell division, mating, and sporulation. For a thorough review of the cell wall and cell wall biogenesis the reader is referred to Orlean (1997).

CELL WALL COMPOSITION AND SYNTHESIS

The major component (80%-90%) of the cell wall is polysaccharide. This includes 3-glucans, mannoproteins, and chitin. /3-Glucan is a glucose homopolymer. In Saccharomyces, one finds (3-1,3 straight chains up to 1500 residues long with some (3-1,6 branches. These long polymers are intertwined to form microfibrils that are interwoven into the meshwork that makes the basic support structure of the cell wall, much like the steel rods in reinforced concrete. Imbedded into this meshwork are the mannoproteins or mannans. These are secreted proteins with large, highly branched, covalently bound carbohydrate side groups consisting mostly of mannose residues but also including glucose and N-acetylglucosamine residues. Some of these glycoproteins are also attached to lipids of the plasma membrane via a GPI anchor (glycosyl phosphatidylinositol) at the C-terminus of the protein. Cell wall proteins include the agglutinins and flocculins that play important roles in cell adhesion. Enzymes such as invertase, a heavily glycosylated protein, are found in the peri-plasmic space. Chitin is a homopolymer of /3-1,4-linked N-acetylglucosamine residues. It is a minor component of the cell wall (l%-2%) and is largely found at the site of bud formation and in bud scars. Many of the cell wall components are cross-linked to one another to form this very complex and interconnecting rigid structure. Figure 3.3 shows the organization of the Saccharomyces cell wall.

Synthesis of the major cell wall components takes place in the ER and Golgi, and transit to the cell surface is achieved via secretory vesicles. Glycosylation and mannosylation of cell wall proteins initiates in the ER and is completed in the Golgi. The same is true for the formation of the GPI anchor. Polymerization of (3-glucan initiates in the ER but continues in the Golgi and completes at the plasma membrane and involves some membrane-bound protein components of these compartments. Chitin synthesis is different. Chitin synthase is a membrane enzyme. It uses an intracellular precursor, UDP-N-acetylglucosamine, to synthesize extracellular chitin by some type of transmembrane process that is as yet not well understood. Since chitin is found in selected regions of the cell wall, chitin synthase must be active only in certain sites and is apparently regulated by the processes of bud site selection.

BUD SCARS, BIRTH SCARS, AND BUDDING PATTERNS

Bud scars are chitin-rich turtleneck-like raised rings that form at the site of bud formation and surround the bud neck. The bud scar remains on the cell wall surface on the mother cell even after the bud has detached. Since an individual cell can divide 25 or more times, the surface of a mature cell will be studded with multiple bud scars. The site on the daughter cell that had been the attachment to the mother is also visible on the surface and is called the birth scar. Both can be seen in Figure 3.1.

Budding patterns differ in haploid versus diploid cells. In haploid cells, both a-and Q-mating type, the new bud forms near the site of the previous bud scar on the

Saccharomyces Cell Wall Glucan

Mannoproteins

-Chitin

N-Glycosidic chain

O-Glycosidic chain

Periplasmic enzymes rfifffffffWfiffffifisTiimiWfffmfffffrf piasma luiunmuuiumnmuutumuuumu membrane

Figure 3.3 Composition and structure of the cell wall. The various components of the Saccharomyces cell wall and their complex intermolecular interactions to form a mesh-like organization are depicted. Taken from Schreuder et at. (1996) with permission from Elsevier Science mother and the birth scar on the daughter, as is shown in Figure 3.1 and depicted in Figure 3.5. This is referred to as an axial budding pattern. In a la diploid cells, the bud forms at or near either pole in the mother but only at the opposite pole in the daughter giving what is referred to as a bipolar budding pattern (Figure 3.4 and depicted in Figure 3.5). A third type of budding pattern, unipolar budding, is seen in cells undergoing pseudohyphal growth. Here the bud always forms in the mother at a site near the attachment of mother and daughter and always at the opposite pole in the daughter cell (Figure 3.4 and depicted in Figure 3.5). In budding yeast, the daughter cell (bud) is smaller than the mother cell at the time of cell separation. Therefore, before it can produce its own bud it must grow to a certain minimal size and does this by lengthening the time spent in G1 of the cell cycle. This G1 delay is evident from the finding that mother cells bud before daughter cells. This is indicated in Figure 3.5.

SCHMOO FORMATION AND MATING

In response to mating pheromone in the medium, haploid cells will arrest cell division in G1 and begin to form a protrusion on the side of highest pheromone concentration. This gives the cell a pearlike shape, jokingly called a schmoo after a defunct syndicated newspaper cartoon character, and the process is called schmoo-ing. The shmoo is seen in Figure 3.6 and one should note that schmoos are unbudded. Schmoos of opposite mating type will attach to one another at the schmoo tip, a region of the cell wall containing a high concentration of agglutinin proteins. As mating progresses, the cell wall breaks down in the region of cell-cell contact and the plasma membranes and the cytoplasms of the two cells fuse (reviewed in Marsh & Rose, 1997). This is followed by fusion of the two nuclei to form the diploid nucleus that then migrates into a new bud produced at the junction of the two cells, as shown in Figure 3.7. Production of this diploid bud does not involve DNA replication or chromosome segregation.

BUD SITE SELECTION AND POLARIZED CELL GROWTH

As can be inferred from the above description, budding patterns and changes in cell morphology such as mating and pseudohyphal differentiation are highly regulated processes. Each requires polarized growth, i.e. preferential growth at a defined position on the cell surface. The site of growth must be selected and materials such as cell wall components and membrane lipids must be directed to the site to allow the growth to occur (reviewed in Madden & Snyder, 1998, and Johnson, 1999).

Axial and bipolar budding are controlled by two separate pathways. Interestingly, bipolar budding appears to be the default pathway because haploid strains carrying mutations in axial budding genes undergo bipolar budding. The function of these proteins is to mark the incipient growth site; they are sometimes referred to as the cytokinesis tag. The 10 nm neck filament system is the cytokinesis tag required for axial budding. It consists of several septin proteins and other proteins including Bud3p, Bud4p, Axllp, and Axl2p. Genetic analysis has identified several components required for bipolar budding but the process is more complex, particularly because it differs in the mother and the bud. It is suggested that the G1 delay in oo

Cerevisiae Bud Diploid

Figure 3.4 Bud scars in diploid cells. SEMs of diploid cells growing in the yeast form (A) and in the pseudohyphal form (B) is used to demonstrate the different patterns of bud site selection. The cell in panel A is undergoing an axial pattern of bud site selection. It is producing its fourth bud at a site in the proximal pole (adjacent to the birth scar). A bud scar is visible in a proximal pole location and two other bud scars are positioned in the distal pole. The cell in panel B has the elongated shape typical of pseudohyphal cells and bud site selection is unipolar. A birth scar is apparent at one end of the cell (to the left) and two bud scars can be seen at the opposite end. Reprinted from Gimeno et al. (1992) with permission from Elsevier Science

Figure 3.4 Bud scars in diploid cells. SEMs of diploid cells growing in the yeast form (A) and in the pseudohyphal form (B) is used to demonstrate the different patterns of bud site selection. The cell in panel A is undergoing an axial pattern of bud site selection. It is producing its fourth bud at a site in the proximal pole (adjacent to the birth scar). A bud scar is visible in a proximal pole location and two other bud scars are positioned in the distal pole. The cell in panel B has the elongated shape typical of pseudohyphal cells and bud site selection is unipolar. A birth scar is apparent at one end of the cell (to the left) and two bud scars can be seen at the opposite end. Reprinted from Gimeno et al. (1992) with permission from Elsevier Science

Axial

Axial

Haploid Saccharomyces Birth Scars

Figure 3.5 Bud site selection patterns in Saccharomyces. The three types of bud site selection patterns observed in Saccharomyces are depicted. Yeast form cells bud by an axial (haploid cells) or a bipolar (diploid cells) pattern. Cells growing in the pseudohyphal form have a unipolar budding pattern. The numbers indicate the order of formation of the particular cell

Figure 3.5 Bud site selection patterns in Saccharomyces. The three types of bud site selection patterns observed in Saccharomyces are depicted. Yeast form cells bud by an axial (haploid cells) or a bipolar (diploid cells) pattern. Cells growing in the pseudohyphal form have a unipolar budding pattern. The numbers indicate the order of formation of the particular cell a

Figure 3.6 The schmoo morphology. Nomarski optics is used to demonstrate the characteristic pear-shaped schmoo morphology formed by haploid cells exposed to the peptide pheromone of the opposite mating type. The cells shown are a-mating type and have been exposed to a-factor, the peptide pheromone produced by cells of the a-mating type. The cells carry another genetic alteration that enhances the effect of pheromone treatment (MAT» barl). Taken from Sprague (1991). Reproduced with permission from Academic Press a-factor

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Responses

  • Crispina
    Do the cell walls of saccharomyces cerevisiae contain chitin?
    7 years ago
  • Idris
    How is the structure of the cell wall of the saccharomyces cerevisiae define its function?
    6 years ago

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