ABSTRACT
The cell wall of yeasts accounts for about 25-30% of the cell dry weight and thus represents a considerable investment of the cell in terms of metabolic energy (Fleet, 1991). It has several functions. Because of its mechanical strength it protects the cell against physical damage and plays a major role in morphogenesis. Other functions are related to the surface layer of glycoproteins, which is generally found in fungal walls (De Nobel et al., 2001b). The proteins of this layer are implanted in the underlying skeletal layer. They may be involved in sexual and nonsexual cell-cell recognition, in adhesion to host cells or to inert surfaces, they may confer hydrophobic properties to the wall, and they may limit the permeability of the cell wall to life-threatening molecules such as cell wall-degrading enzymes secreted by other organisms as defense proteins. They may also be involved in the retention of water and offer protection against dehydration. In the past few years, a more and more detailed molecular description of the cell wall of baker’s yeast has gradually emerged (Lipke and Ovalle, 1998; Kapteyn et al., 1999a; Cabib et al., 2001; Klis et al., 2002). This chapter presents its current status, discusses the biosynthesis and assembly of cell wall polymers, and extends this model to other ascomycetous fungi. Particular emphasis is given to the notion that cell wall formation is an intrinsic part of cell metabolism. Indeed, cell wall construction is tightly controlled both in space and in time, thus affecting cell shape, and the composition and molecular organization of the wall strongly depend on environmental conditions. For an earlier discussion of the yeast cell wall and its biosynthesis, the extensive review by Orlean (1997) is strongly recommended.
