Schizosaccharomyces pombe Rgf 3 p is a specific Rho 1 GEF that regulates cell wall β - glucan biosynthesis through the GTPase Rho 1 p

The fungal cell wall is the essential cellular boundary, controlling many transport processes, cellular metabolism and, indeed, all communications with the extracellular world. Because of its mechanical strength, it allows cells to withstand turgor pressure and consequently prevents cell lysis. In the fission yeast Schizosaccharomyces pombe, the cell wall mainly consists of three polysaccharides, β-1,3-glucan, α-1,3-glucan and galactomannoproteins, all of which form a large complex (for a review, see Duran and Perez, 2004). Their coordinated synthesis represents an essential step for the assembly of a functional cell wall to ensure cell integrity (Ishiguro, 1998). We have used the biosynthesis of β-1,3-glucan as a model to study morphogenesis. It has been suggested that β-1,3-glucan is the first polymer to be synthesized (Osumi et al., 1989; Roh et al., 2002) and that the regulation of this polysaccharide might be a key step in the sequential assembly of the other cell wall components. β-1,3-Glucan comprises ~45% of the cell wall and is the major structural component, as seen by the fact that its enzymatic degradation leads to the solubilization of the other components. The enzymatic system that catalyses the synthesis of this polysaccharide is β-1,3-glucan synthase (GS). GS is composed of at least two fractions: a catalytic moiety of the enzyme and a regulatory component.