6.5 The glucan component

Although chitin provides the crucial mechanical strength of the wall, glucans make up 50–60% of the dry weight of the filamentous fungal cell wall, and between 65% and 90% of the cell wall glucan is the β-1,3-glucan. Glucans with other chemical linkages between their repeating glucose residues have been found in various fungal cell walls, including β-1,6, mixed β-1,3 with β-1,4, α-1,3, and α-1,4-linked glucans. However, the β-1,3-glucan serves as the main structural element to which other cell wall components are covalently attached. As a result, the synthesis of β-1,3-glucan is required for cell wall formation and normal development of fungi (Latgé et al., 2005; Bowman & Free, 2006; Lesage & Bussey, 2006).

As with chitin, glucan polymers are formed by multisubunit enzyme complexes in the plasma membrane, the polymer being extruded into the extracellular space through a vectorial synthesis. The linear polymer is synthesised by the enzyme complex known as β(1,3)-glucan synthase (Douglas, 2001; Latgé et al., 2005). The polysaccharide polymer produced ranges up to 1500 glucose residues long; it becomes insoluble as the degree of polymerisation increases, and enzyme remains attached to the nascent glucan. Synthesis is vectorial, the new glucan chains being extruded into the extracellular (periplasmic) space immediately adjacent to the plasma membrane. This promotes their integration into the cell wall at points of active cell wall synthesis. Glucan synthase complexes, like the chitin synthases, are mainly localised to regions of active extension growth, budding, branching or septation. The carbon-6 positions of approximately 40-50 of the grand total of 1500 glucose residues in a glucan polymer have additional β-1,3-glucans linked to them to generate the β(1,6)-linked branched structure of the mature wall glucan.

A good range of β-1,3-glucan synthase enzymes have been studied: including those from the yeasts Saccharomyces cerevisiae, Schizosaccharomyces pombe, Candida albicans, filamentous Ascomycota Neurospora crassa, Aspergillus nidulans  and A. fumigatus, the pathogenic basidiomycete Cryptococcus neoformans, and even the oomycete Phytophthora. All these enzymes are membrane-bound complexes that use UDP-glucose as a substrate and produce a linear polysaccharide. The reaction is processive, meaning that for every molecule of UDP-glucose hydrolysed, one molecule of glucose is added to the length of the polymer chain.

Genes encoding β-1,3-glucan synthase proteins were first identified in Saccharomyces cerevisiae, which contains two catalytic subunits and one regulatory protein. The S. cerevisiae catalytic proteins are encoded by genes FKS1 and FKS2, both of which are essential for normal wall formation. Disruption of either the FKS1 or FKS2 gene results in mutants with slow growth rate and cell wall defects; simultaneous deletion of both genes is lethal. Clearly, the catalytic subunits have overlapping functions but activity of both is required and β-1,3-glucan is essential for yeast survival. The regulatory protein is a GTPase subunit called Rho1; this is also essential for survival.  The FKS and RHO1 genes are highly conserved in other fungi. A. fumigatus and N. crassa genomes each contain one catalytic subunit gene and one gene encoding the GTPase regulatory subunit; both genes are required for cell viability (Latgé et al., 2005).

Updated December 17, 2016