β-Linked disaccharides stimulate, but do not act as primer for,β(1–3)glucan synthase activity ofNeurospora crassa

α- and β-1,3-Glucan Synthesis and Remodeling

Glucans are characteristic and major constituents of fungal cell walls. Depending on the species, different glucan polysaccharides can be found. These differ in the linkage of the D-glucose monomers which can be either in α- or β-conformation and form 1,3, 1,4 or 1,6 O-glycosidic bonds. The linkages and polymer lengths define the physical properties of the glucan macromolecules, which may form a scaffold for other cell wall structures and influence the rigidity and elasticity of the wall. β-1,3-glucan is essential for the viability of many fungal pathogens. Therefore, the β-1,3-glucan synthase complex represents an excellent and primary target structure for antifungal drugs. Fungal cell wall β-glucan is also an important pathogen-associated molecular pattern (PAMP). To hide from innate immunity, many fungal pathogens depend on the synthesis of cell wall α-glucan, which functions as a stealth molecule to mask the β-glucans itself or links other masking structures to the cell wall. Here, we review the current knowledge about the biosynthetic machineries that synthesize β-1,3-glucan, β-1,6-glucan, and α-1,3-glucan. We summarize the discovery of the synthases, major regulatory traits, and the impact of glucan synthesis deficiencies on the fungal organisms. Despite all efforts, many aspects of glucan synthesis remain yet unresolved, keeping research directed toward cell wall biogenesis an exciting and continuously challenging topic.

Fungal beta(1,3)-D-glucan synthesis

The polysaccharide beta(1,3)-D-glucan is a component of the cell wall of many fungi. Synthesis of the linear polymer is catalysed by UDP-glucose beta(1,3)-D-glucan beta(3)-D-glucosyltransferase. Because this enzyme has a key role in fungal cell-wall synthesis, and because many organisms that are responsible for human mycoses, including Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans, produce walls that are rich in beta(1,3)-glucan, it has been and remains the focus of intensive study. From early characterization of the enzymatic activity in Saccharomyces cerevisiae, advances have been made in purification of the enzyme, identification of essential subunits and description of regulatory circuitry that controls expression and localization of different components of the multisubunit enzyme complex. Progress in each of these areas has been enhanced dramatically by the availability of specific inhibitors of the enzymatic reaction that produces beta(1,3)-glucan. These natural product inhibitors have utility both as tools to dissect the biology of beta(1,3)-glucan synthase and as sources for development of semisynthetic derivatives with clinical utility in treatment of human fungal disease. This review will focus on the biochemistry, genetics and regulation of the enzyme.