Glycoconjugate structure and function in fungal cell walls

Complex cell walls distinguish fungi from animals, and the roles of these walls include osmotic support, selective permeability and interaction with the environment. Fungal walls consist of covalently cross-linked complexes of polysaccharides (usually β-glucans and chitin) with glycoproteins. Several different polysaccharides are covalently cross-linked through glycosidic bonds. They are also cross-linked to two types of glycoproteins (CWPs). The Pir-CWPs are multiply cross-linked through ester bonds between deamidated glutamine residues and hydroxyl groups in the major β-(1→3)-glucan. The GPI(-t)-CWPs are defined by their linkage through transglycosylation of the truncated glycosylphosphatidylinositol (GPI) glycan to β-(1→6)-glucan. These GPI(-t)-CWPs are highly N- and O-mannosylated and may also have ester-glucan cross-links like the Pir-CWPs. GPI(-t)-CWPs have a common modular structure, with an N-terminal secretion signal, then a well-folded functional enzymatic or ligand binding domain, a middle region of tandem repeats, a C-terminal serine- and threonine-rich highly glycosylated "stalk" and a C-terminal GPI addition signal. Their functional regions include enzymes active in cell wall biogenesis and maintenance and nutrient acquisition; transport functions; and cell adhesion proteins that mediate interactions between fungal cells, pathogen and host and in biofilms. Therefore, fungal walls have functions including those assumed by membrane, cytoskeleton and extracellular matrix in other organisms.