6.3 Fundamentals of wall architecture
Electron microscope observations of fungal cell wall architecture in both filamentous fungi and yeast reveal a layered structure. The inner, most electron dense layer composed mainly of chitin and glucan, both of which form intrachain hydrogen bonds and assemble into microfibrils that form a basket-like framework just external to the plasma membrane. It makes eminent sense for the main skeletal element (the microfibrils) to be located in the innermost layer of the wall, as this is where turgor pressure is first exerted. This is covered by an outer layer composed of glycoproteins together with glucans, in filamentous fungi, or mannans, in yeasts (Fig. 2). The glycoprotein, glucan and chitin components are extensively crosslinked to create a complex three-dimensional network, which forms the structural basis of the cell wall. Some of the glycoproteins are bound to the cell through hydrogen bonds; some are attached covalently to the cell wall via disulphide-bonds; and some are covalently linked to the glucans of the cell wall.
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| Fig. 2. Diagrammatic depictions of the structure of fungal walls. A. Interpretation of the filamentous hyphal cell wall (modified from Bowman & Free, 2006). Most of the chitin (blue) is thought to be near to the plasma membrane where it resists the turgor pressure of the protoplast, β1,3-glucan (green) extends throughout the wall. Protein (shown in purple), glucan and chitin components are integrated into the wall by cross linking the components together with N- and O-linked oligosaccharides (yellow). Many of the glycoproteins have GPI anchors (red), which tether them to the plasma membrane; while other glycoproteins are secreted into the wall matrix. B. Generalised cell wall structure of Candida albicans; note that mannans (shown in black) dominate the outer regions of this yeast wall and β1,6-glucan (orange) cross links the components (modified from Odds, Brown & Gow, 2003). Also note that in liquid cultures the wall ‘blends into’ the surrounding aqueous medium because the surface polymers (polypeptide and polysaccharide) are hydrophilic and able to dissolve into the aqueous medium. Walls of aerial hyphae can be chemically modified at the surface by deposition of polyphenols and/or assembly of layers of proteins like hydrophobin (see Section 6.8). Similar diagrammatic representations of the walls of a range of fungal pathogens of humans are shown by Erwig & Gow (2016). |
Two different classes of glucan-linked cell wall glycoproteins have been described:
the ‘protein with internal repeats’ (PIR)-class,
the glycosylphosphatidylinositol (GPI) linked class, which are tethered to the plasma membrane through their (modified) GPI anchor.
GPI anchors and their association with the fungal wall are mentioned in the last paragraph of Section 5.14, Plasma membrane and signalling pathways, CLICK HERE to view the page.
Updated July, 2019
