Transglycosidases and Fungal Cell Wall β-(1,3)-Glucan Branching

Abstract

Fungal cells are endowed with a cell wall that plays a crucial role in the fungal life, by providing mechanical strength and protecting fungal cells from their environment. Chemically, this fungal cell wall consists of different polysaccharides, contributing to 80-95% of the cell wall dry mass. The core cell wall structure is made up of β-(1,6)-branched β-(1,3)-glucan linked to chitin, and is common to all fungal species. Branching leads to β-(1,3)-glucan ramificating, facilitating its cross-linking with chitin as well as other cell wall components resulting in the construction of a functional fungal cell wall. Recently, using Saccharomyces cerevsiae as the model, we showed that the dual activity associated with CAZy family GH72 transglycosidases (http://www.cazy.org/) belonging to the GAS-family, are capable of elongating as well as branching β-(1,3)-glucan, an essential phenomenon during cell wall biogenesis and remodeling. Not only GAS-proteins, but also GEL-family protein from Aspergillus fumigatus (a pathogenic fungus) showed β-(1,3)-glucan elongating-branching activity. Interestingly, this dual activity was shown by only those GH72 family members containing a Carbohydrate Binding Motif-43 (CBM), suggesting that branching activity is universal in the fungal kingdom. Disruption of β-(1,3)-glucan branching resulted either in a sick-phenotype or led to inviability, suggesting that β-(1,3)-glucan branching is an essential phenomenon during fungal cell wall biogenesis. In this commentary, future perspectives on our findings are presented.

Spent Brewer’s Yeast as a Source of Insoluble β-Glucans

“…Beta-glucan synthesis takes place in several cell regions. The process of glucan formation occurs in the plasma membrane and is catalyzed enzymatically by glucan synthase (UDP-glucose; 1,3-β-D-glucan 3-β-D-glucosyltransferase, EC 2.4.1.3) encoded by the FKS gene (1 and 2), which, by the extrusion, incorporates newly formed chains in the cell wall [ 50 ]. The order in which these compounds are found in the cell wall is: β-1,3-glucan → β-1,6-glucan → mannoproteins [ 51 ].…”