Glucan remodelling enzymes : expression, localization and replacement during morphological transitions in fungi

The formation of a core beta(1,3)-glucan is crucial for cell wall functions in fungi. The glucan network results from a vast array of biosynthetic, degradative and remodeling activities. Enzymes classified as GH72 in the CaZy database are glucanosyltransferases that cleave and relegate beta(1,3)-glucan chains. The elongation of branching points in glucan chains creates anchoring sites for cell wall components and is crucial for the maintenance of cell integrity. In general, these enzymes are plasma membrane GPI-anchored proteins but can be also covalently linked to the cell wall. A phylogenetic analysis indicates that GH72 enzymes are uniquely present in the Kingdom Fungi and form redundant families of orthologous proteins suggesting specialized roles among various species. The best characterized enzymes belong to the Gel/Gas/Phr families of Aspergillus fumigatus (7 members), Saccharomyces cerevisiae (5) and Candida albicans (5), respectively. ScGas1p-Gas5p are specialized in cell wall remodelling whereas Gas2p-Gas4p are essential for the formation of the spore wall, a process that takes place in the citoplasm of a diploid cell. ScGas family is regulated at the level of transcription, protein stability and localization. Recently, using a new enzyme assay, we found that optimum pH for activity is different for Gas1p-Gas5p compared to Gas2p-Gas4p. This suggests that different isoforms are required in order to function optimally at the interface between the cell and the extracellular space or the spore surface and the cytosol. We also studied PHR1 and PHR2, two-pH responsive genes of C.albicans. Phr1p-GFP concentrates at the bud periphery, hyphal apex and septum indicating that Phr1p activity is required at sites of active wall synthesis. The effects of defective septum formation on Phr1-GFP localization were studied in detail. The role of Phr1p in septum formation and in the biology of the human fungal pathogen C. albicans is currently under investigation.