Dynamics of glucan-remodeling enzymes during developmental transitions in yeast and fungal pathogens

The cell wall is an essential exoskeleton of fungi and its composition and architecture are highly dynamic. The constituents - mannoproteins, chitin and glucans - are cross-linked and remodeled in the extracellular space by several transglycosylases. Among the best studied are members of the CRH family, responsible of the linkage of chitin to beta(1,3)- or beta(1,6)-glucan, and GAS family, required for the elongation of the beta(1,3)-glucan. These genes have homologs in all the fungal species so far examined and play a crucial role in fungal morphogenesis (1). The five ScGAS genes are differentially regulated during the yeast life cycle. The vegetative growth-specific isoforms (Gas1-Gas3 and Gas5) differ in several features (stability, pH optima, glycosylation degree) with respect to the prospore-specific isoforms (Gas2-Gas4) that are essential for the spore wall assembly (2, 3). Recently, we studied the localization of fluorescent versions of ScGas1p and CaPhr1, the Candida albicans homologous protein. PHR1 is expressed in neutral-alkaline conditions and is required for C. albicans morphogenesis and virulence. Gas1 and Phr1 proteins localize to the site of bud emergence, to the chitin ring at the mother-bud neck and all around the bud. Differently from ScGas1p, that is stabilized by cross-links to the wall of the chitin ring and remained in the bud scars even after generations, Phr1p-GFP did not persist indicating that it is not covalently bound to the wall of the bud scar. By use of mutants and inhibitors we could conclude that Gas1p and Phr1p are components of the secondary septa that are formed at cytokinesis after the deposition of the chitin disk by Chs2p in S. cerevisiae and Chs1p in C. albicans. This result is consistent with the reported function of elongation of beta(1,3)-glucans at the division septum (4). Further studies are underway to characterize the biochemical properties of the C. albicans PHR family (Phr1, Phr2, Phr3,Pga4 and Pga5) using a novel fluorescent transglycosylase assay. Acknowledgement We thank the University of Milano and the Slovak Grant Agency for Science (VEGA) for the financial support.