SUMMARY PARAGRAPH for GAS1
GAS1 encodes a cell wall-bound 1,3-beta-glucanosyltransferase involved in the formation and maintenance of 1,3-beta-glucan, which is the major polysaccharide of the cell wall (reviewed in 6). After 1,3-beta-glucan is synthesized by the plasma membrane 1,3-beta-glucan synthase complex, Gas1p is believed to provide the activity required to elongate and rearrange its side chains. Those chains are subsequently cross-linked with chitin, 1,6-beta-glucan and proteins to form the mesh-like structure of the main layer of the cell wall (7, 8). Consistent with the role of Gas1p in cell wall biogenesis, a gas1 null mutation leads to phenotypes characteristic of a weakened cell wall: cells are abnormally round, have reduced viability, and display increased sensitivity to cell wall-affecting drugs and elevated temperatures (9, 10).
The Gas1p precursor is modified by removal of the C-terminal hydrophobic domain and covalent attachment of the glycosylphosphatidylinositol (GPI) moiety (11, 1). The GPI serves as a membrane anchor and is essential for transport of the protein through the ER and Golgi towards the cell surface. As it moves through the secretory pathway, Gas1p also receives O-linked and N-linked glycosylation (12). The final destination of Gas1p had long been thought to be the plasma membrane, where Gas1p would remain attached to the bilayer via the intact GPI anchor (1). Recent data, however, indicate that Gas1p is covalently attached to the cell wall glucan via a GPI remnant at its C terminus (13, 14). As the most abundant and easily detectable GPI-anchored protein, Gas1p has been widely used as a marker in studies of GPI anchoring (15), as well as ER to Golgi vesicle-mediated transport (16), roles of lipid rafts (17), and protein glycosylation (18).
GAS1, GAS2, GAS3, GAS4, and GAS5 are members of the glycosidase/transglycosidase GH72 family of fungal enzymes involved in cell wall maintenance. They share significant similarity with Aspergillus fumigatus GEL1 and GEL2, and with Candida albicans PHR1 and PHR2. Similar to the most extensively characterized member, Gas1p, the remaining GAS proteins are thought to be cell wall-bound 1,3-beta-glucanosyltransferases involved in cell wall assembly and maintenance (8, 19). Based on their expression patterns, they appear to play partially overlapping roles throughout the development: GAS1 and GAS5 are expressed during vegetative growth, whereas GAS2 and GAS4 are expressed exclusively during sporulation and required for normal spore wall formation (20).
Last updated: 2007-06-01