Immobilization of the Glycosylphosphatidylinositol-anchored Gas1 protein into the chitin ring and septum is required for proper morphogenesis in yeast

Gas1p is a glucan-elongase that plays a crucial role in yeast morphogenesis. It is predominantly anchored to the plasma membrane through a glycosylphosphatidylinositol but a fraction was also found covalently bound to the cell wall. We have used fusions with the green or red fluorescent proteins (GFP or RFP) to determine its localization. Gas1p was present in microdomains of the plasma membrane, at the mother-bud neck and in the bud scars. By exploiting the instability of RFP-Gas1p we identified mobile and immobile pools of Gas1p. Moreover, in chs3 cells the chitin ring and the cross-linked Gas1p were missing but this unveiled an additional unexpected localization of Gas1p along the septum line in cells at cytokinesis. Localization of Gas1p was also perturbed in a chs2 mutant where a remedial septum is produced. Phenotypic analysis of cells expressing a fusion of Gas1p to a trans-membrane domain unmasked new roles of the cell wall-bound Gas1p in the maintenance of the bud neck size and in cell separation. We present evidence that the Crh1p and Crh2p are required for tethering Gas1p to the chitin ring and bud scar. These results reveal a new mechanism of protein immobilization at specific sites of the cell envelope