SUPPRESSOR GENE ANALYSIS REVEALS AN ESSENTIAL ROLE FOR SPHINGOLIPIDS IN TRANSPORT OF GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED PROTEINS IN SACCHAROMYCES-CEREVISIAE

Sphingolipids are normally necessary for growth of Saccharomyces cerev isiae cells, but mutant strains that bypass the need for sphingolipids have been identified. Such bypass mutants fail to grow under stressfu l conditions, including low pH (pH 4.1), when they lack sphingolipids. To begin to understand why sphingolipids seem to be necessary for cop ing with low-pH stress, we screened a genomic library and selected a s uppressor gene, CWP2 (cell wall protein 2), that when present in multi ple copies partially compensates for the lack of sphingolipids and enh ances survival at low pH. To explain these results, we present evidenc e that sphingolipids are required for a normal rate of transport of gl ycosylphosphatidylinositol (GPI)-anchored proteins, including Cwp2 and Gas1/Gpg1, from the endoplasmic reticulum (ER) to the Golgi apparatus . The effect of sphingolipids is specific for transport of GPI-anchore d proteins because no effect on the rate of transport of carboxypeptid ase Y, a non-GPI-anchored protein, was observed. Since the Gas1 protei n accumulated in the ER with a GPI anchor in cells lacking sphingolipi ds, we conclude that sphingolipids are not necessary for anchor attach ment. Therefore, sphingolipids must be necessary for a step in formati on of COPII vesicles or for their transport to the Golgi apparatus. Ou r data identify the Cwp2 protein asa vital component in protecting cel ls from the stress of low pH.