Two endoplasmic reticulum (ER) membrane proteins that facilitate ER-to-Golgi transport of glycosylphosphatidylinositol-anchored proteins

Many eukaryotic cell surface proteins are anchored in the lipid bilayer thr ough glycosylphosphatidylinositol (GPI). GPI anchors are covalently attache d in the endo plasmic reticulum (ER). The modified proteins are then transp orted through the secretory pathway to the cell surface. We have identified two genes in Saccharomyces cevevisiae, LAG1 and a novel gene termed DGT1 ( for "delayed GPI-anchored protein transport"), encoding structurally relate d proteins with multiple membrane-spanning domains. Both proteins are local ized to the ER, as demonstrated by immunofluorescence microscopy. Deletion of either gene caused no detectable phenotype, whereas lag1 Delta dgt1 Delt a cells displayed growth defects and a significant delay in ER-to-Golgi tra nsport of GPI-anchored proteins, suggesting that LAG1 and DGT1 encode funct ionally redundant or overlapping proteins. The rate of GPI anchor attachmen t was not affected, nor was the transport rate of several non-GPI-anchored proteins. Consistent with a role of Lag1p and Dgt1p in GPI-anchored protein transport, lag1 Delta dgt1 Delta cells deposit abnormal, multilayered cell walls. Both proteins have significant sequence similarity to TRAM, a mamma lian membrane protein thought to be involved in protein translocation acros s the ER membrane. In vivo translocation studies, however, did not detect a ny defects in protein translocation in lag1 Delta dgt1 Delta cells, suggest ing that neither yeast gene plays a role in this process. Instead, we propo se that Lag1p and Dgt1p facilitate efficient ER-to-Golgi transport of GPI-a nchored proteins.