Lipid rafts function in biosynthetic delivery of proteins to the cell surface in yeast

Lipid rafts, formed by lateral association of sphingolipids and cholesterol , have been implicated in membrane traffic and cell signaling in mammalian cells. Sphingolipids also have been shown to play a role in protein sorting in yeast. Therefore, we wanted to investigate whether lipid rafts exist in yeast and whether these membrane microdomains have an analogous function t o their mammalian counterparts. We first developed a protocol for isolating detergent-insoluble glycolipid-enriched complexes (DIGs) from yeast cells. Sequencing of the major protein components of the isolated DIGs by mass sp ectrometry allowed us to identify, among others, Gas1p, Pma1p, and Nce2p. U sing lipid biosynthetic mutants we could demonstrate that conditions that i mpair the synthesis of sphingolipids and ergosterol also disrupt raft assoc iation of Gas1p and Pma1p but not the secretion of acid phosphatase. That e ndoplasmic reticulum (ER)-to-Golgi transport of Gas1p is blocked in the sph ingolipid mutant lcb1-100 raised the question of whether proteins associate with lipid rafts in the ER or later as shown in mammalian cells. Using the sec18-1 mutant we found that DIGs are present already in the ER Taken toge ther, our results suggest that lipid rafts are involved in the biosynthetic delivery of proteins to the yeast plasma membrane.