We are interested in aspects of membrane biogenesis concerned with lipid synthesis and transport. Although the biological significance of protein modification by GPI remains enigmatic, emerging clues suggest that GPI-anchored proteins may be markers of membrane structural domains that are functionally important in intracellular membrane traffic and transmembrane signalling. We expect that these studies will contribute to an understanding of an area of membrane biogenesis that has generally been ignored, namely, questions concerning the compartmentation of lipid synthesis, the membrane topology of the lipid biosynthetic enzymes, and, importantly, how lipids are transported across and between bilayers. This glycolipid is modified and elaborated before being covalently attached to proteins containing a carboxyl-terminal GPI signal sequence. Glycero-phospholipids are synthesized on the cytoplasmic face of biogenic membranes and initially located in the cytoplasmic leaflet of the membrane bilayer.
Current models suggest that flip-flop is catalyzed by specific proteins termed flippases.
Following synthesis, sterols are distributed amongst cellular membranes, particularly the plasma membrane which is the major repository of sterols in the cell. The mechanisms by which sterols are transferred from the ER to the plasma membrane are unclear. Studies of cholesterol traffic in mammalian cells have led to the proposal of two distinct hypotheses, one suggesting that sterol traffic involves vesicle carriers and another suggesting that cytoplasmic lipoproteins are involved. We are investigating these hypotheses by studying ergosterol transport in yeast, taking advantage of the vast number of existing yeast strains that are defective in ergosterol biosynthesis and vesicular transport pathways.