The bi-directional translocation of lipids from one side of a biological membrane to the other is termed flip-flop. Lipid flip-flop across the endoplasmic reticulum (ER) membrane is required for protein N-glycosylation and GPI-anchoring. These protein modifications are essential in eukaryotes;for example, their genetic abrogation causes embryonic lethality in mammals and renders yeast unviable. Lipid flip-flop across the ER is also required for membrane biogenesis: phospholipids that are synthesized on the cytoplasmic face of the ER must be translocated to the opposite face to enable the membrane bilayer to grow uniformly. The demand for lipid flip-flop at the ER is likely to be exceptionally high when the ER membrane expands and glycoprotein secretion increases;this occurs, for example, during the differentiation of B-lymphocytes to antibody-secreting plasma B cells. Unassisted flip-flop is extremely slow because of the energy barrier to taking the polar lipid head group through the hydrophobic interior of the membrane, yet lipids flip-flop rapidly across the ER membrane on a time-scale of seconds. This is because the ER possesses specific transport proteins (flippases) that accelerate lipid flipping to a physiologically sufficient rate. Lipid flipping in the ER occurs by an ATP-independent mechanism in which the flippases facilitate 'downhill'transport of lipids;this distinguishes ER flippases from other translocators, typically found in the eukaryotic plasma membrane, that couple ATP hydrolysis to concentrative 'uphill'transport of lipids. We estimate that there are as many as six different ER lipid flippases but none of these have been identified at the molecular level. We developed biochemical reconstitution systems that recapitulate the activity of three of the flippases required for ER membrane bilayer expansion and protein glycosylation. These flippases specifically translocate glycerophospholipids, oligosaccharide diphosphate dolichols and mannose-phosphate dolichol.
Our aim i s to identify these physiologically important translocators with the long-term goal of understanding their mechanism of action. We propose to do this via a two-pronged approach involving protein purification and mass spectrometry on the one hand, and screening of systematic collections of yeast ER membrane proteins on the other. Our purification efforts will be aided by the use of novel affinity matrices. We will also use partially purified flippase preparations to continue our efforts to define the specificity of these proteins. Our published work and preliminary data put us in an excellent position to accomplish these aims.
Flipping of lipids from one side of a biological membrane to the other is necessary for membrane expansion during cell growth, as well as for the biosynthesis of molecules that play critical roles in human and microbial physiology. These molecules include glycoproteins such as the neural cell adhesion molecule, GPI-anchored proteins such as acetylcholinesterase, glycolipids such as the receptor for cholera toxin, components of the cell walls of bacteria and yeast, and the O-antigen of E. coli lipopolysaccharide. We are interested in identifying the transport proteins that catalyze lipid flipping in yeast and mammals and understanding how they work.
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