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Lipid Asymmetry and Transport |
| A remarkable feature of several biological membrane systems is that their phospholipids are asymmetrically distributed across the lipid bilayer, a phenomenon called membrane phospholipid asymmetry. Most of our knowledge on phospholipid asymmetry of lipids has come from studies on human erythrocytes. The aminophospholipids, phosphatidylethanolamine (PE) and in particular phosphatidylserine (PS) are preferentially located in the inner leaflet of the membrane bilayer, while the other major components sphingomyelin (SM) and phosphatidylcholine (PC) are more abundant in the outer leaflet. It is assumed that phospholipids are also distributed asymmetrically in the plasma membrane of most, if not all, nucleated cells. Three distinct mechanisms for transmembrane movement of phospholipids have been described: (i) spontaneous diffusion (ii) facilitated diffusion and (iii) ATP-dependent, active translocation (Fig.1). |
Active transport ATP-dependent (e.g. Aminophospholipid-translocase for PS+PE in plasma membrane) |
Spontanous diffusion ATP-independent |
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| Plasma membrane phospholipid asymmetry plays a crucial role in a variety of biological processes . | Fig.1 Mechanisms of Lipid transmembrane movement | |
| Most of our current knowledge on the kinetics of phospholipid transmembrane movement in cell membranes has been gained using phospholipid analogues in translocation assays. | ||
| The information gleaned from those translocation assays has been used to formulate mathematical models which allow calculation of the transmembrane movement and distribution of phospholipids. | ||
| Results and Conclusions: |
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| Transmembrane movement of phospholipids in the plasma membrane of
myoblasts ...more |
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| Translocation of phospholipids in the plasma membrane of sperm
cells ...more |
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| Redistribution of phospholipids in the plasma membrane of
fibroblasts ...more |
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| Translocation of phospholipids in the plasma
membrane of hepatocytes ...more |
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| Transmembrane movement of phospholipids in the plasma membrane of
yeast cells |
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| The baker's yeast Saccharomyces cerevisiae possesses a gene, the so-called DRS 2 gene, that is thought to encode an aminophospholipid translocase similar to the ATPase II of bovine chromaffin cells (Yoshinori, M. and N.Nelson, 1987, J. Biol. Chem. 262: 9175-9180; Tang X. et al., 1996, Science 272: 1495-1497). However, there is no definitive evidence that yeasts are capable of aminophospholipid-specific transport. We are investigating the transmembrane movement of fluorescently labeled phospholipid analogues in the plasma membrane of Saccharomyces cerevisiae cells. In order to resolve this issue. If there is evidence for aminophospholipid transport we will try to answer the question if the DRS 2 gene product plays a role in aminophospholipid transport. ...back | |
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Relevance of phospholipid asymmetry in endocytosis ...more |
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