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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 |
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| We have investigated the redistribution of spin-labeled phospholipid analogues across the plasma membrane of the heatocytic cell line HepG2. Spin-labeled phosphatidylserine (PS) and phosphatidylethanolamine (PE) moved rapidly to the inner leaflet, whereas phosphatidylcholine (PC) disappeared more slowly from the outer leaflet of the plasma membrane. The fast, inward movement of PS and PE was abolished after ATP-depletion of cells, suggesting the presence of an aminophospholipid translocase in the plasma membrane. The enrichement of PC in the outer leaflet of the plasma membrane could be a determinant of bile lipid composition. The major component of biliary lipids is PC. If bile lipids become recruited, mainly by those from the exoplasmic leaflet, aminophospholipids would be effectively excluded from secretion into the bile due to their almost exclusive localization on the cytoplasmic leaflet of the plasma membrane. ...back | |
| Transmembrane movement of phospholipids in the plasma membrane of
yeast cells ...more |
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Relevance of phospholipid asymmetry in endocytosis ...more |
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