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
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:
 
  Transmembrane movement of phospholipids in the plasma membrane of myoblasts
 
  Myoblasts seem to be an exception to the general feature of phospholipid asymmetry because they contain high amounts of phosphatidylethanolamine (PE) and phosphatidylserin (PS)in the exoplasmic leaflet of their plasma membrane. It has been suggested that the transverse phospholipid distribution may play an essential role during myoblast fusion into myotubes. However it is not known whether myoblasts exhibit an aminophospholipid translocase activity.
We have investigated the transmembrane movement of spin-labeled phospholipid analogues of sphingomyelin (SM), phosphatidylcholine (PC), PE and PS in the plasma membrane of rat L6 myoblasts. In proliferating rat L6 myoblasts a specific internalization of PS and PE was found which is progressively lost during differentiation and myotube formation. ...back
  Translocation of phospholipids in the plasma membrane of sperm cells
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  Redistribution of phospholipids in the plasma membrane of fibroblasts
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  Translocation of phospholipids in the plasma membrane of hepatocytes
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  Transmembrane movement of phospholipids in the plasma membrane of yeast cells
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  Relevance of phospholipid asymmetry in endocytosis
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