Membranes asymmetry and

The simple model does allow an entry point into the study of self-assembly of multicomponent lipid systems, lateral phase separation (clustering), membrane asymmetry, and in particular how these relate to curvature through packing. These form a central class of problems in membrane biology. 

Electroformed GUVs are ideal model objects for performing micromanipulation and local microinjection, thereby creating localized and temporal membrane asymmetry and lateral gradients of the physico-chemical properties of the lipid membrane. 

There are other ways in which the lateral organization (and asymmetry) of lipids in biological membranes can be altered. Eor example, cholesterol can intercalate between the phospholipid fatty acid chains, its polar hydroxyl group associated with the polar head groups. In this manner, patches of cholesterol and phospholipids can form in an otherwise homogeneous sea of pure phospholipid. This lateral asymmetry can in turn affect the function of membrane proteins and enzymes. The lateral distribution of lipids in a membrane can also be affected by proteins in the membrane. Certain integral membrane proteins prefer associations with specific lipids. Proteins may select unsaturated lipid chains over saturated chains or may prefer a specific head group over others. 

Membrane asymmetries in the transverse direction (from one side of the membrane to the other) can be anticipated when one considers that many properties of a membrane depend upon its two-sided nature. Properties that are a consequence of membrane sidedness include membrane transport, which is driven in one direction only, the effects of hormones at the outsides of cells, and the immunological reactions that occur between cells (necessarily involving only the outside surfaces of the cells). One would surmise that the proteins involved in these and other interactions must be arranged asymmetrically in the membrane. 

The properties of membranes commonly studied by fluorescence techniques include motional, structural, and organizational aspects. Motional aspects include the rate of motion of fatty acyl chains, the head-group region of the phospholipids, and other lipid components and membrane proteins. The structural aspects of membranes would cover the orientational aspects of the lipid components. Organizational aspects include the distribution of lipids both laterally, in the plane of the membrane (e.g., phase separations), and across the membrane bilayer (phospholipid asymmetry) and distances from the surface or depth in the bilayer. Finally, there are properties of membranes pertaining to the surface such as the surface charge and dielectric properties. Fluorescence techniques have been widely used in the studies of membranes mainly since the time scale of the fluorescence lifetime coincides with the time scale of interest for lipid motion and since there are a wide number of fluorescence probes available which can be used to yield very specific information on membrane properties. 

Reviews of the role of aminophospholipid translocase and scramblase (Schlegel et al, 2000) and the consequences of the appearance of phosphatidylserine on the cell surface (Williamson et al, 2001) in apoptosis of thymocytes have been published. The precise relationship between membrane phospholipid asymmetry and apoptosis is currently a topic of considerable interest. 

HammUl, A.K., Uhr, J.W. and Scheuermann, R.H., 1999, Annexin V staining due to loss of membrane asymmetry can be reversible and precede commitment to apoptotic death. 

McIntyre, J.C. and Sleight, R.G., 1991, Fluorescence assay for phospholipid membrane asymmetry. Biochemistry, 30 11819-11827. 

Rothman, J.E. and Lenard, J., 1977, Membrane asymmetry. The nature of membrane... 

Verkleij, A.J. and Post, J.A., 2000, Membrane phospholipid asymmetry and signal transduction. J. Memb. Biol. 178 1-10. 

Wu, G. and Hubbel, W.L., 1993, Phosphohpid asymmetry and transmembrane diffusion in photoreceptor disc membranes. Biochemistry, 32 879-888. 

Zachowski, A., 1993. PhosphoUpids in animal eukaryotic membranes transverse asymmetry and movement. Biochem. J., 294 1-14... 

Thus the concept of membrane asymmetry was also supported by a comparison of Schleicher and Schuell menbrane performance with that of homogeneovis menbranes. 

Membrane Properties. The performance range of ammonia-modified membranes in low pressure operation is indicated in Figure 6 along with the performance of the reference membrane (I, reference membrane IV, ammonia-modified membrane). The lower boundary of the performance range refers to a solvent-to-polymer ratio of 3, the upper boundary to a ratio of 4. While the salt rejection towards univalent ions of the ammonia-modified membrane is limited to below 80 %, the maximum low pressure flux is over 15 m /m d (approaching 400 gfd) at a sodium chloride rejection of the order of 10 %. This membrane thus exhibits the flux capability of an ultrafiltration membrane while retaining the features of reverse osmosis membranes, viz. asymmetry and pressure resistance. 

T5. Thornberry, N. A., andLazebnik, Y, Caspases Enemies within. SciencelSl, 1312—1316(1998). T6. Tsujimoto, Y., and Shimizu, S., Bcl-2 family Life-or-death switch. FEBSLett. 466,6-10 (2000). V1. Van den Eijnde, S. M., Boshart, L., Reutelingsperger, C. R, De Zeeuw, C. I., and Vermeij-Keers, C., Phosphatidylserine plasma membrane asymmetry in vivo A pancellular phenomenon which alters during apoptosis. Cell Death Differ. 4, 311-316 (1997). 

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