Membrane asymmetry

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. 

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... 

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

Prelytic DNA fragmentation Release of various factor (e.g., cytochrome C) into cytoplasm by mitochondria Caspase cascade is activated Alterations in membrane asymmetry... 

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). 

V4. Van Engeland, M., Ramaekers, F. C., Schutte, B., and Reutelingsperger, C. R, A novel assay to measure loss of plasma membrane asymmetry during apoptosis of adherent cells in culture. Cytometry 24, 131-139 (1996). 

Finally, the third step of substrate permeability across the plasma membrane involves partitioning of the substrate from the opposite interface (desorption). This process involves membrane partitioning and the same factors that determine adsorption also determine desorption but for desorption versus adsorption, the relationships are reversed (101). It is important to note that because of membrane asymmetry (between inner and outer leaflets) present in all cells, the processes of adsorption and desorption may be vastly different depending on the direction of substrate transport (from external milieu to cytosol or vice versa). Consequently, differences in adsorption and desorption can lead to differences in substrate permeability across inner and outer leaflets, as shown for doxorubicin (189). Indeed, it has been hypothesized that direction of substrate transport may affect how P-gp effluxes its substrates (190). 

Membrane asymmetry changes can be detected by flow cytometry using a fluorescent marker (e.g. fluorescein, FITC) conjugated to annexin V, a protein that has high affinity for phosphatidylserine. When using a fluorescence microscope, this technique can be quantitative if a hemocyt-ometer is used. 

Membranes are structurally and functionally asymmetric, as exemplified by the restriction of sugar residues to the external surface of mammalian plasma membranes. Membranes are dynamic structures in which proteins and lipids diffuse rapidly in the plane of the membrane (lateral diffusion), unless restricted by special interactions. In contrast, the rotation of lipids from one face of a membrane to the other (transverse diffusion, or flip-flop) is usually very slow. Proteins do not rotate across bilayers hence, membrane asymmetry can be preserved. The degree of fluidity of a... 

Analysis of the complete yeast genome revealed the presence of 16 proteins that clearly belong to the P-type ATPase family. More detailed sequence analysis suggests that 2 of these proteins transport H+ ions, 2 transport Ca +, 3 transport Na+, and 2 transport metals such as Cu2+. In addition, 5 members of this family appear to participate in the transport of phospholipids with amino acid head groups. These latter proteins assist in the maintenance of membrane asymmetry by transporting lipids such as phosphatidyl serine from the outer to the irmer leaflet of the bilayer membrane (Figure 13.6). Such enzymes have been termed "flippases."... 

Figure 13.6. P-Type ATPases Can Transport Lipids. Flippases are enzymes that maintain membrane asymmetry by "flipping" phospholipids (displayed with a red head group) from the outer to the inner layer of the membrane.

The addition of the carbohydrate introduces a significant energy barrier to the flip-flop because a hydrophilic carbohydrate moiety would need to be moved through a hydrophobic environment. This energetic barrier enhances membrane asymmetry. 

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. 

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