Transmembrane asymmetry

Malewicz B, Valiyaveettil IT, Jacob K, Byun HS, Mattjus P, Baumann WJ, Bittman R, Brown RE. The 3-hydroxy group and 4,5-trans double bond of sphingomyelin are essential for modulation of galactosylceramide transmembrane asymmetry. Biophys. J. 2005 88 2670-2680. 

The transmembrane asymmetry of acyl lipids raises several interesting questions. The most intriguing ones concern the origin of this asymmetry and the reciprocal influence of proteins and acyl lipids on their respective organization. Another way to put it is to ask whether the insertion of new proteins in the thylakoid membrane induces a new or maintains the same acyl lipid asymmetry. The knowledge of the distribution of acyl lipids in prothylakoids may provide answers to the above questions [9,10]. Frothylakoids are the precursors of mature thylakoids but their function and protein composition are quite different, e.g. prothylakoids are devoid of... 

Although helpful, this schematic representation raises two issues. First, the plasma membrane also contains sphingolipids (inverted cones with 0 < P < 1/3) and cholesterol (cones with P = 1.21), and it is still assumed to adopt a bilayer structure. Second, the lipid composition of each leaflet of the plasma membrane is specific, and this transmembrane asymmetry induces a curvature that would not occur if all lipids were cylindrical. Solving these problems will help us to figure out how a plasma membrane is really organized at the lipid level. 

Photosystem II Inhibitors. The PSII complex usually is assumed to be that stmctural entity capable of light absorption, water oxidation, plastoquiaone reduction, and generation of transmembrane charge asymmetry and the chemical potential of hydrogen ions (41). The typical PSII complex... 

Methods used to demonstrate the existence of membrane phospholipid asymmetry, such as chemical labelling and susceptibility to hydrolysis or modification by phospholipases and other enzymes, are rmsuitable for dynamic studies because the rates of chemical and biochemical reactions are of a different order compared to the transmembrane translocahon of the phospholipids. Indirect methods have therefore been developed to measure the translocation rate which are consequent on the loss of membrane phospholipid asymmetry. Thus time scales appropriate to rates of lipid scrambling under resting conditions or when the forces preserving the asymmetric phospholipid distribution are disturbed can be monitored. Generally the methods rely on detecting the appearance of phosphatidylserine on the surface of cells. Methods of demonstrating Upid translocation in mammalian cells has been the subject of a recent review (Bevers etal., 1999). 

Rauch, C. and Farge, R, 2000, Endocytosis switch controlled by transmembrane osmotic pressure and phospholipid number asymmetry. Biophys. J., 78 3036-3047. 

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

Zachowski, A., Henry, J.P. and Devaux, P.F., 1989, Control of transmembrane hpid asymmetry in chromaffin granules by an ATP-dependent protein. Nature, 340 75-76... 

The octanol/buffer represents a partition coefficient between two bulk phases it is less affected by the structure of the analyte and therefore it cannot be used to predict the exact value of liposome membrane-to-buffer Xp, which is also affected by the geometry of the analyte (41 4). However, it is accepted and established that the octanol-to-buffer can help to predict transmembrane passive diffusion (40). In the case of liposomes such as Doxil, in which the internal aqueous phase (intraliposome aqueous phase) is different from the external liposome aqueous medium due to large differences in the composition and pH of these two aqueous phases, there are two different liposome membrane-to-aqueous phase partition coefficients this is referred to as asymmetry in the membrane-to-aqueous media partition coefficient. 

The biological function of lipid asymmetry and of proteins involved in the transmembrane traffic of lipids is multiple. Rapid reorientation of phospholipids in ery-... 

One of the most intriguing features of the bacterial reaction centre is the asymmetry of trans-membrane electron transfer. Although the BChl, BPhe and UQjq cofactors are arranged in two approximately symmetrical trans-membrane branches, only the so-called A-branch is used for transmembrane electron transfer. The factors determining this functional asymmetry continue to be the subject of great interest, as the reaction centre presents a chain of cofactors that catalyses electron transfer with great efficiency, and a similar chain of cofactors that is much less effective. This functional asymmetry is due to small but crucial differences in the structure of the proteinxofactor system along the two branches. 

Although the two-fold symmetry displayed by the reaction centre is striking, it is only a pseudo-symmetry, because differences in the amino aeid sequences of the L and M subunits result in small differences in the positions and relative orientations of equivalent cofactors on the two branehes, and in differences of the protein environment of equivalent cofactors. The root cause of the functional asymmetry that is observed when electron transfer is monitored is therefore asymmetry in the detailed structure of the cofactor protein system on the two branches. Assuming that the transmembrane electron transfer process can basically be described as a non-adiabatic electron transfer reaction according to the Marcus equation, this... 

P.F. Devaux. 1992. Protein involvement in transmembrane lipid asymmetry Annu. Rev. Biophys. Biomol. Struct 21 417-439. (PubMed)... 

In experiments designed to explore whether there is protein transverse asymmetry in membranes, it was shown that when trypsin was used to treat intact erythrocytes, the carbohydrate groups of tiie transmembrane protein glycophorin were released from the N-terminus of this protein (as glycopeptides). This was taken as evidence that the N-terminus of glycophorin must be on the exterior side of the membrane because ... 

---