Rafts, domains

Recently, due to increased interest in membrane raft domains, extensive attention has been paid to the cholesterol-dependent liquid-ordered phase in the membrane (Subczynski and Kusumi 2003). The pulse EPR spin-labeling DOT method detected two coexisting phases in the DMPC/cholesterol membranes the liquid-ordered and the liquid-disordered domains above the phase-transition temperature (Subczynski et al. 2007b). However, using the same method for DMPC/lutein (zeaxanthin) membranes, only the liquid-ordered-like phase was detected above the phase-transition temperature (Widomska, Wisniewska, and Subczynski, unpublished data). No significant differences were found in the effects of lutein and zeaxanthin on the lateral organization of lipid bilayer membranes. We can conclude that lutein and zeaxanthin—macular xanthophylls that parallel cholesterol in its function as a regulator of both membrane fluidity and hydrophobicity—cannot parallel the ability of cholesterol to induce liquid-ordered-disordered phase separation. 

FIGURE 10.12 The mole ratio of carotenoid/phospholipid and carotenoid/total lipid (phospholipid + cholesterol) in raft domain (detergent-resistant membrane, DRM) and bulk domain (detergent-soluble membrane, DSM) isolated from membranes made of raft-forming mixture (equimolar ternary mixture of dioleoyl-PC (DOPC)/sphingomyelin/cholesterol) with 1 mol% lutein (LUT), zeaxanthin (ZEA), P-cryptoxanthin (P-CXT), or P-carotene (P-CAR). 

FIGU RE 10.13 Schematic drawing of the distribution of xanthophyll molecules between raft domain (DRM) and bulk domain (DSM) in lipid bilayer membranes. For this illustration, the xanthophyll partition coefficient between domains is the same as obtained experimentally for raft-forming mixture. However, to better visualize the observed effect in the drawing, the number of lipid molecules was decreased and the total number of xanthophyll molecules was increased about 10 times. (From Wisniewska, A. and Subczynski, W.K., Free Radio. Biol. Med., 40, 1820, 2006. With permission.)... 

Despite the weakness and short-range nature of protein-lipid and lipid-lipid interactions, cells have nevertheless evolved means of laterally assembling into membrane-mi-crodomains. Sphingolipid-cholesterol rafts serve to recmit a specific set of membrane proteins and exclude others [24]. Caveolae are deeply invaginated raft domains that are stabilized by caveolin protein oligomers (binding cholesterol) [25]. 

Targeting of proteins to specialized domains of a membrane are less well understood. These include caveolae and lipid rafts, domains that are high in cholesterol and sphingolipids and which function in endocytosis and in cell signaling. A recent proposal is that proteins with hydrophobic surfaces needed in these domains become coated with a lipid "shell" before entering the membrane.6173... 

Lipid raft domains of plasma membranes are enriched in cholesterol and sphingolipids. As a consequence, compounds that extract or sequester cholesterol, such as fS-cyclodextrins, nystatin, and filipin, can block selectively endocytosis of cholera toxin, GPI-linked proteins, and other receptors that associate with lipid rafts and caveolae. However, cholesterol is also critical for CME, secretion of proteins, and the actin network. Therefore, conditions designed to affect selectively raft-mediated endocytosis by perturbing cholesterol levels must be carefully controlled to avoid disrupting other mechanisms of endocytosis (40). 

Chiantia S Kahya N, SchwUle, P. Raft domain formation driven by short- and long-chain ceramide a combined AFM and PCS... 

Gaus K, Gratton E, Kable EP, Jones AS, Gelissen I, Kritharides L, Jessup W. Visualizing lipid structure and raft domains in living cells with two-photon microscopy. Proc. Natl. Acad. Sci. U.S.A. 2003 100 15554-15559. 

Secondly, a separation of phases was observed (Fig. 3b,d). The feature of three component mixtures was small circular domains (Fig. 3b), probably, raft domains. Indeed, the domains that are essentially different in fiiction have been observed by AFM in the contact mode, which were practically indiscernible in the topography image. 

The influence of CHL and Q3P on film morphology in monolayer mixtures with DPPC has been studied. Monolayers of DPPC as well as it mixtures tvith cholesterol, transferred by HP method, showed a molecularly smooth structure of uniform thickness. The addition of Q3P or CHL to DPPC, as investigated by AFM phase measurements, showed that a marked phase separation occurs in DPPC/Q3P mixtures or DPPC/SM/CHL films at small concentration of the alcohols, proving raft domain formation in the case of DPPC/SM/CHL films. 

Muller, G., Jung, C., Wied, S., Welte, S., Jordan, H., and Frick, W. Redistribution of glycolipid raft domain components induces insulin-mimetic signaling in rat adipocytes. Mol. Cell Biol., 2001,... 

G. Muller and S. Welte, Lipid raft domains are the targets for the insulin-independent blood glucose-decreasing activity of the sulfonylurea ghmepiride, Rec. 

Recent studies suggest that detergent-resistant subdomains of the plasma membrane ( rafts , discussed previously) may be involved in formation of PrP . Consistent with this idea, both PrP and PrP are found in raft domains isolated biochemically (Gorodinsky and Harris, 1995 Taraboulos et al, 1995 Naslavsky et al., 1996 Vey et al., 1996 Naslavsky et al., 1999). In addition, pharmacological depletion of cellular cholesterol, which is known to disrupt rafts, inhibits PrP formation (Taraboulos et al, 1995), whereas sphingolipid depletion, which does not alter the raft localization of PrP, actually enhances PrP production (Naslavsky et al, 1999). Finally, artificially constructed transmembrane forms of PrP, which are excluded from rafts, are poor substrates for conversion to PrP (Kaneko et al., 1997). 

Our kinetic studies of mutant PrPs synthesized in CHO cells suggest that individual steps in formation of PrP may take place in at least two different cellular locations (Fig. 5). Because mutant PrPs become PIPLC-resistant within minutes of synthesis in pulse-labeling experiments, this early step must take place in the ER. Consistent with this conclusion, acquisition of PIPLC resistance is not affected by treatment of cells with brefeldin A or by incubation at 18°C, manipulations that block exit of proteins beyond the Golgi (Daude et al, 1997). In contrast, detergent insolubility and protease resistance, which do not develop until later times of chase, and are reduced by brefeldin A and 18°C incubation, are likely to be acquired after arrival of the protein at the cell surface, either on the plasma membrane itself or in endocytic compartments. Raft domains may be involved in these changes (unpublished data). 

Fig. 6. Model of the cellular pathways involved in generation of PrP. In the infectious manifestation of prion diseases, extracellular PrP in the form of a prion particle (1) interacts with PrP on the cell surface, possibly in detergent-resistant rafts, catalyzing its conversion to PrP (2). Conversion may also occur after uptake of the proteins into an endosomal compartment (3). Once formed, some PrP c accumulates in lysosomes (4), although the protein is probably found in a number of other cellular locations as well. In familial prion disorders, mutant PrP is converted spontaneously to the PrP state via a series of biochemical intermediates, the earliest of which is a PIPLC-resistant form generated in the ER (5). Mutant PrP molecules are subsequently delivered to the cell surface, where they become detergent-insoluble (6) and then protease-resistant (7), possibly in raft domains. Steps 6 and 7 could also occur in endocytic organelles. (Reprinted with permission from Harris, 1999).

The model reproduces the most prominent phase transitions of phospholipid monolayers [78] and bilayers [80]. In particular, it reproduces a main transitirm from a fluid membrane phase (L to a tilted gel phase Lpi) with an intermediate ripple phase Pp ), in agreement with experiments. The elastic parameters have been studied in the fluid phase and are in reasonable agreement with those of saturated DPPC (dipalmitoyl-phosphatidylcholine) bilayers. Recently, the Lenz model has been supplemented with a simple cholesterol model [81]. Cholesterol molecules are taken to be shorter and stiffer than lipids, and they have a slight affinity to lipids. Mixtures of lipids and cholesterol were found to develop nanoscale raft domains... 

Meinhardt S, Vink R, Schmid F (2013) Monolayer curvature stabilizes nanoscale raft domains in mixed lipid bilayers. Proc Natl Acad Sci USA 12 4476-4481... 

Dermine, J.F., Duclos, S., Garin, J., St-Louis, F., Rea, S., Parton, R.G. and Desjardins, M. (2001) Flotillin-1 -enriched lipid raft domains accumulate on maturing phagosomes. J. Biol. Chem. 276, 18507-18512. 

Figure 6.2 Lipid raft structure. The central raft domain contains lipids in the liquid ordered phase, in which the chains are extended. This domain is also enriched in cholesterol, and many types of protein are bound to such domains...

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