Transition vesicles

Two mechanisms have been proposed to explain the transport of phospholipids from the ER to other cellular membranes protein-mediated transfer and a vesicular process. Several experiments have demonstrated that water-soluble proteins, known as phospholipid exchange proteins, can bind to specific phospholipid molecules and transfer them to another bilayer. Vesicular transport of phospholipids and membrane proteins in structures known as transition vesicles from the ER to the Golgi complex is not clearly understood. However, evidence of transfer of luminal material from the ER to the Golgi cistemae clearly supports vesicular transport. 

Phospholipid translocator proteins, phospholipid exchange proteins, and transition vesicles are involved in the complicated process of membrane synthesis and delivery of membrane components to their cellular destinations. 

Figure 5. System of transition vesicles for delivery of endoplasmic reticulum (ER) membranes to the plasma membrane. A hierarchy of decreasing complexity is illustrated ranging from a primary vesicle coupled to a stacked Golgi apparatus to a simple continuum. At least one discontinuous, vesicular compartment if necessary to avoid the disastrous consequences of direct continuity that would allow free entry of the extracellular medium into the cell.

The cytoplasmic vesicle or primary vesicle (transition vesicle) which provides structural and functional continuity between systems of transition elements also provides a link between smooth ER and more complex systems of transitional elements. According to Merriam (19), the cytoplasmic vesicle represents a "dynamic pool of cellular membrane systems whose common point of Interconversion Is the general cytoplasmic vesicle." Merriam continues, "It Is of course possible that the general vesicles are homologous with the endoplasmic reticulum of other cell types where a continuity between the reticulum and other membrane systems may be apparent at a single moment in time."... 

In our laboratory, studies of lipid transfer in leek seedlings in vivo, have already shown the existence of a vesicular process for the transfer of phospholipids and particularly of very long chain fatty acid-containing lipids [6]. This process follows the vesicular endoplasmic reticulum- Golgi apparatus- plasma membrane pathway. Using the cell-free system developed by Morre and coworkers, we have reconstituted in vitro the vesicular transfer of some phospholipids between the ER and the GA. This transfer is ATP and cytosol-dependent, is N Ethyl Maleimide and temperature sensitive and specific for the ER as donor and the GA as acceptor. The phospholipids transferred via an ATP-dependent manner in vitro between the ER and the GA were phosphatidylcholine (PC +79%), phosphatidylethanolamine (PE +67%) and phosphatidylserine (PS +123%) [7]. All those results are in favour of a vesicular transport of phospholipids between the ER and the GA of leek seedlings, and brought us to purify these transition vesicles issued from the ER. 

The optimal conditions for the formation of the transition vesicles in vitro have first been determined. For these reasons, we have checked whether a sustained lipid synthesis in vitro could increase the ATP-dependent transfer of membrane material i.e. the amount of vesicles formed. For that, the reaction mixture already described [7] was completed with CDP-choline (12,5 lM), CDP-ethanolamine (12,5p.M), C16 0 (2,5 lM), C18 l (7,5(lM) and CoA (SOOiiM). We have measured in vitro the ATP-dependent and ATP-independent lipid transfer between the ER and the Golgi in the presence or absence of the components described above, for three times of incubation 5, 15 or 30 minutes. The results are presented in the figure 1. 

Therefore, it is likely that the transition vesicles isolated correspond, at least to some extent, to those that are operative in the transfer of phospholipids between the ER and the Golgi apparatus. 

Micellar structure has been a subject of much discussion [104]. Early proposals for spherical [159] and lamellar [160] micelles may both have merit. A schematic of a spherical micelle and a unilamellar vesicle is shown in Fig. Xni-11. In addition to the most common spherical micelles, scattering and microscopy experiments have shown the existence of rodlike [161, 162], disklike [163], threadlike [132] and even quadmple-helix [164] structures. Lattice models (see Fig. XIII-12) by Leermakers and Scheutjens have confirmed and characterized the properties of spherical and membrane like micelles [165]. Similar analyses exist for micelles formed by diblock copolymers in a selective solvent [166]. Other shapes proposed include ellipsoidal [167] and a sphere-to-cylinder transition [168]. Fluorescence depolarization and NMR studies both point to a rather fluid micellar core consistent with the disorder implied by Fig. Xm-12. 

Phospholipid molecules form bilayer films or membranes about 5 nm in thickness as illustrated in Fig. XV-10. Vesicles or liposomes are closed bilayer shells in the 100-1000-nm size range formed on sonication of bilayer forming amphiphiles. Vesicles find use as controlled release and delivery vehicles in cosmetic lotions, agrochemicals, and, potentially, drugs. The advances in cryoelec-tron microscopy (see Section VIII-2A) in recent years have aided their characterization [70-72]. Additional light and x-ray scattering measurements reveal bilayer thickness and phase transitions [70, 71]. Differential thermal analysis... 

The interest in vesicles as models for cell biomembranes has led to much work on the interactions within and between lipid layers. The primary contributions to vesicle stability and curvature include those familiar to us already, the electrostatic interactions between charged head groups (Chapter V) and the van der Waals interaction between layers (Chapter VI). An additional force due to thermal fluctuations in membranes produces a steric repulsion between membranes known as the Helfrich or undulation interaction. This force has been quantified by Sackmann and co-workers using reflection interference contrast microscopy to monitor vesicles weakly adhering to a solid substrate [78]. Membrane fluctuation forces may influence the interactions between proteins embedded in them [79]. Finally, in balance with these forces, bending elasticity helps determine shape transitions [80], interactions between inclusions [81], aggregation of membrane junctions [82], and unbinding of pinched membranes [83]. Specific interactions between membrane embedded receptors add an additional complication to biomembrane behavior. These have been stud-... 

The other class of phenomenological approaches subsumes the random surface theories (Sec. B). These reduce the system to a set of internal surfaces, supposedly filled with amphiphiles, which can be described by an effective interface Hamiltonian. The internal surfaces represent either bilayers or monolayers—bilayers in binary amphiphile—water mixtures, and monolayers in ternary mixtures, where the monolayers are assumed to separate oil domains from water domains. Random surface theories have been formulated on lattices and in the continuum. In the latter case, they are an interesting application of the membrane theories which are studied in many areas of physics, from general statistical field theory to elementary particle physics [26]. Random surface theories for amphiphilic systems have been used to calculate shapes and distributions of vesicles, and phase transitions [27-31]. 

FIG. 16 Phase diagram of fluid vesicles as a function of pressure increment p and bending rigidity A. Solid lines denote first-order transitions, dotted lines compressibility maxima. The transition between the prolate vesicles and the stomatocytes shows strong hysteresis efifects, as indicated by the error bars. Dashed line (squares) indicates a transition from metastable prolate to metastable disk-shaped vesicles. (From Gompper and KroU 1995 [243]. Copyright 1995 APS.)... 

Discuss the effects on the lipid phase transition of pure dimyris-toyl phosphatidylcholine vesicles of added (a) divalent cations, (b) cholesterol, (c) distearoyl phosphatidylserine, (d) dioleoyl phosphatidylcholine, and (e) integral membrane proteins. 

The development of monoalkyl phosphate as a low skin irritating anionic surfactant is accented in a review with 30 references on monoalkyl phosphate salts, including surface-active properties, cutaneous effects, and applications to paste and liquid-type skin cleansers, and also phosphorylation reactions from the viewpoint of industrial production [26]. Amine salts of acrylate ester polymers, which are physiologically acceptable and useful as surfactants, are prepared by transesterification of alkyl acrylate polymers with 4-morpholinethanol or the alkanolamines and fatty alcohols or alkoxylated alkylphenols, and neutralizing with carboxylic or phosphoric acid. The polymer salt was used as an emulsifying agent for oils and waxes [70]. Preparation of pharmaceutical liposomes with surfactants derived from phosphoric acid is described in [279]. Lipid bilayer vesicles comprise an anionic or zwitterionic surfactant which when dispersed in H20 at a temperature above the phase transition temperature is in a micellar phase and a second lipid which is a single-chain fatty acid, fatty acid ester, or fatty alcohol which is in an emulsion phase, and cholesterol or a derivative. 

Zheng Y, Lin Z, Zakin JL, Talmon Y, Davis HT, Scriven LE (2000) Cryo-TEM imaging the flow induced transition from vesicles to threadlike micelles. J Phys Chem B 104(22) 5263-5271... 

H. Noguchi and G. Gompper, Shape transitions of fluid vesicles and red blood cells in capillary flows, Proc. Natl. Acad. Sci. USA 102, 14159 (2005). 

The phase transition of bilayer lipids is related to the highly ordered arrangement of the lipids inside the vesicle. In the ordered gel state below a characteristic temperature, the lipid hydrocarbon chains are in an all-trans configuration. When the temperature is increased, an endothermic phase transition occurs, during which there is a trans-gauche rotational isomerization along the chains which results in a lateral expansion and decrease in thickness of the bilayer. This so-called gel to liquid-crystalline transition has been demonstrated in many different lipid systems and the relationship of the transition to molecular structure and environmental conditions has been studied extensively. 

The DSC spectra confirm that the fluid phase of the polymerized vesicles remains and the phase transitions are retained with the introduction of the spacer group. As can been seen in Figure 8 of the DSC spectrum of the monomeric lipid, there is a peak around 28°C which corresponds to the phase transition of monomeric lipid. As the result of the presence of the spacer group, a similar phase transition can also be observed clearly in the spectrum of the polymerized lipid as shown in Figure 9, but the transition temperature is increased to 36°C by the presence of the polymer chains. 

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