Membrane lattice

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

Such a spectrum is interpreted in terms of exciton interactions between the retinals within the trimer clusters (41,42,279-282) which characterize the purple membrane lattice (cf. Section I-B). Although this conclusion is generally accepted, the details of the exciton-interaction model may have to be revised. For example, the current exciton models (41,42) predict a low-intensity, out-of-the-membrane-plane optical transition on the high-energy side of the main absorption band, which is in varianceo with linear dichroism spectra (254). Moreover, the 9- to 12-A distance between chromophores predicted by these models are not compatible with recent neutron diffraction data (283). 

After a few minutes illumination ( light-adaptation ) bacteriorhodopsin immobilized in the purple membrane lattice contains 100% aW-trans retinal [73-78]. Light-adapted solubilized bacteriorhodopsin [79,80] and halorhodopsin [81-83], on the other hand, contain a mixture of about % aW-trans and /3 n-cii retinal. Dark-adaptation which takes minutes or hours, depending on conditions, results in thermally stable mixtures of /3 a -tmns and % 13-c/s chromophores in all cases. The dark-adapted 13-cis chromophores are stable because the overall shape of retinal is not very different from that of the a -trans chain, the C=N bond having assumed the syn rather than the anti configuration [84,85]. 

The purple membrane lattice can be dissociated with mild detergents to yield bacteriorhodopsin monomers [45,167-169]. Dissociation will take place [167,169] without loss of the chromophore in both Triton X-100 and octylglucoside at pH 5, and virtually all of the lipids can be removed after this treatment by gel filtration in deoxycholate [170]. The significance of the lattice structure in the purple membrane is not very clear, but it appears from whole cell studies that crystalline bacteriorhodopsin is more effective in photophosphorylation than the monomeric pigment [171]. Remarkably, sodium dodecyl sulfate-denatured bacteriorhodopsin, with extensive loss of secondary structure, could be renatured to yield a product similar to native bacteriorhodopsin, which will spontaneously recrystallize [172]. 

Dissociation of the purple membrane lattice, followed by incorporation of the bacteriorhodopsin into liposomes [327,356,357], yields a functional proton transport system. Thus, bacteriorhodopsin monomers will translocate protons upon illumination. 

The structure of the PLB has been related to that of cubic phcises[7], discussed in Chapters 4 and 5. However, as we shall see, a description of these membrane morphologies as equilibrium phases seems to be applicable, if at all, in only a few cases that we have encountered. Independently of Larsson et al. [7], Linder and Staehelin [14] also suggested that a certain "membrane lattice" in a parasitic protozoa did indeed correspond to an infinite periodic minimal surface. However, no further structural details, such as the symmetry or form of IPMS, were deduced or discussed. Some ten additional examples of membrane assemblies displaying cubic symmetries have been pointed out [15,16] but no structural details were inferred. To the best of our knowledge, the above references ([7, 14-16]) are the only reports in which membrane assemblies have been related to the structure of IP. There are. 

PnUnaa Zoomastigopliorea Leptomonai collosoma Membrane lattice (14)... 

A. Antiglomerular basement membrane antibodies Some xenobiotics may induce an immune-type glomerulonephritis associated with the occurrence of antibodies against some constituents of the glomerular basement membrane. These antibodies may be directly responsible for the nephropathy or they may be produced as a consequence of non- specific alterations of the normal glomerular basement membrane lattice. 

We propose the unsaturated lipids of biological membranes to provide the cells with an electronic conduction band. A striking feature of the unsaturated lipids in biomembranes is the very constant location of ethylenic cw-double bond in monounsaturated acyl chains between carbon atoms 9 and 10. A noteworthy exception is nervonic add (cw-15-24 1), which is abundant in central nervous system membranes. A microviscosity barrier has been observed in bilayers of dioleoylphosphatidylchohne at the depth of the oleic acid double bonds.Phospholipids belong to lyotropic liquid crystals and possess remarkable short- and long-range order. In a cell membrane lattice the local concentration of ester carbonyls and acyl chain ethylenic double bonds as well as the local concentration of cholesterol C=C bonds is very high. 

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. 

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]. 

T. Charitat, B. Fourcade. Lattice of passages connecting membranes. J Phys II France 7 15-35, 1997. 

Specialized regions of internalization from the plasma membrane, coated with a polyhedral lattice of the protein clathrin. It is in these regions that the first step of the process of endocytosis takes place, with the formation of clathrin-coated endocytic vesicles. 

In this context it is interesting to note that archaea, which possess S-layers as exclusive cell wall components outside the cytoplasmic membrane (Fig. 14), exist under extreme environmental conditions (e.g., high temperatures, hydrostatic pressure, and salt concentrations, low pH values). Thus, it is obvious one should study the effect of proteinaceous S-layer lattices on the fluidity, integrity, structure, and stability of lipid membranes. This section focuses on the generation and characterization of composite structures that mimic the supramolecular assembly of archaeal cell envelope structures composed of a cytoplasmic membrane and a closely associated S-layer. In this biomimetic structure, either a tetraether... 

---