Eukaryotic cell membranes

Phosphatidylinositol (abbreviated Ptdlns, or PI) is a minor class of phospholipids composed of glycerol, fatty acids and inositol. Pis are found in the cytosolic side of eukaryotic cell membranes. They are substrates fora large number of enzymes which are involved in cell signalling. 

Each eukaryotic cell membrane has a somewhat different hpid composition, though phospholipids are the major class in all. 

Here, we discuss a solid-state 19F-NMR approach that has been developed for structural studies of MAPs in lipid bilayers, and how this can be translated to measurements in native biomembranes. We review the essentials of the methodology and discuss key objectives in the practice of 19F-labelling of peptides. Furthermore, the preparation of macroscopically oriented biomembranes on solid supports is discussed in the context of other membrane models. Two native biomembrane systems are presented as examples human erythrocyte ghosts as representatives of eukaryotic cell membranes, and protoplasts from Micrococcus luteus as membranes... 

Sloppy feeding by grazers also leads to lysis of microbial and eukaryotic cell membranes with a similar outcome to viral lysis, i.e., conversion of POM into DOM as the cellular matrix is released into seawater. 

Thus, the fat globules are surrounded, at least initially, by a membrane typical of eukaryotic cells. Membranes are a conspicuous feature of all cells and may represent 80% of the dry weight of some cells. They serve as barriers separating aqueous compartments with different solute composition and as the structural base on which many enzymes and transport systems are located. Although there is considerable variation, the typical composition of membranes is about 40% lipid and 60% protein. The lipids are mostly polar (nearly all the polar lipids in cells are located in the membranes), principally phospholipids and cholesterol in varying proportions. Membranes contain several proteins, perhaps up to 100 in complex membranes. Some of the proteins, referred to as extrinsic or peripheral, are loosely attached to the membrane surface and are easily removed by mild extraction procedures. The intrinsic or integral proteins, about 70% of the total protein, are tightly bound to the lipid portion and are removed only by severe treatment, e.g. by SDS or urea. 

The phosphoinositides constitute 2-8% of the lipid of eukaryotic cell membranes but are metabolized more rapidly than are other lipids.265 278 279 283 - 285 A simplified picture of this metabolism is presented in Fig. 11-9. Phosphatidylinositol is converted by the consecutive action of two kinases into phosphatidylinositol 4,5-bisphosphate.286287 The InsP3 released from this precursor molecule by receptor-stimulated phospholipase C is thought to mobilize calcium ions by... 

Lipids have several important functions in animal cells, which include serving as structural components of membranes and as a stored source of metabolic fuel (Griner et al., 1993). Eukaryotic cell membranes are composed of a complex array of proteins, phospholipids, sphingolipids, and cholesterol. The relative proportions and fatty acid composition of these components dictate the physical properties of membranes, such as fluidity, surface potential, microdomain structure, and permeability. This in turn regulates the localization and activity of membrane-associated proteins. Assembly of membranes necessitates the coordinate synthesis and catabolism of phospholipids, sterols, and sphingolipids to create the unique properties of a given cellular membrane. This must be an extremely complex process that requires coordination of multiple biosynthetic and degradative enzymes and lipid transport activities. 

FIGURE 2.12 (a) The structure of a bilayer lipid membrane (b) the structure of the glyceryl esters that are major components of bacterial and eukaryotic cell membranes and (c) the structure of the glyceryl ethers that are major components of archaean cell membranes. R is a hydrophilic chain of CH and CH2 atoms. 

Thus far, the interactions of phospholipid head groups have been considered, because the model was applied toward rationalizing the membranolysis of eukaryotic cells such as erythrocytes, and PLs are the primary constituents of eukaryotic cell membranes. A reasonable question to ask at this time is whether the above results are relevant to prokaryotic membranes. Although PLs constitute a smaller proportion of the prokaryotic cell wall and cell membrane, the other constituent molecules such as liopolysaccharides and teichoic acids, are also amphiphilic. The general structure of a hydrophilic portion attached to a hydrophobic tail is common... 

Glycosylphosphatidyl inositol anchors constitute a class of glycolipids that link proteins and glycoproteins via their C-terminus to eukaryotic cell membranes. The first structure of a GPI anchor, that of Trypanosoma brucei, was published by Ferguson et al. [262]. Since then quite a few examples of GPI anchors were described, allowing the definition of the core structure depicted inO Scheme 37 [263]. 

Although phospholipids are critical for the formation of the classic bilayer structure of membranes, eukaryotic cell membranes require other components, including sterols. Here, we focus on cholesterol, the principal sterol in animal cells and the most abundant single lipid in the mammalian plasma... 

Soon after the 1988 communication on cephalostatin 1, it was predicted41 that the compound acts on the cell membrane. Steroids are components of eukaryotic cell membranes, where they incorporate into one half of the phospholipid bilayer and provide rigidity. Taking into account the dimeric nature of the cephalostatins, these steroids may now traverse the full length of the bilayer (for example, cephalostatin 1 is 30 A x 9 A x 5 A) and adversely affect its properties. A number of other highly oxygenated marine natural products (e.g. brevetoxin, palytoxin) are also membrane active agents. 

Cecropins. Cecropins (169) were isolated from the larval hemolymph of the giant silkworm moth, Hyalophora cecropia, on the basis of their antibacterial activity subsequent to bacterial injection. Cecropins are also produced by Drosophila (170) and the larva of the tobacco homworm, Manduca sexta (110). Similar compounds called sarcotoxins (171) are produced by the flesh fly, Sarcophaga. Cecropins (169) are specific for prokaryotic cell lysis in contrast to melittin, which lyses eukaryotic cell membranes as well. Cecropins are characterized structurally by a concentration of basic... 

These alkaloids do not contain functional groups such as alkylation sites, Michael acceptors, intercalators, or redox-active qui-nones commonly associated with cytotoxicity, while their scarcity has hindered investigation of the mechanism of action. An early proposal, [16] taking into account the steroidal and dimeric nature of the alkaloids, was that these compounds span the lipid bilayer (cephalostatin 1 is 30 A long) and perturb the eukaryotic cell membrane. 

Glycoproteins are ones in which carbohydrates are covalently bonded to proteins. They play a role in eukaryotic cell membranes, frequently as recognition sites for external molecules. Antibodies (immunoglobulins) are glycoproteins. 

A biological membrane is a structure particularly suitable for study by the LB technique. The eukaryotic cell membrane is a barrier that serves as a highway and controls the transfer of important molecules in and out of the cell (Roth etal., 2000). The cell membrane consists of a bilayer or a two-layer LB film (Tien etal, 1998). Lipid bilayers are composed of a variety of amphiphilic molecules, mainly phospholipids and sterols which in turn consist of a hydrophobic tail, and a hydrophilic headgroup. The complexity of the biomembrane is such that frequently simpler systems are used as models for physical investigations. They are based on the spontaneous self-organization of the amphiphilic lipid molecules when brought in contact with an aqueous medium. The three most frequently used model systems are monolayers, black lipid membranes, and vesicles or liposomes. 

Magee, A.I. Schlesinger, M.J. (1982) Biochim. Biophys. Acta, 649, 279-289. Fatty acid acylation of eukaryotic cell membrane proteins. 

---