Cell surface membrane, experimental

Figure 9.5 Schematic illustration of the experimental analysis of molecular networks of the cell surface membrane (for details see text).

Experimentally, the cholesterol content of cell surface membranes may be modified in vivo e.g. in the guinea pig by dietary means [131] or by modification of the culture media of micro-organisms [104]. In vitro, cholesterol modulation has also been achieved by the use of inhibitors of sterol synthesis [95] or liposomes [97,103,132-134]. 

Altman, P. L., and Katz, D. D., eds., 1976, Changes in cell surface membranes associated with malignant transformation, in Cell Biology, pp. 134-138, Federation of American Societies for Experimental Biology, Bethesda, Md. 

Bacteria have been Implicated in the formation of N-nitroso compounds under a wide variety of conditions representing both vitro and vivo situations Mechanisms of participation and/or catalysis Include a) decrease of the pH of the system, b) reduction of nitrate to nitrite, c) adsorption of amine onto the cell surface or cytoplasmic membrane, d) actual enzymatic formation. The literature of the field will be reviewed and experimental evidence which tests the above mechanisms will be presented ... 

One experimental tool in this direction is provided by some enveloped animal viruses which mature at the cell surface of infected cells (K Sri inen and Renkonen, 1977 Lenard, 1978). Such viruses include influenza virus, Semliki Forest virus (SFV), Sindbis virus, and vesicular stomatitis virus (VSV). They are extremely simple in makeup and hence are very well characterized. They can be tagged with biochemical probes in many different ways. They infect many animal cells in culture, and after infection turn the cells into factories for the production of virus progeny. The protein-synthesizing machinery of the host cell is programmed by the viral RNA to make viral proteins exclusively and these include the viral surface glycoproteins. These are synthesized with signal peptides and inserted into the ER membrane (Katz et ai, 1977 Garoff et... 

Possible Role of Long-Range VDW Forces Between Cell Membrane Surfaces. The above discussion has been based on the model of macro-molecular bridging, and the Eb is caused by the interaction between the bridging macromolecule and the cell surface. Recent theoretical and experimental studies (41,42) have shown that VDW attractive forces can be exerted between cell surfaces over distances in the range found in RBC rouleaux. 

How the hydrophilic a-LTX inserts into lipid membranes and makes cation-permeable pores is not fully known, but an in-depth insight into the mechanisms of channel formation has been gained by combining cryo-EM, biochemical and biophysical studies with toxin mutagenesis. a-LTX pore formation consists of at least three steps toxin tetramerisation, interaction with a specific cell-surface receptor and, finally, membrane insertion. Many experimental procedures can affect some of these steps and thereby prevent or assist channel formation. 

Interestingly, the diffusional behavior of membrane proteins measured experimentally by FRAP, FCS, or single particle tracking in cells is more complex than predicted by this model. This technique is described best for the case of cell surface proteins, as assessed by FRAP. Such measurements indicate that diffusion is typically much slower than one would expect based on membrane viscosity. In cell membranes, typical values of D for transmembrane proteins are approximately 0.05 pm /s or less, which is much slower than observed in artificial membranes composed of purified lipids. In addition, a significant fraction of proteins is often immobile over the timescale of diffusion experiments (4, 5). Furthermore, diffusional mobilities vary among proteins, and sometimes they differ for the same protein expressed in different cell lines (4, 5). Deviations from pure diffusion are more readily apparent when the trajectories... 

The surface of cells is constantly renewed the new plasma membraiK is synthraized in the cell and translocated to the surface while parts of the existing membrane are internalized in the endocytotic process, illustrated in Fig. 4. During the endocytotic process the polymer that was adsorbed on the cell surface and a small portion of the polymer in the surrounding medium enter into cells these polymers are enclosed in endocytotic vesicles. The internalized polymers stay firmly assodated with the cells. When the internalized poly-9-vinyladenine was followed for three cell divisions, the amount correlated with the number of viable cells within experimental error. Thus,no degradation or excretion of polynucleotide analogs from cells occurred. The cells containing radioactive polymer were also fractionated and (fistribution of radioactivity in aU these fractions was measured. Results confirm that a substantial amount of the polymer was in an enveloped form, probably in endocytotic vesides and lyM)somes ... 

The potential value of C2 toxin as a research tool depends on the vulnerability of cells. The experimental approach is straightforward when cells are susceptible to natural poisoning, because the two components of C2 toxin can be added to the exterior of cells and the enzymatic component will find its way to the cell interior (see for example Miyake and Ohishi, 1987 Reuner et al., 1987 Zepeda eta/., 1988). The approach is more problematic when cells are resistant to natural poisoning. In theory, resistance could be due to absence of cell surface receptors, absence of a mechanism for productive internalization, or absence of an intracellular substrate, but thus far only an absence of receptors (Fritz ef al., 1995) and an absence of substrate (Aktories et al., 1986) have been described. Cells without receptors can be rendered susceptible by using techniques that produce artificial internalization (e.g., permeabilizing the cell membrane or microinjection see Muller efal., 1992). Cells that do not have substrate are permanently resistant to poisoning. 

Albanese J., Dainiak N., Modulation of intercellular communication mediated at the cell surface and on extracellular, plasma membrane-derived vesicles by ionizing radiation. Experimental Hematology, 2003,31,455-464. 

In this paper, we examine the Interactions of pyran copolymer with model biomembranes of two kinds 1) the human red blood cell membrane (or red cell "ghost") and 11) multilamellar suspensions (liposomes) of dlpalmltoylphosphatldylchollne (DFPC), a pure synthetic phospholipid. Each of these systems offers advantages In studies of polymer-cell surface Interaction The red cell membrane, idille complex. Is still the most readily Isolated and best understood of the membranes of nonnal human cells, and Its molecular architecture Is, In a general way at least, typical of such membranes. The pure phospholipids provide a much simpler biomembrane model, with the prospect of yielding more complete Interpretation of experimental observations. 

A EXPERIMENTAL FIGURE 5-38 Small vesicles can be purified by binding of antibody specific for a vesicle surface protein and linkage to bacterial cells. In this example, a suspension of membranes from rat liver is incubated with an antibody specific for clathrin, a protein that coats the outer surface of certain cytosolic vesicles. To this mixture is added a suspension of Staphylococcus aureus bacteria whose surface membrane contains protein A, which binds to the Fc constant... 

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