Rapid lateral diffusion of phospholipids in rabbit sarcoplasmic reticulum

Differences in membrane fluidity and structure in contact-inhibited and transformed cells

Studies on contact-inhibited mouse embryo fibroblast 3T3 cells and 3T3 cells transformed by oncogenic RNA and DNA viruses and by a chemical carcinogen have demonstrated differences in plasma membrane architecture. Spin-label and freeze-fracture ultrastructural studies have shown that contact-inhibited cells have ordered membrane lipids and aggregated intramembranous particles, whereas transformed cells have fluid membrane lipids and randomly distributed intramembranous particles. These findings suggest a model for how changes in the cell membrane may account for some of the characteristic differences observed between contact-inhibited and transformed cells.

Spin label translational diffusion in solid tristearin

Translational diffusion of the intermediate chain length spin label 7N14 has been detected and studied in a lipid environment which is in the bulk solid state. Under favorable circumstances this can occur at temperatures as much as 50 degrees C below the optical melting point. Translational diffusion allows 7N14 molecules to coalesce into impurity pools of high spin label concentration. Two other spin labels, 2N3 and 14N27, do not show a tendency to form such impurity pools. While 2N3 undergoes rapid tumbling at temperatures far below the melting point of the tristearin matrix, the molecules remain in an isolated state with no evidence of spin exchange. 14N27 is restricted in rotational motion in the solid matrix and also does not form impurity pools.

Reconstitution of a calcium pump using defined membrane components

A (Mg(2+) + Ca(2+))ATPase (ATP phosphohydrolase, EC 3.6.1.3) has been purified from sarcoplasmic reticulum using a single step centrifugation procedure. The preparation is >95% pure by weight and contains only 25-30% of the lipid associated with the enzyme in native sarcoplasmic reticulum. The purified enzyme is unable to accumulate Ca(2+). Using a sedimentation-substitution technique, >98% of the lipid associated with the purified enzyme can be replaced by dioleoyl lecithin without grossly affecting the ATPase activity of the enzyme. The Ca(2+) pump can be restored to this dioleoyl lecithin-substituted enzyme by addition of excess sarcoplasmic reticulum lipids in the presence of cholate. Removal of the cholate by dialysis generates a system which accumulates Ca(2+) at a rate and to a level comparable to native sarcoplasmic reticulum. Significant reconstitution of the Ca(2+) pump can also be achieved using excess dioleoyl lecithin, but since the full expression of the capacity to accumulate Ca(2+) requires the presence of oxalate, these vesicles would appear to be more leaky than those reconstituted with an excess of sarcoplasmic reticulum lipids. Of about 90 lipid molecules which are associated with one molecule of ATPase in native sarcoplasmic reticulum, an average of less than one lipid molecule remains in these reconstituted systems. We have therefore achieved a fully functional Ca(2+) pump containing essentially a single protein and exogenous lipid.

Viscosity of cellular protoplasm

The protoplasmic viscosity was studied by using a small spin label having high permeability and broad solubility properties and nickel chloride as an extracellular spin-subtracting agent to localize signal inside cells. The viscosity is variable and in some cells is many times that of water or phospholipids, suggesting that lateral diffusion in biological membranes is important to cell function.

Direct evidence for a colchicine-induced impairment in the mobility of membrane components

Freeze-etch electron microscopy was used to show that colchicine interacts with membranes of the ciliate protozoan Tetrahymena pyriformis. Colchicine impairs the temperature-induced translational and vertical mobility of the membrane-intercalating particles of the freeze-fractured alveolar membranes lying just below the plasma membranes.

Membrane structure: some general principles

The arrangement of lipids and some proteins in the erythrocyte membrane has been discussed. The conclusions from this are listed here as a set of general guidelines for the structure of membranes of higher organisms: some of these rules may be wrong. But at this stage it seems useful to sharpen our thoughts in this way and thereby focus attention on various specific points. 1) The basis of a membrane is a lipid bilayer with (i) choline phospholipids and glycolipids in the external half and (ii) amino (and possibly some choline) phospholipids in the cytoplasmic half. There is effectively no lipid exchange across the bilayer (unless enzymatically catalyzed) (68). 2) Some proteins extend across the bilayer. Where this is so, they will in general have carbohydrate on their surface remote from the cytoplasm. This carbohydrate may prevent the protein diffusing out of the membrane into the cytoplasm; it acts as a lock on the protein. 3) Just as lipids do not flip-flop, proteins do not rotate across the membrane. Lateral motion or rotation of lipids and proteins in the plane of the bilayer may be expected. 4) Most membrane protein is associated with the inner, cytoplasmic, urface of the membrane. Proteins are not usually associated exclusively with the outer half of the lipid bilayer. 5) Membrane proteins are a special class of cytoplasmic proteins, not of secreted proteins.

Lateral phase separations in membrane lipids and the mechanism of sugar transport in Escherichia coli

Changes in slope of Arrhenius plots for transport can, in some instances, be detected at two different temperatures for cells that have a relatively simple fatty-acid composition in the membrane lipids. These characteristic temperatures correlate with the characteristic temperatures that define changes of state in membrane phospholipids as revealed by the paramagnetic resonance of the spin label TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl). The higher of these characteristic temperatures is that at which the formation of solid patches of membrane lipids is first detected. The lower is the end point of the course of lateral phase separations, at which all the membrane lipids are in a solid phase. For cells enriched for elaidic acid, the rate of transport increase by as much as 2-fold as the temperature is decreased by less than 1 degrees , at the higher characteristic temperature. At this characteristic temperature, lateral phase separations begin in the membrane phospholipids. This is also the temperature where one predicts a striking increase in the lateral compressibility of the membrane lipids. These data are thus interpreted to indicate that a component of the transport system vertically penetrates one or both monolayer faces of the membrane during transport, or that some other event involving the lateral compression of the phospholipids is important for transport.

Fluidity of the surface of cultured muscle fibers. Rapid lateral diffusion of marked surface antigens

Fluorescent antibody fragments of anti-muscle plasma membrane antibody bound as small fluorescent spots when applied by micropipetting to cultured myotubes. The spots were observed to enlarge with time. The rate of enlargement of fluorescent spots was greater when fragments were applied than when divalent antibody was used. It was also greater at 23 degrees -25 degrees C than at 0 degrees -4 degrees C. With glutaraldehyde-fixed cells no increase in the size of the spots was seen. The observations are consistent with the spread of fluorescent spots due to diffusion of surface protein antigens within the plane of a fluid membrane. From measurements of spot size against time, a diffusion constant of 1-3 x 10(-9) cm(2) s(-1) can be calculated for muscle plasma membrane proteins of mol wt approximately 200,000. This value is consistent with other observations on the diffusion of surface antigens and of labeled lipid molecules in synthetic and natural membranes.

Fusion of phospholipid vesicles with viable Acholeplasma laidlawii

When vesicles of dipalmitoyl phosphatidylcholine produced by sonication are mixed with Acholeplasma laidlawii in neutral buffer, a phenomenon occurs that is most readily explained in terms of fusion of lipid vesicle membrane with the mycoplasma cytoplasmic membrane. The mycoplasma can readily accumulate large quantities of the foreign lipid without loss of viability. The added lipids do not remain in patches but diffuse laterally throughout the mycoplasma membrane.