The Composition and Architecture of Membranes:- Lipids and Proteins Diffuse Laterally in the Bilayer

Individual lipid molecules can move laterally in the plane of the membrane by changing places with neighboring lipid molecules (Fig. 11–16c). A molecule in one mono layer, or leaflet, of the lipid bilayer—the outer leaflet of the erythrocyte plasma membrane, for example—can diffuse laterally so fast that it circumnavigates the erythrocyte in seconds. This rapid lateral diffusion within the plane of the bilayer tends to randomize the positions of individual molecules in a few seconds. Lateral diffusion can be shown experimentally by attaching fluorescent probes to the head groups of lipids and using fluorescence microscopy to follow the probes over time (Fig. 11–17). In one technique, a small region (5 m2) of a cell surface with fluorescence-tagged lipids is bleached by intense laser radiation so that the irradiated patch no longer fluoresces when viewed in the much dimmer light of the fluorescence microscope. However, within milliseconds, the region recovers its fluorescence as unbleached lipid molecules diffuse into the bleached patch and bleached lipid molecules diffuse away from it. The rate of fluorescence recovery after photobleaching, or FRAP, is a measure of the rate of lateral diffusion of the lipids. Using the FRAP technique, researchers have shown that some membrane lipids diffuse laterally by up to 1 m/s. Another technique, single particle tracking, allows one to follow the movement of a single lipid molecule in the plasma membrane on a much shorter time scale. Results from these studies confirm the rapid lateral diffusion within small, discrete regions of the cell surface and show that movement from one such region to a nearby region is inhibited; lipids behave as though corralled by fences that they can occasionally jump (Fig. 11–18). Many membrane proteins seem to be afloat in a sea of lipids. Like membrane lipids, these proteins are free to diffuse laterally in the plane of the bilayer and are in constant motion, as shown by the FRAP technique with fluorescence-tagged surface proteins. Some membrane proteins associate to form large aggregates (“patches”) on the surface of a cell or organelle in which individual protein molecules do not move relative to one another; for example, acetylcholine receptors (see Fig. 11–51) form dense patches on neuron plasma membranes at synapses. Other membrane proteins are anchored to internal structures that prevent their free diffusion. In the erythrocyte membrane, both glycophorin and the chloride-bicarbonate exchanger (p. 395) are tethered to spectrin, a filamentous cytoskeletal protein (Fig. 11–19). One possible explanation for the pattern of lateral diffusion of lipid molecules shown in Figure 11–18 is that membrane proteins immobilized by their association with spectrin are the “fences” that define the regions of relatively unrestricted lipid motion.

FIGURE 11–17 Measurement of lateral diffusion rates of lipids by fluorescence recovery after photobleaching (FRAP). The lipids in the outer leaflet of the plasma membrane are labeled by reaction with a membrane-impermeant fluorescent probe (red), so the surface is uniformly labeled when viewed with a fluorescence microscope. A small area is bleached by irradiation with an intense laser beam, leaving that area nonfluorescent. With the passage of time, labeled lipid molecules diffuse into the bleached region, and it again becomes fluorescent. From the time course of fluorescence return to this area, the diffusion coefficient for the labeled lipid is determined. The rates are typically high; a lipid moving at this speed could circumnavigate E. coli in one second. (The FRAP method can also be used to measure the lateral diffusion of membrane proteins.)

FIGURE 11–18 Hop diffusion of individual lipid molecules. The motion of a single fluorescent lipid molecule in a cell surface is recorded on video by fluorescence microscopy, with a time resolution of 25 µs (equivalent to 40,000 frames/s). The track shown here represents a molecule followed for 56 ms (a total of 2,250 frames); the trace begins in the purple area and continues through blue, green, and orange. The pattern of movement indicates rapid diffusion within a confined region (about 250 nm in diameter, shown by a single color), with occasional hops into an adjoining region. This finding suggests that the lipids are corralled by molecular fences that they occasion ally jump.

FIGURE 11–19 Restricted motion of the erythrocyte chloride bicarbonate exchanger and glycophorin. The proteins span the mem brane and are tethered to spectrin, a cytoskeletal protein, by another protein, ankyrin, limiting their lateral mobilities. Ankyrin is anchored in the membrane by a covalently bound palmitoyl side chain (see Fig. 11–14). Spectrin, a long, filamentous protein, is cross-linked at junctional complexes containing actin. A network of cross-linked spectrin molecules attached to the cytoplasmic face of the plasma membrane stabilizes the membrane against deformation. This network of anchored membrane proteins may be the “corral” suggested by the experiment shown in Figure 11–18; the lipid tracks shown here are con fined to subregions defined by the tethered membrane proteins.

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مواضيع ذات صلة

Membrane Dynamics:- Transbilayer Movement of Lipids Requires Catalysis

Membrane Dynamics:- Acyl Groups in the Bilayer Interior Are Ordered to Varying Degrees

The Composition and Architecture of Membranes:- Covalently Attached Lipids Anchor Some Membrane Proteins

The Composition and Architecture of Membranes:- The Topology of an Integral Membrane Protein Can Be Predicted from Its Sequence

The Composition and Architecture of Membranes: -Integral Proteins Are Held in the Membrane by Hydrophobic Interactions with Lipids

The Composition and Architecture of Membranes:- All Biological Membranes Share Some Fundamental Properties

The Composition and Architecture of Membranes:- Each Type of Membrane Has Characteristic Lipids and Proteins

Working with Lipids:- Mass Spectrometry Reveals Complete Lipid Structure

Working with Lipids:- Gas-Liquid Chromatography Resolves Mixtures of Volatile Lipid Derivatives

Working with Lipids:- Adsorption Chromatography Separates Lipids of Different Polarity

Working with Lipids:- Lipid Extraction Requires Organic Solvents

Lipids as Signals, Cofactors, and Pigments: -Plants Use Phosphatidylinositols, Steroids, and Eicosanoidlike Compounds in Signaling