Lateral diffusion, of membrane proteins

Frye and Edidin s experiment demonstrating lateral diffusion of membrane proteins, (a) Proteins on the plasma membranes of human and mouse cells were labeled with dyes that fluoresced at different wavelengths. (b) The two populations of cells were mixed and infected with a virus that causes cells to fuse. At short times after mixing, red and green fluorescence from the original cells was seen in separate parts of the membranes of the fused cells, (c) Within about 30 min, the two populations of proteins had become intermingled over the entire surface. 

Transformed and tumorigenic cells are different from normal cell lines in that they are not usually anchorage dependent. They exhibit a spherical shape, increased life span and lateral diffusivity of membrane proteins, decreased cell receptors and membrane proteins, and a different cytoskeletal structure. The decrease in the concentration of the cell adhesion molecules in the cell membrane of these cells causes the anchorage independence. Transformed cell lines also do not assemble a normal ECM. It is important to note that some cell lines (e.g., lymphocytes) that are normally anchorage dependent can be induced and then adapted to become anchorage independent. This is of tremendous importance to recombinant protein production as discussed later, because the scale-up of suspension cultures is easier than that of anchorage-dependent cell lines. 

Because the lipid bilayer is fluid, there can be rapid lateral diffusion of membrane proteins through the lipid bilayer but membrane proteins, like membrane lipids, do not "flip-flop" across the membrane or turn in the membrane like a revolving door of a department store. 

Cherry, R.J., 1979, Rotational and lateral diffusion of membrane proteins, Biochim. Biophys. Acta, 559 289. 

A continuous lipidic cubic phase is obtained by mixing a long-chain lipid such as monoolein with a small amount of water. The result is a highly viscous state where the lipids are packed in curved continuous bilayers extending in three dimensions and which are interpenetrated by communicating aqueous channels. Crystallization of incorporated proteins starts inside the lipid phase and growth is achieved by lateral diffusion of the protein molecules to the nucleation sites. This system has recently been used to obtain three-dimensional crystals 20 x 20 x 8 pm in size of the membrane protein bacteriorhodopsin, which diffracted to 2 A resolution using a microfocus beam at the European Synchrotron Radiation Facility. 

Alkyl chain heterogeneities cause cell membrane bilayers to remain in the fluid state over a broad temperature range. This permits rapid lateral diffusion of membrane lipids and proteins within the plane of the bilayer. The lateral diffusion rate for an unconstrained phospholipid in a bilayer is of the order of 1 mm2 s 1 an integral membrane protein such as rhodopsin would diffuse 40nm2 s 1. 

Phospholipid molecules in the plasma membrane diffuse rapidly enough to go from one end of an average-sized animal cell to the other in a few minutes. In a bacterial cell, such a trip would take only a few seconds. Integral membrane proteins move more slowly than phospholipids, as we expect in view of their greater mass. Diffusion of membrane proteins plays essential roles in many biochemical processes, including the cellular uptake of lipoproteins (chapter 18), responses of cells to hormones (chapter 24), immunological reactions (supplement 3), vision (supplement 2), and the transport of nutrients and ions. As we see in a later section, however, some membrane proteins cannot move about rapidly because they are attached to cytoskeletal scaffolds. 

Chen Y, Lagerholm BC, Yang B, Jacobson K. Methods to measure the lateral diffusion of membrane lipids and proteins. Methods 2006 39 147-153. 

Biological membranes are not rigid, static structures. On the contrary, lipids and many membrane proteins are constantly in lateral motion, a process called lateral diffusion. The rapid lateral movement of membrane proteins has been visuali ied by means of fluorescence microscopy using the technique of fluorescence recovery after photohleaching (FRAP Figure 12.29). First, a cell-surface component is specifically labeled with a fluorescent... 

Zhang, F., Crise, B., Su, B., Rose, J. K., Bothwell, A., and Jacobson, K., Lateral diffusion of membrane-spanning glycosylphosphatidylinositol-linked proteins Towards establishing rules governing the lateral mobility of membrane proteins. J. Cell Biol. 115, 75 (1991). 

For cell membranes to be effective permeability barriers, they must be flexible and allow relatively free motion of proteins that are embedded in or linked to them. Integral membrane proteins often diffuse laterally, and many receptor-mediated solute-uptake pathways involve endocytosis that entails phospholipid rearrangement in the membrane. Hormone secretion and other protein trafficking processes involve exo-cytosis and it is usual for membrane vesicles to fuse with each other in a process that also involves the lateral diffusion of membrane constituents. The activity of some receptors is strongly linked to the extent of fluidity of the membrane around them. 

FIGURE 1. Schematic representation of the fluid-mosaic model of a cell membrane showing protein molecules incorporated into a lipid bilayer structure. Lateral diffusion of the proteins and lipid molecules occurs, but the lipids very rarely migrate from one side of the membrane to the other. A transmembrane electrical field arises from the action of vectorial ion pumps (ATPase proteins) in producing ionic concentration differences across the membrane, and from the presence of membrane surface charges. Surface redox reactions may affect this membrane field. 

Edidin, M., Zagyansky, Y., and Lardner, T. (1976). Measurement of membrane protein lateral diffusion in single cells. Science 191, 466—468. 

The fluidity of lipid bilayers permits dynamic interactions among membrane proteins. For example, the interactions of a neurotransmitter or hormone with its receptor can dissociate a transducer protein, which in turn will diffuse to interact with other effector proteins (Ch. 19). A given effector protein, such as adenylyl cyclase, may respond differently to different receptors because of mediation by different transducers. These dynamic interactions require rapid protein diffusion within the plane of the membrane bilayer. Receptor occupation can initiate extensive redistribution of membrane proteins, as exemplified by the clustering of membrane antigens consequent to binding bivalent antibodies [8]. In contrast to these examples of lateral mobility, the surface distribution of integral membrane proteins can be fixed by interactions with other proteins. Membranes may also be partitioned into local spatial domains consisting of networks... 

ACTIN REGULATORY PROTEINS Lateral diffusibility of molecules within membranes,... 

At the present time, the rates of lateral diffusion of phospholipids and membrane proteins in the solid phase of pure phospholipids is not known. It is hoped that such diffusion constants can be obtained by one of the transient methods mentioned earlier. It is likely that these diffusion rates will be found to be quite low. 

As indicated in my report, we now know the rates of lateral diffusion of phospholipids in lipid bilayers in the fluid state, and in a few cases the rates of lateral diffusion of proteins in fluid lipids are also known. At the present time nothing is known about the rates of lateral diffusion of phospholipids in the crystalline, solid phases of the substances. As mentioned in my report, there are reasons to suspect that the rates of lateral diffusion of phospholipids in the solid solution crystalline phases of binary mixtures of phospholipids may be appreciable on the experimental time scale. Professor Ubbelohde may well be correct in pointing out the possibility of diffusion caused by defects. However, such defects, if present, apparently do not lead to significant loss of the membrane permeability barrier, except at domain boundaries. 

Lipids and proteins can diffuse laterally within the plane of the membrane, but this mobility is limited by interactions of membrane proteins with internal cytoskeletal structures and interactions of lipids with lipid rafts. One class of lipid rafts consists of sphingolipids and cholesterol with a subset of membrane proteins that are GPI-linked or attached to several long-chain fatty acyl moieties. 

The cytoskeletons of other eukaryotic cells typically include both microtubules and microfilaments, which consist of long, chainlike oligomers of the proteins tubulin and actin, respectively. Bundles of microfilaments often lie just underneath the plasma membrane (fig. 17.22). They participate in processes that require changes in the shape of the cell, such as locomotion and phagocytosis. In some cells, cytoskeletal microfilaments appear to be linked indirectly through the plasma membrane to peripheral proteins on the outer surface of the cell (fig. 17.23). Among the cell surface proteins connected to this network is fibronectin, a glycoprotein believed to play a role in cell-cell interactions. The lateral diffusion of fibronectin is at least 5,000 times slower than that of freely diffusible membrane proteins. 

A. H. Stolpen, D. E. Golan, and J. S. Pober, Tumor necrosis factor and immune interferon act in concert to slow the lateral diffusion of proteins and lipids in human endothelial cell membranes, J. Cell Biol. 707 781-789 (1988). 

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