Lateral movements of membrane

Edidin, M., Kuo, S. C., and Sheetz, M. P. (1991) Lateral movements of membrane glycoproteins restricted by dynamic cytoplasmic barriers. Science 254,1379-1382. 

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... 

Another technique, referred to as fluorescence recovery after photobleaching (FRAP), is also used to observe lateral diffusion. Cell plasma membranes are uniformly labeled with a fluorescent marker. Using a laser beam, the fluorescence in a small area is destroyed (or bleached ). Using video equipment, the lateral movement of membrane components into and out of the bleached area can be tracked as a function of time. 

The fluidity of a membrane is difficult to define but it is known to increase the rate of lateral movement of proteins in the membrane and the activity of some membrane proteins, such as ion channels and transporters of fuels (Chapter 5). Fluidity depends, in part, upon the amount and the degree of unsaturation of the fatty acids that are present... 

Membrane fluidity regulates lateral movement of proteins and lipids in the bilayer. 

Note, however that the concepts about the lipid membrane as the isotropic, structureless medium are oversimplified. It is well known [19, 190] that the rates and character of the molecular motion in the lateral direction and across the membrane are quite different. This is true for both the molecules inserted in the lipid bilayer and the lipid molecules themselves. Thus, for example, while it still seems possible to characterize the lateral movement of the egg lecithin molecule by the diffusion coefficient D its movement across the membrane seems to be better described by the so-called flip-flop mechanism when two lipid molecules from the inner and outer membrane monolayers of the vesicle synchronously change locations with each other [19]. The value of D, = 1.8 x 10 8 cm2 s 1 [191] corresponds to the time of the lateral diffusion jump of lecithin molecule, Le. about 10 7s. The characteristic time of flip-flop under the same conditions is much longer (about 6.5 hours) [19]. The molecules without long hydrocarbon chains migrate much more rapidly. For example for pyrene D, = 1.4x 10 7 cm2 s1 [192]. 

Cry toxins do not form spontaneous oligomers in solution in vitro [115], suggesting that oligomer formation in vivo may occur either after the toxin binds or by lateral movement of monomers in the membrane. Gomez et al. [61] recently showed that toxin binding to receptor promotes proteolytic activation of CrylAb by removing helices al and o2 of domain I, a prerequisite for toxin oligomerization. 

Morrot G, Cribier S, Devaux PF, Gcldwcrlh D, Davoust J, Bureau JF, et al. Asymmetric lateral movement of phospholipids in the human erythrocyte membrane. Proc Natl Acad Sci USA 1986 83 6863-6867. 

Without a transmembrane domain, the lateral movement of GPI-anchored proteins is not restricted by interactions with the cytoskeleton. Many GPI-anchored proteins are receptors or adhesion molecules and freedom of movement in the membrane may be advantageous for the interactions with their ligands. GPI anchors, then, may impart an increased lateral mobility to their linked proteins. 

A EXPERIMENTAL FIGURE 5-6 Fluorescence recovery after photobleaching (FRAP) experiments can quantify the lateral movement of proteins and lipids within the plasma membrane, (a) Experimental protocol. Step H Cells are first labeled with a fluorescent reagent that binds uniformly to a specific membrane lipid or protein. Step B A laser light is then focused on a small area of the surface, irreversibly bleaching the bound reagent and thus reducing the fluorescence in the illuminated area. Step B In time, the fluorescence of the bleached patch increases as unbleached fluorescent surface molecules diffuse into it and bleached ones diffuse outward. The extent of recovery of fluorescence in the bleached patch is... 

The lateral movements of specific plasma-membrane proteins and lipids can be quantified by a technique called fluo rescence recovery after photobleaching (FRAP). With this method, described In Figure 5-6, the rate at which membrane llpid or protein molecules move—the diffusion coefficient— can be determined, as well as the proportion of the molecules that are laterally mobile. 

Teissie and coworkers detected rapid lateral movement of protons on a phospholipid monolayer-water interface by a number of measurements fluorescence from a pH indicator dye near the membrane surface, electrical surface conductance, and surface potentiaL These investigators found that the conduction of protons along the surface is considerably faster than proton conduction in the bulk phase (2 to 3 min versus 40 min for a comparable distance in their measurement setup). This novel conduction mechanism is proton-specific, as confirmed by a radioactive electrode measurement as well as by replacement with deuterated water. It is a consequence of cooperativity between neighboring phospholipid molecules the conduction mechanism disappears when phospholipid molecules are not in contact with each other. 

In 1972, S. J. Singer and G. L. Nicolson proposed the fluid mosaic model for membrane structure, which suggested that membranes are dynamic structures composed of proteins and phospholipids. In this model, the phospholipid bilayer is a fluid matrix, in essence, a two-dimensional solvent for proteins. Both lipids and proteins are capable of rotational and lateral movement. 

Just how fast can proteins move in a biological membrane Many membrane proteins can move laterally across a membrane at a rate of a few microns per minute. On the other hand, some integral membrane proteins are much more restricted in their lateral movement, with diffusion rates of about 10 nm/sec or even slower. These latter proteins are often found to be anchored to the cytoskeleton (Chapter 17), a complex latticelike structure that maintains the cell s shape and assists in the controlled movement of various substances through the ceil. 

---