A cell can also create membrane domains without using intercellular junctions. As we already discussed, regulated protein–protein interactions in membranes are thought to create nanoscale raft domains that function in signaling and membrane trafficking. A more extreme example is seen in the mammalian spermatozoon, a single cell that consists of several structurally and functionally distinct parts covered by a continuous plasma membrane. When a sperm cell is examined by immunofluorescence microscopy with a variety of antibodies, each of which reacts with a specific cell-surface molecule, the plasma membrane is found to consist of at least three distinct domains (Figure 10–35). Some of the membrane molecules are able to diffuse freely within the confines of their own domain. The molecular nature of the “fence” that prevents the molecules from
Figure 10–34 How membrane molecules can be restricted to a particular membrane domain.
In this drawing of an epithelial cell, protein A (in the apical domain of the plasma membrane) and protein B (in the basal and lateral domains) can diffuse laterally in their own domains but are prevented from entering the other domain, at least partly by the specialized cell–cell junction called a tight junction. lipid molecules in the outer (extracellular) monolayer of the plasma membrane are likewise unable to diffuse between the two domains; lipids in the inner (cytosolic) monolayer, however, are able to do so (not shown). the basal lamina is a thin mat of extracellular matrix that separates epithelial sheets from other tissues (discussed in chapter 19).
leaving their domain is not known. Many other cells have similar membrane fences that confine membrane protein diffusion to certain membrane domains. The plasma membrane of nerve cells, for example, contains a domain enclosing the cell body and dendrites, and another enclosing the axon; it is thought that a belt of actin filaments tightly associated with the plasma membrane at the cell-body–axon junction forms part of the barrier.
Figure 10–36 shows four common ways of immobilizing specific membrane proteins through protein–protein interactions.
the cortical cytoskeleton gives Membranes Mechanical Strength and restricts Membrane protein Diffusion
As shown in Figure 10–36B and C, a common way in which a cell restricts the lateral mobility of specific membrane proteins is to tether them to macromolecular assemblies on either side of the membrane. The characteristic biconcave shape of a red blood cell (Figure 10–37), for example, results from interactions of its plasma membrane proteins with an underlying cytoskeleton, which consists mainly of a meshwork of the filamentous protein spectrin. Spectrin is a long, thin, flexible rod about 100 nm in length. As the principal component of the red cell cytoskeleton, it maintains the structural integrity and shape of the plasma membrane, which is the red cell’s only membrane, as the cell has no nucleus or other organelles. The spectrin cytoskeleton is riveted to the membrane through various membrane proteins. The final result is a deformable, netlike meshwork that covers the entire cytosolic surface of the red cell membrane (Figure 10–38). This spectrin-based cytoskeleton enables the red cell to withstand the stress on its membrane as it is forced through narrow capillaries. Mice and humans with genetic abnormalities in spectrin are anemic and have red cells that are spherical (instead of concave) and fragile; the severity of the anemia increases with the degree of spectrin deficiency.