Integral membrane proteins can serve as receptors
All communication between a cell and its environment must involve or at least pass through the plasma membrane. For the purposes of this discussion, we define communication rather broadly as the exchange of any signal between the cell and its surroundings. Except for lipid-soluble signaling molecules such as steroid hormones, essentially all communication functions served by the plasma membrane occur via membrane proteins. From an engineering perspective, membrane proteins are perfectly situated to transmit signals because they form a single, continuous link between the two compartments that are separated by the membrane.
Ligand-binding receptors comprise the group of transmembrane proteins that perhaps most clearly illustrate the concept of transmembrane signaling (Fig. 2-7A). For water-soluble hormones such as epinephrine to influence cellular behavior, their presence in the ECF compartment must be made known to the various intracellular mechanisms whose behaviors they modulate. The interaction of a hormone with the extracellular portion of the hormone receptor, which forms a high-affinity binding site, produces conformational changes within the receptor protein that extend through the membrane-spanning domain to the intracellular domain of the receptor. As a consequence, the intracellular domain either becomes enzymatically active or can interact with cytoplasmic proteins that are involved in the generation of so-called second messengers. Either mechanism completes the transmission of the hormone signal across the membrane. The transmembrane disposition of a hormone receptor thus creates a single, continuous communication medium that is capable of conveying, through its own structural modifications, information from the environment to the cellular interior. The process of transmembrane signal transduction is discussed in Chapter 3.
FIGURE 2-7 Integral membrane proteins that transmit signals from the outside to the inside of a cell. A, The ligand may be a hormone, a growth factor, a neurotransmitter, an odorant, or another local mediator. B, An integrin is an adhesion molecule that attaches the cell to the extracellular matrix.
Integral membrane proteins can serve as adhesion molecules
Cells can also exploit integral membrane proteins as adhesion molecules that form physical contacts with the surrounding extracellular matrix (i.e., cell-matrix adhesion molecules) or with their cellular neighbors (i.e., cell-cell adhesion molecules). These attachments can be extremely important in regulating the shape, growth, and differentiation of cells. The nature and extent of these attachments must be communicated to the cell interior so that the cell can adapt appropriately to the physical constraints and cues that are provided by its immediate surroundings. Numerous classes of transmembrane proteins are involved in these communication processes. The integrins are examples of matrix receptors or cell-matrix adhesion molecules. They comprise a large family of transmembrane proteins that link cells to components of the extracellular matrix (e.g., fibronectin, laminin) at adhesion plaques (see Fig. 2-7B). These linkages produce conformational changes in the integrin molecules that are transmitted to their cytoplasmic tails. These tails, in turn, communicate the linkage events to various structural and signaling molecules that participate in formulating a cell's response to its physical environment.
In contrast to matrix receptors, which attach cells to the extracellular matrix, several enormous superfamilies of cell-cell adhesion molecules attach cells to each other. These cell-cell adhesion molecules include the Ca2+-dependent cell adhesion molecules (cadherins) and Ca2+-independent neural cell adhesion molecules (N-CAMs). The cadherins are glycoproteins (i.e., proteins with sugars attached) with one membrane-spanning segment and a large extracellular domain that binds Ca2+. The N-CAMs, on the other hand, generally are members of the immunoglobulin superfamily. The two classes of cell-cell adhesion molecules mediate similar sorts of transmembrane signals that help organize the cytoplasm and control gene expression in response to intercellular contacts. Some cell-cell adhesion molecules belong to the GPI-linked class of membrane proteins (see p. 13). These polypeptides lack a transmembrane and cytoplasmic tail. Interactions mediated by this unique class of adhesion molecules may be communicated to the cell interior via lateral associations with other membrane proteins.
Adhesion molecules orchestrate processes that are as diverse as the directed migration of immune cells and the guidance of axons in the developing nervous system. Loss of cell-cell and cell-matrix adhesion is a hallmark of metastatic tumor cells.
Integral membrane proteins can carry out the transmembrane movement of water-soluble substances
As noted above, a pure phospholipid bilayer does not have the permeability properties that are normally associated with animal cell plasma membranes. Pure phospholipid bilayers also lack the ability to transport substances uphill (i.e., against electrochemical gradients; p. 105). Transmembrane proteins endow biological membranes with these capabilities. Ions and other membrane-impermeable substances can cross the bilayer with the assistance of transmembrane proteins that serve as pores, channels, carriers, and pumps. Pores and channels serve as conduits that allow water, specific ions, or even very large proteins to flow passively through the bilayer. Carriers can either facilitate the transport of a specific molecule across the membrane or couple the transport of a molecule to that of other solutes. Pumps use the energy that is released through the hydrolysis of ATP to drive the transport of substances into or out of cells against energy gradients. Each of these important classes of proteins is discussed in Chapter 5.
Channels, carriers, and pumps succeed in allowing hydrophilic substances to cross the membrane by creating a hydrophilic pathway in the bilayer. Previously, we asserted that membrane-spanning segments are as hydrophobic as the fatty acids that surround them. How is it possible for these hydrophobic membrane-spanning domains to produce the hydrophilic pathways that permit the passage of ions through the membrane? The solution to this puzzle appears to be that the α helices that make up these membrane-spanning segments are amphipathic. That is, they possess both hydrophobic and hydrophilic domains.
For each α helix, the helical turns produce alignments of amino acids that are spaced at regular intervals in the sequence. Thus, it is possible to align all the hydrophilic or hydrophobic amino acids along a single edge of the helix. In amphipathic helices, hydrophobic amino acids alternate with hydrophilic residues at regular intervals of approximately three or four amino acids (recall that there are ~3.6 amino acids per turn of the helix). Thus, as the helices pack together, side by side, the resultant membrane protein has distinct hydrophilic and hydrophobic surfaces. The hydrophobic surfaces of each helix will face either the membrane lipid or the hydrophobic surfaces of neighboring helices. Similarly, the hydrophilic surfaces of each helix will face a common central pore through which water-soluble substances can move. Depending on how the protein regulates access to this pore, the protein could be a channel, a carrier, or a pump. The mix of hydrophilic amino acids that line the pore presumably determines, at least in part, the nature of the substances that the pore can accommodate. In some instances, the amphipathic helices that line the pore are contributed by several distinct proteins—or subunits—that assemble into a single multimeric complex. Figure 2-8 shows an example of a type of K+ channel discussed on page 184. This channel is formed by the apposition of four identical subunits, each of which has six membrane-spanning segments. The pore of this channel is created by the amphipathic helices as well as by short, hydrophilic loops (P loops) contributed by each of the four subunits.
FIGURE 2-8 Amphipathic α helices interacting to form a channel through the cell membrane. This is an example of a potassium channel.
Integral membrane proteins can also be enzymes
Ion pumps are actually enzymes. They catalyze the hydrolysis of ATP and use the energy released by that reaction to drive ion transport. Many other classes of proteins that are embedded in cell membranes function as enzymes as well. Membrane-bound enzymes are especially prevalent in the cells of the intestine, which participate in the final stages of nutrient digestion and absorption (see pp. 916–918). These enzymes—located on the side of the intestinal cells that faces the lumen of the intestine—break down small polysaccharides into single sugars, or break down polypeptides into shorter polypeptides or amino acids, so that they can be imported into the cells. By embedding these enzymes in the plasma membrane, the cell can generate the final products of digestion close to the transport proteins that mediate the uptake of these nutrient molecules. This theme is repeated in numerous other cell types. Thus, the membrane can serve as an extremely efficient two-dimensional reaction center for multistep processes that involve enzymatic reactions or transport.
Many of the GPI-linked proteins are enzymes. Several of the enzymatic activities that are classically thought of as extracellular markers of the plasma membrane, such as alkaline phosphatase, 5′-nucleotidase, and carbonic anhydrase IV (see p. 828), are anchored to the external leaflet of the bilayer by covalent attachment to a GPI.
Integral membrane proteins can participate in intracellular signaling
Some integral proteins associate with the cytoplasmic surface of the plasma membrane by covalently attaching to fatty acids or prenyl groups that in turn intercalate into the lipid bilayer (see Fig. 2-5F). The fatty acids or prenyl groups act as hydrophobic tails that anchor an otherwise soluble protein to the bilayer. These proteins are all located at the intracellular leaflet of the membrane bilayer and often participate in intracellular signaling pathways. The family of lipid-linked proteins includes the small and heterotrimeric GTP-binding proteins, kinases, and oncogene products (see Chapter 3). Many of these proteins are involved in relaying the signals that are received at the cell surface to the effector machinery within the cell interior. Their association with the membrane, therefore, brings these proteins close to the cytoplasmic sides of receptors that transmit signals from the cell exterior across the bilayer. The medical relevance of this type of membrane association is beginning to be appreciated. For example, denying certain oncogene products their lipid modifications—and hence their membrane attachment—eliminates their ability to induce tumorigenic transformation.
Peripheral membrane proteins participate in intracellular signaling and can form a submembranous cytoskeleton
Peripheral membrane proteins attach loosely to the lipid bilayer but are not embedded within it (see p. 13). Their association with the membrane can take one of two forms. First, some proteins interact via ionic interactions with phospholipid head groups. Many of these head groups are positively or negatively charged and thus can participate in salt bridges with adherent proteins.
For a second group of peripheral membrane proteins, attachment is based on the direct binding of peripheral membrane proteins to the extracellular or cytoplasmic surfaces of integral membrane proteins (see Fig. 2-5A). This form of attachment is epitomized by the cytoskeleton. For instance, the cytoplasmic surface of the erythrocyte plasma membrane is in close apposition to a dense meshwork of interlocking protein strands known as the subcortical cytoskeleton. It consists of a long, fibrillar molecule called spectrin, short polymers of the cytoskeletal protein actin, and other proteins, including ankyrin and band 4.1 (Fig. 2-9).
FIGURE 2-9 Attachments of the cell membrane to the submembranous cytoskeleton in red blood cells. Integral membrane proteins form the bridges that link the cell membrane to the interlocking system of proteins that comprise the subcortical cytoskeleton.
Two closely related isoforms of spectrin (α and β) form dimers, and two of these dimers assemble head to head with one another to form spectrin heterotetramers. The tail regions of spectrin bind the globular protein band 4.1, which in turn can bind to actin fibrils. Each actin fibril can associate with more than one molecule of band 4.1 so that, together, spectrin, actin, and band 4.1 assemble into an extensive interlocking matrix. The protein known as ankyrin binds to spectrin as well as to the cytoplasmic domain of band 3, the integral membrane protein responsible for transporting Cl– and ions across the erythrocyte membrane. Thus, ankyrin is a peripheral membrane protein that anchors the spectrin-actin meshwork directly to an integral membrane protein of the erythrocyte.
The subcortical cytoskeleton provides the erythrocyte plasma membrane with strength and resilience. People who carry mutations in genes encoding components of the cytoskeleton have erythrocytes lacking the characteristic biconcave disk shape. These erythrocytes are extremely fragile and are easily torn apart by the shear stresses (see p. 415) associated with circulation through capillaries. It would appear, therefore, that the subcortical cytoskeleton forms a scaffolding of peripheral membrane proteins whose direct attachment to transmembrane proteins enhances the bilayer's structural integrity.
The subcortical cytoskeleton is not unique to erythrocytes. Numerous cell types, including neurons and epithelial cells, have submembranous meshworks that consist of proteins very similar to those first described in the erythrocyte. In addition to band 3, transmembrane proteins found in a wide variety of cells (including ion pumps, ion channels, and cell adhesion molecules) bind ankyrin and can thus serve as focal points of cytoskeletal attachment. In polarized cells (e.g., neurons and epithelial cells), the subcortical cytoskeleton appears to play a critically important role in organizing the plasma membrane into morphologically and functionally distinct domains.