Biochemistry, 4th Edition P30 pot

9.2 What Are the Structure and Chemistry of Membrane Proteins? 253 charged groups (amino or guanidinium) extending beyond to associate with nega- tively charged phosphate groups. This behavior, with the side chain pointing up out of the membrane core, has been termed snorkeling (Figure 9.16). If a Phe residue occurs near the lipid–water interface, it is typically arranged with the aromatic ring oriented toward the membrane core. This is termed antisnorkeling. Membrane Protein Structures Show Many Variations on the Classical Themes Although it revealed many insights of membrane protein structure, bacterio- rhodopsin gave a relatively limited view of the structural landscape. Many mem- brane protein structures obtained since bacteriorhodopsin (and a few others) have provided a vastly more complex picture to biochemists. For example, the structures of a homodimeric chloride ion transport protein and a glutamate transport protein show several novel structural features (Figure 9.17). In addition to several trans- membrane helices that lie perpendicular to the membrane plane (like those of bac- teriorhodopsin), these structures each contain several long, severely tilted helices that span the membrane. Both these proteins also contain several reentrant loops, con- sisting of a pair of short α-helices and a connecting loop that together penetrate part way into the membrane core. There are also regions of nonhelical polypeptide deep in the membrane core of these proteins, with helical segments on either side that extend to the membrane surface (Figure 9.17). Finally, most membrane protein structures are relatively stable; that is, trans- membrane helices do not flip in and out of the membrane, and they do not flip across the lipid bilayer, inverting their orientation. However, a few membrane pro- teins can in fact change their membrane orientation. Aquaporin-1 is a protein that functions normally with six transmembrane ␣-helices. When this protein is first in- serted into its membrane, it has only four transmembrane ␣-helices (Figure 9.18a). One of these, the third transmembrane helix (TM3), reorients across the mem- brane, pulling helices 2 and 4 into the membrane. Similarly, a glycoprotein of the hepatitis B virus is initially inserted into the viral membrane with its N-terminal do- main lying outside. During the viral maturation process, about half of these glyco- proteins rearrange (Figure 9.18b), with the N-terminal segment moving across the membrane as TM4 creates a new transmembrane segment. Some Proteins Use ␤-Strands and ␤-Barrels To Span the Membrane The ␣-helix is not the only structural motif by which a protein can cross a membrane. Some in- tegral transmembrane proteins use structures built from ␤-strands and ␤-sheets to diminish the polar character of the peptide backbone as it crosses the nonpolar Lys 111 Phe 72 Snorkeling Antisnorkeling 0 15 Ϫ15 FIGURE 9.16 Snorkeling and antisnorkeling behavior in membrane proteins.The SdhC subunit of succinate dehydrogenase (pdb id ϭ 1NEK). Lys 111 snorkels away from the membrane core and Phe 72 antisnorkels toward the membrane core. (Adapted from Liang, J.,Adamian, L.,and Jackups, R.,Jr., 2005. The membrane-water interface region of membrane proteins: Structural bias and the anti-snorkeling effect. Trends in Biochemical Sciences 30:355–357.) (a) (b) FIGURE 9.17 Not all the embedded segments of membrane proteins are transmembrane and oriented per- pendicular to the membrane plane. (a) The glutamate transporter homolog (pdb id ϭ 1XFH).“Reentrant” helices (orange) and interrupted helices (red) are shown. Several of the transmembrane helices deviate signifi- cantly from the perpendicular. (b) The E. coli ClC chloride transporter (pdb id ϭ 1KPK).Few of the transmem- brane helices are perpendicular to the membrane plane. 254 Chapter 9 Membranes and Membrane Transport TM5 TM6 TM1 TM3 C C N N Pre-S 50% TM1 N N TM2 TM3 TM4 TM5 TM6 C TM3 TM2 TM1 C TM3 TM4 TM2 TM1 TM4 (a) Aquaporin-1 (b) Hepatitis B virus FIGURE 9.18 Dynamic insertion of helical segments of membrane proteins. (a) Aquaporin-1.The second and fourth transmembrane helices insert properly across the membrane only after reorientation of the third trans- membrane helix. (b) The large envelope glycoprotein of the hepatitis B virus.The N-terminal “pre-S”domain translocates across the endoplasmic reticulum membrane in a slow process in 50% of the molecules. (Adapted from von Heijne, G., 2006. Nature Reviews Molecular and Cell Biology 7:909–918.) (g) (f) (e)(d)(c)(b)(a) FIGURE 9.19 Some proteins traverse the membrane with ␤-barrel structures. Several examples are shown, including (a) maltoporin from S. typhimurium (pdb id ϭ 2MPR), (b) ferric enterobactin receptor (pdb id ϭ 1FEP), (c) TolC, an outer membrane protein from E. coli (pdb id ϭ 1EK9), (d) the translocator domain of the NalP autotransporter of N. meningitides (pdb id ϭ 1UYN), (e) the translocator domain of the Hia autotransporter from H. influenzae (pdb id ϭ 2GR8), (f) the outer membrane cobalamin transporter from E.coli, in a complex with the 100 Å coiled coil of colicin E3 (pdb id ϭ 1UJW), (g) the fatty acid trans- porter FadL from E.coli (pdb id ϭ 1T16). 9.2 What Are the Structure and Chemistry of Membrane Proteins? 255 membrane core. These ␤-barrel structures (Figure 9.19) maximize hydrogen bond- ing and are highly stable. The barrel interior is large enough to accommodate wa- ter molecules and often structures as large as peptide chains, and most barrels are literally water filled. How does the ␤-barrel structure tolerate water on one surface (the inside) and the nonpolar membrane core on the other? In all transmembrane ␤-barrels, polar and nonpolar residues alternate along the ␤-strands, with polar residues facing the center of the barrel and nonpolar residues facing outward, where they can interact with the hydrophobic lipid milieu of the membrane. Porin proteins found in the outer membranes (OMs) of Gram-negative bacte- ria such as Escherichia coli, and also in the outer mitochondrial membranes of eu- karyotic cells, span their respective membranes with large ␤-barrels. A good ex- ample is maltoporin, also known as LamB protein or lambda receptor, which participates in the entry of maltose and maltodextrins into E. coli. Maltoporin is active as a trimer. The 421-residue monomer forms an 18-strand ␤-barrel with antiparallel ␤-strands connected to their nearest neighbors either by long loops or by ␤-turns (Figure 9.20; see also Figure 9.19a). The long loops are found at the end of the barrel that is exposed to the cell exterior, whereas the turns are located on the intracellular face of the barrel. Three of the loops fold into the center of the barrel. ␤-barrels can also be constructed from multiple subunits. The ␣-hemolysin toxin (Figure 9.21) forms a 14-stranded ␤-barrel with seven identical subunits that each contribute two antiparallel ␤-strands connected by a short loop. Staphylococcus aureus secretes monomers of this toxin, which bind to the plasma membranes of host Cell surface Outer membrane Periplasmic space – OOC NH 3 + FIGURE 9.20 The arrangement of the peptide chain in maltoporin from E. coli. Membrane Side view Axial view ACTIVE FIGURE 9.21 The structure of the heptameric channel formed by Staphylococcus aureus ␣-hemolysin. Each of the seven subunits con- tributes a ␤-sheet hairpin to the transmembrane chan- nel (pdb id ϭ 7AHL). Test yourself on the concepts in this figure at www.cengage.com/login. Go to CengageNOW at www.cengage.com/login and click BiochemistryInteractive to discover how a ␤-sheet is expoilted by maltoporin. 256 Chapter 9 Membranes and Membrane Transport blood cells. Upon binding, the monomers oligomerize to form the 7-subunit struc- ture. The channel thus formed facilitates uncontrolled permeation of water, ions, and small molecules, destroying the host cell. Why have certain proteins evolved to use ␤-strands instead of ␣-helices as membrane-crossing devices? Among other reasons, there is an advantage of genetic economy in the use of ␤-strands to traverse the membrane instead of ␣-helices. An ␣-helix requires 21 to 25 amino acid residues to span a typical biological membrane; a ␤-strand can cross the same membrane with 9 to 11 residues. Therefore, a given amount of genetic information could encode a larger number of membrane-spanning segments using a ␤-strand motif instead of ␣-helical arrays. Transmembrane Barrels Can also Be Formed with ␣-Helices Many bacteria, in- cluding E. coli, produce extracellular polysaccharides, some of which form a discrete structural layer—the capsule, which shields the cell, allowing it to evade or counter- act host immune systems. In E. coli, the components of this polysaccharide capsule are synthesized inside the cell and then transported outward through an octameric outer membrane protein called Wza. To cross the outer membrane, Wza uses a novel ␣-helical barrel (Figure 9.22). Wza is composed of three novel domains that, with the ␣-helical barrel, form a large central cavity that accommodates the transported poly- saccharides. The transmembrane ␣-helices of Wza are amphiphilic, with hydrophobic outer surfaces that face the lipid bilayer and hydrophilic inner surfaces that face the water-filled pore. Lipid-Anchored Membrane Proteins Are Switching Devices Certain proteins are found to be covalently linked to lipid molecules. For many of these proteins, covalent attachment of lipid is required for association with a mem- brane. The lipid moieties can insert into the membrane bilayer, effectively anchor- ing their linked proteins to the membrane. Some proteins with covalently linked lipid normally behave as soluble proteins; others are integral membrane proteins and remain membrane associated even when the lipid is removed. Covalently Side view Cytosol Axial view Monomer FIGURE 9.22 The structure of Wza, an octameric membrane protein that anchors the peptidoglycan layer and the outer membrane of Gram-negative bacteria.The structure contains a central barrel constructed from ␣-helical segments (pdb id ϭ 2J58). 9.2 What Are the Structure and Chemistry of Membrane Proteins? 257 bound lipid in these latter proteins can play a role distinct from membrane an- choring. In many cases, attachment to the membrane via the lipid anchor serves to modulate the activity of the protein. Another interesting facet of lipid anchors is that they are transient. Lipid anchors can be reversibly attached to and detached from proteins. This provides a “switch- ing device” for altering the affinity of a protein for the membrane. Reversible lipid anchoring is one factor in the control of signal transduction pathways in eukaryotic cells (see Chapter 32). Four different types of lipid-anchoring motifs have been found to date. These are amide-linked myristoyl anchors, thioester-linked fatty acyl anchors, thioether-linked prenyl anchors, and amide-linked glycosyl phosphatidylinositol anchors. Each of these anchoring motifs is used by a variety of membrane proteins, but each nonetheless exhibits a characteristic pattern of structural requirements. Amide-Linked Myristoyl Anchors Myristic acid may be linked via an amide bond to the ␣-amino group of the N-terminal glycine residue of selected proteins (Figure 9.23a). The reaction is referred to as N-myristoylation and is catalyzed by myristoyl–CoAϺprotein N-myristoyltransferase, known simply as NMT. N-Myristoyl– anchored proteins include the catalytic subunit of cAMP-dependent protein kinase, the pp60 src tyrosine kinase, the phosphatase known as calcineurin B, the ␣-subunit of Gpro- teins (involved in GTP-dependent transmembrane signaling events), and the gag pro- teins of certain retroviruses (including the HIV-1 virus that causes AIDS). Thioester-Linked Fatty Acyl Anchors A variety of cellular and viral proteins contain fatty acids covalently bound via ester linkages to the side chains of cysteine and some- times to serine or threonine residues within a polypeptide chain (Figure 9.23b). This type of fatty acyl chain linkage has a broader fatty acid specificity than N-myristoylation. Myristate, palmitate, stearate, and oleate can all be esterified in this way, with the C 16 and C 18 chain lengths being most commonly found. Proteins anchored to membranes via fatty acyl thioesters include G-protein–coupled receptors, the surface glycoproteins of sev- eral viruses, the reggie proteins of nerve axons, and the transferrin receptor protein. Thioether-Linked Prenyl Anchors As noted in Chapter 8, polyprenyl (or simply prenyl) groups are long-chain polyisoprenoid groups derived from isoprene units. Prenylation of proteins destined for membrane anchoring can involve either far- nesyl or geranylgeranyl groups (Figure 9.23c and d). The addition of a prenyl group A DEEPER LOOK Exterminator Proteins—Biological Pest Control at the Membrane Control of biological pests, including mosquitoes, houseflies, gnats, and tree-consuming predators like the eastern tent caterpillar, is frequently achieved through the use of microbial membrane pro- teins. For example, several varieties of Bacillus thuringiensis produce proteins that bind to cell membranes in the digestive systems of in- sects that consume them, creating transmembrane ion channels. Leakage of Na ϩ , K ϩ , and H ϩ ions through these membranes in the insect gut destroys crucial ion gradients and interferes with diges- tion of food. Insects that ingest these toxins eventually die of star- vation. B. thuringiensis toxins account for more than 90% of sales of biological pest control agents. B. thuringiensis is a common Gram-positive, spore-forming soil bacterium that produces inclusion bodies, microcrystalline clus- ters of many different proteins. These crystalline proteins, called ␦-endotoxins, are the ion channel toxins that are sold commer- cially for pest control. Most such endotoxins are protoxins, which are inactive until cleaved to smaller, active proteins by proteases in the gut of a susceptible insect. One such crystalline protoxin, lethal to mosquitoes, is a 27-kD protein, which is cleaved to form the active 25-kD toxin in the mosquito. This toxin has no effect on membranes at neutral pH, but at pH 9.5 (the pH of the mosquito gut) the toxin forms cation channels in the gut membranes. This 25-kD protein is not toxic to tent caterpillars, but a larger, 130-kD protein in the B. thuringiensis inclusion bodies is cleaved by a caterpillar gut protease to produce a 55-kD toxin that is active in the caterpillar. Remarkably, the strain of B. thuringiensis known as azawai produces a protoxin with dual specificity: In the caterpillar gut, this 130-kD protein is cleaved to form a 55-kD toxin active in the caterpillar. However, when the same 130-kD protoxin is con- sumed by mosquitoes or houseflies, it is cleaved to form a 53-kD protein (15 amino acid residues shorter than the caterpillar toxin) that is toxic to these latter organisms. Understanding the molecu- lar basis of the toxicity and specificity of these proteins and the means by which they interact with membranes to form lethal ion channels is a fascinating biochemical challenge with far-reaching commercial implications. CH 2 HN C O C O C O S CH 2 (a) N-Myristoylation (b) S-Palmitoylation – OOC COO – NH 3 + Extracellular side Cytoplasmic side S H 2 C HC C O HN S H 2 C HC C O O CH 3 HN (c) Farnesylation (d) Geranylgeranylation O CH 3 NH 3 + NH 3 + S S (e) Vesicular stomatitis glycoprotein E P M M MGal GNGal Gal Gal I P Acetylcholinesterase E P I P P E Thyroglobulin 1 E P I P E P I P Glycolipid A E Gal I = Ethanolamine = Galactose = Mannose = Glucosamine = Inositol Key: M M GN M M M GN M M M M GN M M GN FIGURE 9.23 Certain proteins are anchored to biological membranes by lipid anchors. Shown are (a) the N-myristoyl motif, (b) the S-palmitoyl motif, (c) the far- nesyl motif, (d) the geranylgeranyl motif, and (e) several cases of the glycosyl phosphatidylinositol (GPI) motif. 9.2 What Are the Structure and Chemistry of Membrane Proteins? 259 HUMAN BIOCHEMISTRY Prenylation Reactions as Possible Chemotherapy Targets The protein called p21 ras , or simply Ras, is a small GTP-binding protein involved in cell signaling pathways that regulate growth and cell division. Mutant forms of Ras cause uncontrolled cell growth, and Ras mutations are involved in one-third of all human cancers. Because the signaling activity of Ras is dependent on prenylation, the prenylation reaction itself, as well as the proteo- lysis of the -AAX motif and the methylation of the prenylated Cys residue, have been considered targets for development of new chemotherapy strategies. Farnesyl transferase from rat cells is a heterodimer consisting of a 48-kD ␣-subunit and a 46-kD ␤-subunit. In the structure shown here, helices 2 to 15 of the ␣-subunit are folded into seven short, coiled coils that together form a crescent-shaped envelope par- tially surrounding the ␤-subunit. Twelve helices of the ␤-subunit form a novel barrel motif that creates the active site of the enzyme. Farnesyl transferase inhibitors, one of which is shown here, are po- tent suppressors of tumor growth in experimental animals. Mutations that inhibit prenyl transferases cause defective growth or death of cells, raising questions about the usefulness of prenyl transferase inhibitors in chemotherapy. However, Victor Boyartchuk and his colleagues at the University of California, Berkeley, and Acacia Biosciences have shown that the protease that cleaves the -AAX motif from Ras following the prenylation re- action may be a better chemotherapeutic target. They have iden- tified two genes for the prenyl protein protease in the yeast Sac- charomyces cerevisiae and have shown that deletion of these genes results in loss of proteolytic processing of prenylated proteins, in- cluding Ras. Interestingly, normal yeast cells are unaffected by this gene deletion. However, in yeast cells that carry mutant forms of Ras and that display aberrant growth behaviors, deletion of the protease gene restores normal growth patterns. If these remark- able results translate from yeast to human tumor cells, inhibitors of CAAX proteases may be more valuable chemotherapeutic agents than prenyl transferase inhibitors. S S S Ras CMSCKC COO – Ras CMSCKCVLS COO – Farnesyl pyrophosphate Additional modification (methylation and palmitoylation) Farnesyl transferase Ras CMSCKCVLS COO – VLS PPSMT Ras CMSCKCVLS COO – Ras CMSCKC C S O OCH 3 PPSEP Endoplasmic reticulum membrane Plasma membrane H O O SO 2 CH 3 OH N O N H H 2 N HS 2(S)-{(S)-[2(R)-amino-3-mercapto]propylamino- 3(S)-methyl}pentyloxy-3-phenylpropionyl- methioninesulfone methyl ester ᮡ The structure of the farnesyl transferase heterodimer (pdb id ϭ 1JCQ). A novel barrel structure is formed from 12 helical segments in the ␤-subunit (yellow). The ␣-subunit (green) consists largely of seven successive pairs of ␣-helices that form a series of right-handed antiparallel coiled coils running along the bottom of the structure. These “helical hairpins” are arranged in a double-layered, right- handed superhelix resulting in a crescent-shaped subunit that envelopes part of the subunit. ᮡ This substance, also known as I-739,749, is a farnesyl transferase inhibitor that is a potent tumor growth suppressor. ᮡ The farnesylation and subsequent processing of the Ras protein. Follow- ing farnesylation by the FTase, the carboxy-terminal VLS peptide is removed by a prenyl protein-specific endoprotease (PPSEP) in the ER; then a prenyl- protein-specific methyltransferase (PPSMT) donates a methyl group from S-adenosylmethionine (SAM) to the carboxy-terminal S-farnesylated cys- teine. In addition, palmitates are added to cysteine residues near the C-terminus of the protein (not shown). 260 Chapter 9 Membranes and Membrane Transport typically occurs at the cysteine residue of a carboxy-terminal CAAX sequence of the target protein, where C is cysteine, A is any aliphatic residue, and X can be any amino acid. As shown in Figure 9.23c and d, the result is a thioether-linked farnesyl or geranylgeranyl group. Once the prenylation reaction has occurred, a specific protease cleaves the three carboxy-terminal residues, and the carboxyl group of the now terminal Cys is methylated to produce an ester. All of these modifications ap- pear to be important for subsequent activity of the prenyl-anchored protein. Pro- teins anchored to membranes via prenyl groups include yeast mating factors, the p21 ras protein (the protein product of the ras oncogene; see Chapter 32), and the nu- clear lamins, structural components of the lamina of the inner nuclear membrane. Glycosyl Phosphatidylinositol Anchors Glycosyl phosphatidylinositol, or GPI, groups are structurally more elaborate membrane anchors than fatty acyl or prenyl groups. GPI groups modify the carboxy-terminal amino acid of a target protein via an ethanolamine residue linked to an oligosaccharide, which is linked in turn to the in- ositol moiety of a phosphatidylinositol (Figure 9.23e). The oligosaccharide typically consists of a conserved tetrasaccharide core of three mannose residues and a glu- cosamine, which can be altered by addition of galactosyl side chains of various sizes and extra phosphoethanolamines, N-acetylgalactose, or mannosyl residues (Figure 9.23e). The inositol moiety can also be modified by an additional fatty acid, and a va- riety of fatty acyl groups are found linked to the glycerol group. GPI groups anchor a wide variety of surface antigens, adhesion molecules, and cell surface hydrolases to plasma membranes in various eukaryotic organisms. GPI anchors have not yet been observed in prokaryotic organisms or plants. 9.3 How Are Biological Membranes Organized? Membranes Are Asymmetric and Heterogeneous Structures Biological membranes are asymmetric and heterogeneous structures. The two mono- layers of the lipid bilayer have different lipid compositions and different comple- ments of proteins. The membrane composition is also different from place to place across the plane of the membrane. There are clusters of particular kinds of lipids, par- ticular kinds of proteins, and a variety of specific lipid-protein associations and aggregates, all of which serve the functional needs of the cell. We say that both the lipids and the proteins of membranes exhibit lateral heterogeneity and transverse asymmetry. Lateral heterogeneity arises when lipids or proteins of particular types cluster in the plane of the membrane. Transverse asymmetry refers to different lipid or protein compositions in the two leaflets or monolayers of a bilayer membrane. Many properties of a membrane depend on its two-sided nature. Properties that are a consequence of membrane “sidedness” include membrane transport, which is driven in one direction only; the effects of hormones at the outsides of cells; and the immunological reactions that occur between cells (necessarily involving only the out- side surfaces of the cells). The proteins involved in these and other interactions must be arranged asymmetrically in the membrane. Lipid transverse asymmetry can be seen in the typical animal cell, where the amine-containing phospholipids are enriched in the cytoplasmic leaflet of the plasma membrane, and the choline-containing phospholipids and sphingolipids are enriched in the outer leaflet (Figure 9.24). In the erythrocyte, for example, phosphatidylcholine (PC) comprises about 29% of the total phospholipid in the membrane. Of this amount, 76% is found in the outer monolayer and 24% is found in the inner monolayer. Asymmetric lipid distributions are important to cells in several ways. The carbohy- drate groups of glycolipids (and of glycoproteins) always face the outside of plasma membranes, where they participate in cell recognition phenomena. Asymmetric lipid distributions may also be important to various integral membrane proteins, which may prefer particular lipid classes in the inner and outer monolayers. 9.4 What Are the Dynamic Processes That Modulate Membrane Function? 261 Loss of transverse lipid asymmetry has dramatic (and often severe) consequences for cells and organisms. For example, appearance of PS in the outer leaflet of the plasma membrane triggers apoptosis, the programmed death of the cell. Similarly, ag- ing erythrocytes and platelets slowly externalize PS, culminating in engulfment by macrophages. Many disease states, including diabetes and malaria, involve microvas- cular occlusions that may result in part from alterations of transverse lipid asymmetry. 9.4 What Are the Dynamic Processes That Modulate Membrane Function? Lipids and Proteins Undergo a Variety of Movements in Membranes Motions of lipids and proteins in membranes underlie many cell functions. Lipid movements (Figure 9.25) range from bond vibrations (at 10 12 per sec), to bilayer undulations (1 to 10 6 per sec), to transverse motion—called “flip-flop” (roughly one Gauche-trans isomerization 10 10 /sec Rotational diffusion 10 8 /sec Lateral diffusion 10 7 /sec Undulations 1Ϫ10 6 /sec Bond vibrations 10 12 /sec Protrusion 10 9 /sec Flip-flop 10 – 4 Ϫ10 3 /sec FIGURE 9.25 Lipid motions in the membrane and their characteristic frequencies. (Adapted from Gawrisch, K., 2005. The dynamics of membrane lipids. In The Structure of Biological Membranes, Chapter 4, Figure 4.1,Yeagle, P. L.,ed., 2005.Boca Raton: CRC Press.) 0 100 100 Distribution Outer leaflet Inner leaflet Outer leaflet (a) (b) Inner leaflet SM PC PS PE PI PIP PIP 2 PA 27 29 13 27 3 11% 11% 14% 44% 1% 1% 5% 45% 42% 26% SM PC PE ϭ ϭ ϭ sphingomyelin phosphatidylcholine PS PI/PA ϭ ϭ phosphatidylserine phosphatidylinositol/ phosphatidic acid phosphatidylethanolamine FIGURE 9.24 Phospholipids are distributed asymmetrically in most membranes, including the human erythro- cyte membrane, as shown here. (a) The distribution of phospholipids across the inner and outer leaflets of human erythrocytes.The x-axis values show, for each lipid type, its percentage of the total phospholipid in the membrane. (b) The phospholipid compositions of the inner and outer leaflets.All percentages in (a) and (b) are weight percentages. (Adapted from Zachowski, A.,1993. Phospholipids in animal eukaryotic membranes:Transverse asymmetry and movement. Biochemical Journal 294:1–14; and from Andreoli,T. E., 1987. Membrane Physiology, 2nd ed. Chapter 27,Table I. New York: Springer.) 262 Chapter 9 Membranes and Membrane Transport per day to one per sec). Lateral movement of lipids (in the plane of the membrane) is rapid. Adjacent lipids can change places with each other on the order of 10 7 /sec. Thus, a typical phospholipid can diffuse laterally in a membrane at a linear rate of several microns per second. At that rate, a phospholipid molecule travels from one end of a bacterial cell to the other in less than a second or traverse a typical animal cell in a few minutes. Many membrane proteins move laterally (through the plane of the membrane) at a rate of a few microns per minute. On the other hand, some integral membrane proteins are more restricted in their lateral movement, with diffusion rates of about 10 nm per sec or even slower. Slower protein motion is likely for proteins that asso- ciate and bind with each other and for proteins that are anchored to the cytoskele- ton, a complex latticelike structure that maintains the cell’s shape and assists in the controlled movement of various substances through the cell. Flippases, Floppases, and Scramblases: Proteins That Redistribute Lipids Across the Membrane Proteins that can “flip” and “flop” phospholipids from one side of a bilayer to the other have also been identified in several tissues (Figure 9.26). Three classes of such proteins are known: 1. ATP-dependent flippases that transport PS, and to a lesser extent PE, from the outer leaflet to the inner leaflet of the plasma membrane 2. ATP-dependent floppases that transport a variety of amphiphilic lipids, especially cholesterol, PC, and sphigomyelin from the inner leaflet to the outer leaflet 3. bidirectional, Ca 2+ -activated (but ATP-independent) scramblases that function to randomize lipids and thus degrade transverse asymmetry These proteins reduce the half-time for phospholipid movement across a mem- brane from days to a few minutes or less. Approximately one ATP is consumed per lipid transported by flippases and floppases. Energy-dependent lipid flippase activ- ity is essential for the creation and maintenance of transverse lipid asymmetries. A number of diseases have been linked to defects in flippases and floppases. Tangier disease causes accumulation of high concentrations of cholesterol in various tissues and leads to cardiovascular problems. Infants with respiratory distress syndrome produce low amounts of lung surfactant (a mix of lipids) and typically die a few days after birth. Both of these diseases involve flippase or floppase defects. Membrane Lipids Can Be Ordered to Different Extents The phospholipids and sterols of membranes can adopt different structures de- pending on the exact lipid and protein composition of the membrane and on the temperature. At low temperatures, bilayer lipids are highly ordered, forming a gel phase with the acyl chains nearly perpendicular to the plane of the membrane plane (Figure 9.27). In this state—called the solid-ordered state (or S o state)—the lipid chains are tightly packed and undergo relatively little motion. The lipid chains are in their fully extended conformation, the surface area per lipid is minimal, and the Lipid molecule diffuses to flippase protein Flippase Floppase Scramblase Ca 2 + Flippase protein Flippase flips lipid to opposite side of bilayer Lipid diffuses away from flippase 123 (b)(a) ANIMATED FIGURE 9.26 (a) Phospholipids can be flipped, flopped, or scrambled across a bilayer membrane by the action of flippase, floppase, and scramblase proteins. (b) When, by normal diffusion through the bilayer, the lipid encounters one of these proteins, it can be moved quickly to the other face of the bilayer. See this figure animated at www.cengage.com/login. . subunit. ᮡ This substance, also known as I-739,749, is a farnesyl transferase inhibitor that is a potent tumor growth suppressor. ᮡ The farnesylation and subsequent processing of the Ras protein.