11
Cell membranes are fluid, and thus proteins can diffuse laterally within the lipid bilayer. However, sometimes the cell needs to localize proteins to a particular membrane domain. Name three mechanisms that a cell can use to restrict a protein to a particular place in the cell membrane.
1. The protein can be attached to the cell cortex inside the cell. 2. The protein can be attached to the extracellular matrix outside the cell. 3. The protein can be attached to other proteins on the surface of a different cell. 4. The protein can be restricted by a diffusion barrier, such as that set up by specialized junctional proteins at a tight junctions.
Sodium dodecyl sulfate (SDS) and Triton X-100 are both detergents that can be used to lyse cells. A. If the goal is to study the activity of membrane proteins after cell lysis, explain why SDS would not be a good choice. B. How does Triton X-100 work in cell lysis, and why is it a better choice of detergent to help you extract proteins?
A. SDS is a strong ionic detergent. When cells are exposed to SDS, membrane proteins are not only extracted from the membrane, they are completely unfolded. After denaturation, they cannot be studied as functional molecules. b. Triton X-100 has a smaller nonpolar portion and a polar but uncharged end, allowing it to mimic more closely the type of solvation effect of the membrane lipids. Triton X-100 forms a shell around the hydrophobic portion of the protein without disrupting the existing structure. This makes it possible to then place the protein into a new, synthetic membrane bilayer for study
Glycolipids are found on the surface of healthy cells, and contribute to the cell's defense against chemical damage and infectious agents. A. In which organelle are sugar groups added to membrane lipids? B. By what mechanism are glycolipids transported to the plasma membrane and presented to the extracellular environment? Draw a diagram to support your answer to part B.
A. golgi B. Membranes that contain newly synthesized glycolipids bud from the Golgi apparatus to form vesicles. These vesicles then fuse with the plasma membrane. The glycolipids that were facing the lumen of the Golgi will now face the extracellular environment (Figure A11-26).
While many prokaryotic cells have a single membrane bilayer, all eukaryotic cells have a complex system of internal membrane-bound compartments. How might it be advantageous for the cell to have these additional compartments?
Compartmentalization using intracellular membranes allows eukaryotic cells to separate a variety of cell processes. Although this requires a higher degree of coordination, the cell also gains a more stringent degree of control over these processes (examples include: the separation of transcription and translation; the separation of enzymes involved in protein modifications for secreted versus cytosolic substrates; the separation of proteolytic events in the lysosomes versus the cytosol; the separation of anaerobic metabolism in the cytosol and aerobic metabolismin the mitochondria).
Although membrane proteins contribute roughly 50% of the total mass of the membrane, there are are in cellular about 50 times more lipid molecules than there protein molecules membranes. Explain this apparent discrepancy.
Membrane proteins are much larger molecules than the membrane lipids. Thus, fewer are required to represent the same total mass contributed by the lipid components of the membrane. By this estimation, the molecular weight of the average membrane protein is 50 times that of the average membrane lipid.
Thermal motion promotes lateral position exchanges between lipid molecules within a monolayer. In an artificial bilayer, this movement has been estimated to be ~2 µm/second. This represents the entire length of a bacterial cell. Do you expect the lateral movement of a lipid molecule within a biological membrane to be equally fast? Explain your answer.
No. Although the rate of movement may be similar, it will most likely be slower in a biological membrane. An artificial bilayer is primarily phospholipids. Biological membranes contain a large number of protein components and specialized membrane domains that could limit the rate of lateral diffusion.
Anemia, a condition that results in individuals with a low red blood cell count, can be caused by a number of factors. Why do individuals with defects in the spectrin protein often have this condition?
Spectrin is the primary protein in the cortex of red blood cells. A defect in the spectrin protein directly affects the strength and shape of the cortex. Red blood cells that contain mutated spectrin molecules have an irregular shape and are prone to lysis as a result of cortical fragility, leading to a smaller population of red blood cells.
A group of membrane proteins can be extracted from membranes only by using detergents. All the proteins in this group have a similar amino acid sequence at their C-terminus: -KKKKKXXC (where K stands for lysine, X stands for any amino acid, and C stands for cysteine). This sequence is essential for their attachment to the membrane. What is the most likely way in which the C-terminal sequence attaches these proteins to the membrane? (a) The cysteine residue is covalently attached to a membrane lipid. (b) The peptide spans the membrane as an α helix. (c) The peptide spans the membrane as part of a β sheet. (d) The positively charged lysine residues interact with an acidic integral membrane protein.
a
Membrane lipids are capable of many different types movement. Which of these not occur spontaneously in biological membranes? (a) switching between lipid layers (b) lateral movement (c) rotation (d) flexing of hydrocarbon chains
a
Membrane synthesis in the cell requires the regulation of growth for both halves of the bilayer the other. and the selective retention of certain types of lipids on one side or Which group of enzymes accomplishes both of these tasks? (a) flippases (b) phospholipases (c) convertases (d) glycosylases
a
Most animal fats form a solid at room temperature, while plant fats remain liquid at room temperature. Which of the following is a feature of lipids in plant membranes that best explains this difference? (a) unsaturated hydrocarbons (b) longer hydrocarbon tails (c) higher levels of sterols (d) larger head groups
a
Plasma membranes are extremely thin and fragile, requiring an extensive support network of fibrous proteins. This network is called the ____________. (a) cortex. (b) attachment complex. (c) cytoskeleton. (d) spectrin.
a
Porin proteins form large, barrel-like channels in the membrane. Which of the following is not true about these channels? (a) They are made primarily of α helices. (b) They are made primarily of β sheets. (c) They cannot form narrow channels. (d) They have alternating hydrophobic and hydrophilic amino acids.
a
The endothelial cells found closest to the site of an infection express proteins called lectins. Each lectin binds to a particular ____________ that is presented on the surface of a target cell. (a) oligosaccharide (b) aminophospholipid (c) polysaccharide (d) sphingolipid
a
The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which a budding yeast cell designates the site of new bud formation during cell division? (a) proteins are tethered to the cell cortex (b) proteins are tethered to the extracellular matrix (c) proteins are tethered to the proteins on the surface of another cell (d) protein movement is limited by the presence of a diffusion barrier
a
Water molecules readily form hydrogen bonds with other polar molecules, and when they encounter nonpolar molecules they must form hydrogen-bonding networks with neighboring water molecules. Which of the following molecules will cause a "cage" of water to form? (a) 2-methylpropane (b) acetone (c) methanol (d) urea
a
Which of the following phenomena will be observed if a cell's membrane is pierced? (a) the membrane reseals (b) the membrane collapses (c) a tear is formed (d) the membrane expands
a
Which of the following phospholipid precursors is the most hydrophobic? a) triacylglycerol (b) diacylglycerol (c) phosphate (d) glycerol
a
Which type of lipids are the most abundant in the plasma membrane? (a) phospholipids (b) glycolipids (c) sterols (d) triacylglycerides
a
A bacterium is suddenly expelled from a warm human intestine into the cold world outside. Which of the following adjustments might the bacterium make to maintain the same level of membrane fluidity? (a) Produce lipids with hydrocarbon tails that are longer and have fewer double bonds. (b) Produce lipids with hydrocarbon tails that are shorter and have more double bonds. (c) Decrease the amount of cholesterol in the membrane. (d) Decrease the amount of glycolipids in the membrane.
b
Formation of a lipid bilayer is energetically favorable. How does this arrangement result in higher entropy for the system, and thus make bilayer formation energetically favorable? (a) Polar head groups form a hydrogen-bonding network at the interface with water. (b) Water molecules form cagelike structures around hydrophobic molecules. (c) Hydrogen bonds form between neighboring polar head groups in the bilayer. (d) Fatty acid tails are highly saturated and flexible.
b
In the photosynthetic archaean Halobacterium halobium, a membrane transport protein called bacteriorhodopsin captures energy from sunlight and uses it to pump protons out of the cell. The resulting proton gradient serves as an energy store that can later be tapped to generate ATP. Which statement best describes how bacteriorhodopsin operates? (a) The absorption of sunlight triggers a contraction of the β barrel that acts as the protein's central channel, squeezing a proton out of the cell. (b) The absorption of sunlight triggers a shift in the conformation of the protein's seven, membrane spanning α helices, allowing a proton to leave the cell. (c) The absorption of sunlight triggers a restructuring of bacteriorhodopsin's otherwise unstructured core to form the channel through which a proton can exit the cell. (d) The absorption of sunlight triggers the activation of an enzyme that generates atp
b
It is possible to follow the movement of a single molecule or a small group of molecules. This requires the use of antibodies linked to small particles of gold, which appear as dark spots when tracked through video microscopy. What is this method called? What does the abbreviation stand for? (a) SDS (b) SPT single particle tracking (c) GFP (d) FRAP
b
Membrane proteins, like membrane lipids, can move laterally by exchanging positions with other membrane components. Which type of membrane proteins is expected to be the least mobile, based on their function? (a) channels (b) anchors (c) receptors (d) enzymes
b
Red blood cells have been very useful in the study of membranes and the protein components that provide structural support. Which of the following proteins is the principal fibrous protein in the cortex of the red blood cell? (a) tubulin (b) attachment proteins (c) actin (d) spectrin
b
The amino acid sequences below represent the sequences of transmembrane helices. The characteristics of α helices that form a channel are different from those that form a single transmembrane domain. Select the helix that forms a single transmembrane domain. (a) VGHSLSIFTLVISLGIFVFF (b) IMIVLVMLLNIGLAILFVHF (c) ILHFFHQYMMACNYFWMLCE (d) VTLHKNMFLTYILNSMIIII
b
The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which focal adhesions are formed to promote cell motility? (a) proteins are tethered to the cell cortex (b) proteins are tethered to the extracellular matrix (c) proteins are tethered to the proteins on the surface of another cell (d) protein movement is limited by the presence of a diffusion barrier
b
The the presence of plasma membrane serves many functions, many of which depend on specialized membrane proteins. Which of the following roles of the plasma membrane could still occur if the bilayer were lacking these proteins? (a) intercellular communication (b) selective permeability (c) cellular movement (d) import/export of molecules
b
There are two properties of phospholipids that affect how tightly they pack together: the length of the hydrocarbon chain and the number of double bonds. The degree of packing, in turn, influences the relative mobility of these molecules in the membrane. Which of the following would yield the most highly mobile phospholipid (listed as number of carbons and number of double bonds, respectively)? (a) 24 carbons with 1 double bond (b) 15 carbons with 2 double bonds (c) 20 carbons with 2 double bonds (d) 16 carbons with no double bonds
b
We know the detailed molecular structure and mechanism of action of the transmembrane protein of to pump bacteriorhodopsin. This protein uses sunlight as the source energy ______ out of the cell. (a) ATP + (b) H (c) K+ Na+ (d)
b
Where does most new membrane synthesis take place in a eukaryotic cell? (a) in the Golgi apparatus (b) in the endoplasmic reticulum (c) in the plasma membrane (d) in the mitochondria (e) on ribosomes
b
Both glycoproteins and proteoglycans contribute to the carbohydrate layer on the surface of the cell. Which of the following is not true of glycoproteins? (a) They can be secreted into the extracellular environment. (b) They have only one transmembrane domain. (c) They have long carbohydrate chains. (d) They are recognized by lectins.
c
Membranes undergo spontaneous rearrangement if torn. Which of the following would happen if a cell membrane had a large tear a) the membrane reseals b) a cap forms on the edge of the membrane c) vesicles form d) micelles form
c
New membrane phospholipids are synthesized by enzymes bound to the _____________ side of the _________________ membrane. (a) cytosolic, mitochondrial (b) luminal, Golgi (c) cytosolic, endoplasmic reticulum (d) extracellular, plasma
c
The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which an antigen- presenting cell triggers an adaptive immune response? (a) proteins are tethered to the cell cortex (b) proteins are tethered to the extracellular matrix (c) proteins are tethered to the proteins on the surface of another cell (d) protein movement is limited by the presence of a diffusion barrier
c
The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which neutrophils are recruited by endothelial cells? (a) proteins are tethered to the cell cortex (b) proteins are tethered to the extracellular matrix (c) proteins are tethered to the proteins on the surface of another cell (d) protein movement is limited by the presence of a diffusion barrier
c
Three phospholipids X, Y, and Z are distributed in the plasma membrane as indicated in Figure Q11-14. For which of these phospholipids does a flippase probably exist? (a) X only (b) Z only (c) X and Y (d) Y and Z
c
Unlike soluble, cytosolic proteins, membrane proteins are more difficult to purify. Which of the following substances is most commonly used to help purify a membrane protein? (a) high salt solution (b) sucrose (c) detergent (d) ethanol
c
Which of the following statements about the carbohydrate coating of the cell surface is false? (a) It is not usually found on the cytosolic side of the membrane. (b) It can play a role in cell-cell adhesion. (c) The arrangement of the oligosaccharide side chains is highly ordered, much like the peptide bonds of a polypeptide chain. (d) Specific oligosaccharides can be involved in cell-cell recognition.
c
Which of the following statements is true? (a) Phospholipids will spontaneously form liposomes in nonpolar solvents. (b) In eukaryotes, all membrane-enclosed organelles are surrounded by one lipid bilayer. (c) Membrane lipids diffuse within the plane of the membrane. (d) Membrane lipids frequently flip-flop between one monolayer and the other.
c The remaining answers are false. Phospholipids form bilayers only in polar solvents. Nuclei and mitochondria are enclosed by two membranes. Membrane lipids rarely flip- flop between one monolayer and the other.
Cholesterol serves several essential functions in mammalian cells. Which of the following is not influenced by cholesterol? (a) membrane permeability (b) membrane fluidity (c) membrane rigidity (d) membrane thickness
d
Diversity among the oligosaccharide chains found in the carbohydrate coating of the cell surface can be achieved in which of the following ways? (a) varying the types of sugar monomers used (b) varying the types of linkages between sugars (c) varying the number of branches in the chain (d) all of the above
d
Membrane curvature is influenced by the differential lipid composition of the two membrane monolayers. Which factor do you think has the largest impact on the curvature of biological membranes? (a) amount of cholesterol (b) charge of the lipid head group (c) length of the hydrocarbon tails (d) size of the lipid head group
d
Some lipases are able to cleave the covalent bonds between the glycerol backbone and the attached fatty acid. What final products do you expect to accumulate through the action of the enzyme monoacylglycerol lipase? (a) phosphoglycerol and free fatty acid (b) sterol and glycerol (c) free phosphate and glycerol (d) glycerol and free fatty acid
d
The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes the process by which nutrients are taken up at the apical surface of the epithelial cells that line the gut and released from their basal and lateral surfaces? (a) proteins are tethered to the cell cortex (b) proteins are tethered to the extracellular matrix (c) proteins are tethered to the proteins on the surface of another cell (d) protein movement is limited by the presence of a diffusion barrier
d
We can estimate the relative mobility of a population of molecules along the surface of a living cell by fluorescently labeling the molecules of interest, bleaching the label in one small area, and then measuring the speed of signal recovery as molecules migrate back into the bleached area. What is this method called? What does the abbreviation stand for? (a) SDS (b) SPT (c) GFP (d) FRAP flurescence recovery after photobleaching
d
Which of the following membrane lipids does not contain a fatty acid tail? (a) phosphatidylcholine (b) a glycolipid (c) phosphatidylserine (d) cholesterol
d
Even though proteins can form channels across biological membranes using either α helices or the physical βsheets, channels made of αhelices are more versatile. Explain constraints on β-barrel structures and why these constraints do not apply to channels made of α helices.
β-Barrel structures are composed of individual β strands that form a β sheet that needs to be curved to make the structure of a pore in the membrane. The physical constraints are due to very specific positioning of each strand to maintain the necessary hydrogen-bonding network within the sheet. The relative positions of α helices can vary and still form strong interactions with other helices in the transmembrane region of a protein.
