Chapter 11: Membrane Proteins

11-27 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.) anchors

11-28 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.) The cysteine residue is covalently attached to a membrane lipid

11-32 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.) they are primarily made of a-helices

11-34 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) IMIVLVMLLNIGLAILFVHF

11-35 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.) detergent

11-37 For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used; each word or phrase should be used only once.

There are several ways that membrane proteins can associate with the cell membrane. Membrane proteins that extend through the lipid bilayer are called __________________ proteins and have __________________ regions that are exposed to the interior of the bilayer. On the other hand, membrane-associated proteins do not span the bilayer and instead associate with the membrane through an α helix that is __________________. Other proteins are __________________ attached to lipid molecules that are inserted in the membrane. __________________ membrane proteins are linked to the membrane through noncovalent interactions with other membrane-bound proteins.

amphipathic hydrophilic noncovalently

cortical hydrophobic peripheral

covalently integral transmembrane

detergent micelle unfolded

transmembrane

hydrophobic

amphipathic

covalently

Peripheral

11-38 We know the detailed molecular structure and mechanism of action of the transmembrane protein bacteriorhodopsin. This protein uses sunlight as the source of energy to pump ______ out of the cell.

(a) ATP

(b) H+

(c) K+

(d) Na+

(b.) H+

11-39 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.) the absorption of sunlight triggers a shift in the conformation of the protein's seven, membrane spanning a-helices, allowing a proton to leave the cell

11-40 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.) cortex

11-41 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

(d.) spectrin

11-44 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.) proteins are tethered to the cell cortex

11-45 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.) proteins are tethered to the extracellular matrix

11-46 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.) proteins are tethered to the proteins on the surface of another cell

11-47 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.) protein movement is limited by the presence of a diffusion barrier

11-48 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.) proteins are tethered to the proteins on the surface of another cell

11-49 Consider the apical location of a particular protein expressed in epithelial cells, illustrated in Figure Q11-49A. Which type of defect described below is the most likely to cause the redistribution of that protein around the entire cell, shown in Figure Q11-49B?

Figure Q11-49

(a) a nonfunctional protein glycosylase

(b) the deletion of a junctional protein

(c) the truncation of a protein found in the extracellular matrix

(d) a nonfunctional flippase

(b.) the deletion of a junctional protein

11-50 Consider the apical location of a particular protein expressed in epithelial cells, illustrated in Figure Q11-50A. When a molecule that chelates calcium is added to the cell culture medium, you observe a redistribution of that protein around the entire cell, shown in Figure Q11-50B. Which is most likely to be true about the role of calcium in maintaining an apical distribution of protein A?

Figure Q11-50

(a) calcium is required to maintain the structural integrity of the junctional complex

(b) calcium is required for the binding of the junctional proteins to the cell cortex

(c) calcium is a structural component of protein A

(d) calcium inhibits intracellular transport of protein A

(a.) calcium is required to maintain the structural integrity of the junctional complex

11-51 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.) all of the above

11-52 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.) the arrangement of the oligosaccharide side chains is highly ordered, much like the peptide bonds of a polypeptide chain

11-54 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.) they have long carbohydrate chains

11-55 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.) oligosaccharide

11-56 You have isolated two mutants of a normally pear-shaped microorganism that have lost their distinctive shape and are now round. One of the mutants has a defect in a protein you call A and the other has a defect in a protein you call B. First, you grind up each type of mutant cell and normal cells separately and separate the plasma membranes from the cytoplasm, forming the first cell extract. Then you set aside a portion of each fraction for later testing. Next, you wash the remaining portion of the membrane fractions with a low concentration of urea (which will unfold proteins and disrupt their ability to interact with other proteins) and centrifuge the mixture. The membranes and their constituent proteins form a pellet, and the proteins liberated from the membranes by the urea wash remain in the supernatant. When you check each of the fractions for the presence of A or B, you obtain the results given below.

Which of the following statements are consistent with your results (more than one answer may apply)?

(a) Protein A is an integral membrane protein that interacts with B, a peripheral membrane protein that is part of the cell cortex.

(b) Protein B is an integral membrane protein that interacts with A, a peripheral membrane protein that is part of the cell cortex.

(c) Proteins A and B are both integral membrane proteins.

(d) The mutation in A affects its ability to interact with B.

(a.) Protein A is an integral membrane protein that interacts with B, a peripheral membrane protein that is part of the cell cortex

(d.) the mutation in A affects its ability to interact with B