Molecular Biology of the Cell 7e · Part 1 of 2 · 10 MCQs per part · 60 total questions
Part 1 of 2
The Lipid Bilayer and Membrane Composition
Every cell is bounded by a membrane that both defines it and controls what enters and leaves. Understanding how a simple sheet of lipid molecules, just 5 nm thick, can carry out the extraordinary range of functions attributed to biological membranes is one of the triumphs of twentieth-century cell biology.
10.1 The Lipid Bilayer
Biological membranes are built from phospholipid bilayers. Phospholipids are amphipathic molecules with a hydrophilic glycerophosphate head and two hydrophobic fatty acid tails. In aqueous solution they spontaneously self-assemble into bilayers, with tails sequestered from water and heads exposed.
The three major membrane lipid classes are: glycerophospholipids (e.g., phosphatidylcholine, phosphatidylserine), sphingolipids (e.g., sphingomyelin), and sterols (cholesterol in animal cells).
Key term
Amphipathic molecule
A molecule with both hydrophilic (water-loving) and hydrophobic (water-fearing) regions; the amphipathic nature of phospholipids drives bilayer self-assembly.
Bilayer thickness is approximately 5 nm. The bilayer is impermeable to most ions and polar molecules but freely permeable to small nonpolar molecules (O₂, CO₂, N₂) and small uncharged polar molecules (H₂O, urea).
10.2 Membrane Fluidity and Cholesterol
Membrane fluidity is determined by lipid composition. Unsaturated fatty acids (cis double bonds introduce kinks) increase fluidity; longer saturated chains decrease fluidity. Cholesterol has a dual role: it broadens the temperature range over which the bilayer is fluid, preventing both crystallisation at low temperature and excess fluidity at high temperature.
The transition temperature (Tm) separates the ordered gel phase from the disordered fluid (liquid-crystalline) phase.
✒
Pause & Recall
What is the role of cholesterol in regulating membrane fluidity?
Cholesterol intercalates between phospholipids, restraining movement at high temperatures (reducing fluidity) and preventing tight packing at low temperatures (preventing gel formation), so it buffers fluidity across physiological temperature ranges.
10.3 The Fluid Mosaic Model
The Singer-Nicolson fluid mosaic model (1972) describes proteins as embedded in or associated with a fluid lipid bilayer, free to diffuse laterally unless constrained.
FRAP (fluorescence recovery after photobleaching) directly demonstrates lateral diffusion of lipids and some membrane proteins by bleaching a patch of fluorescently labelled membrane and monitoring recovery as unbleached molecules diffuse in.
Lipid flip-flop (transverse diffusion) is extremely slow (t½ ~ days) for phospholipids in the absence of enzymes. Flippases (ATP-driven, move PS/PE to inner leaflet), floppases (move PC/SM to outer leaflet), and scramblases (bidirectional, Ca²⁷-activated) maintain lipid asymmetry.
10.4 Membrane Asymmetry
The two leaflets of the plasma membrane have distinct lipid compositions. In the outer leaflet: phosphatidylcholine (PC), sphingomyelin (SM), glycolipids. In the inner leaflet: phosphatidylserine (PS), phosphatidylethanolamine (PE), PI(4,5)P₂.
PS exposure on the outer leaflet is a signal for apoptosis (recognition by macrophages) and platelet activation.
Key term
Flippase
A P4-type ATPase that uses ATP hydrolysis to flip specific phospholipids (PS, PE) from the outer to the inner leaflet, establishing and maintaining bilayer asymmetry.
Practice Questions — Part 1Score: 0 / 10
1. Which of the following best describes the structure of a phospholipid?
Phospholipids are amphipathic with a polar head and nonpolar tails — the basis of bilayer self-assembly.
2. The transition temperature (Tm) of a lipid bilayer would be LOWEST for membranes containing:
Unsaturation, not length, is the main driver of lowering Tm. Polyunsaturated fatty acids have multiple cis double bonds that prevent tight packing, drastically lowering Tm.
3. What experimental technique directly demonstrated the lateral diffusion of membrane proteins?
FRAP bleaches a patch of fluorescently labelled membrane and monitors recovery as unbleached molecules diffuse in, directly demonstrating lateral mobility. FRAP is the key technique here.
4. Which lipid is found predominantly in the INNER leaflet of the plasma membrane?
Lipid asymmetry: PS and PE are inner leaflet; PC, SM, glycolipids are outer leaflet. PS is actively maintained in the inner leaflet by flippases; its appearance on the outer leaflet signals apoptosis.
5. Cholesterol's effect on membrane fluidity can best be described as:
Cholesterol has a complex, temperature-dependent effect on membrane fluidity. It broadens the liquid-crystalline phase by intercalating between phospholipids, acting as a fluidity buffer.
6. Which enzyme class moves phospholipids from the outer to the inner leaflet of the plasma membrane using ATP?
Flippases move lipids inward; floppases move outward; scramblases are bidirectional and Ca²⁷-activated. P4-type ATPases (flippases) use ATP to flip PS and PE from the outer to the inner leaflet.
7. Which of the following can cross a pure phospholipid bilayer most rapidly by simple diffusion?
Ions and polar molecules require channels or carriers; O₂ crosses freely. Small nonpolar molecules like O₂ dissolve readily in the hydrophobic core and diffuse freely across the bilayer.
8. The fluid mosaic model proposed by Singer and Nicolson describes biological membranes as:
The fluid mosaic model emphasises both the fluidity of the lipid bilayer and the mosaic distribution of proteins. The 1972 Singer-Nicolson model established the modern view of the membrane as a two-dimensional fluid mosaic.
9. Exposure of phosphatidylserine on the outer leaflet of a cell's plasma membrane is a signal for:
PS on the outer leaflet flags the cell for phagocytic clearance — it is a key marker of apoptosis. PS exposure is an 'eat-me' signal recognised by macrophage receptors such as TIM-4 and MERTK, triggering phagocytosis of apoptotic cells.
10. Which of the following is NOT a property of glycerophospholipids?
Glycerophospholipids have a glycerol backbone, two fatty acids, and a phosphate head — no direct carbohydrate attachment. Carbohydrate chains attached to lipids define glycolipids (e.g., glycosphingolipids), not glycerophospholipids.
Part 1 complete! Score: 0 / 10
Section B · Recall Questions · Part 1
Type your answer, then click Check to reveal the sample answer.
B1
What forces drive phospholipid self-assembly into a bilayer?
Sample answer: Phospholipids are amphipathic: the hydrophobic effect drives the fatty acid tails away from water, while the hydrophilic heads remain in contact with water. Entropy of the surrounding water is maximised when hydrophobic tails are clustered together, spontaneously forming a bilayer.
B2
Describe how unsaturated fatty acids affect membrane fluidity.
Sample answer: Cis double bonds in unsaturated fatty acids introduce kinks that prevent tight packing of lipid tails, increasing membrane fluidity and lowering the gel-to-liquid transition temperature (Tm). More unsaturation = more fluid at a given temperature.
B3
What is FRAP and what does it measure?
Sample answer: FRAP (fluorescence recovery after photobleaching) bleaches a small region of fluorescently labelled membrane with a laser pulse, then monitors fluorescence recovery over time as unbleached molecules diffuse into the bleached zone. The rate and extent of recovery reveal the diffusion coefficient and mobile fraction of the labelled molecule.
B4
Explain membrane lipid asymmetry — which lipids are in each leaflet?
Sample answer: The outer leaflet is enriched in PC, sphingomyelin, and glycolipids. The inner leaflet contains PS, PE, and PI(4,5)P₂. This asymmetry is maintained by flippases, floppases, and scramblases and has functional consequences (e.g., PS signals apoptosis when it appears on the outer leaflet).
B5
What distinguishes a flippase from a scramblase?
Sample answer: Flippases (P4-ATPases) use ATP to move specific phospholipids (PS, PE) directionally from outer to inner leaflet. Scramblases are Ca²⁷-activated enzymes that move lipids bidirectionally (randomly) across the bilayer, disrupting asymmetry — this occurs during apoptosis and platelet activation.
B6
Why are ions such as Na⁺ unable to cross a lipid bilayer by simple diffusion?
Sample answer: Ions carry a charge and are surrounded by a shell of water molecules. To cross the hydrophobic core of the bilayer, an ion would have to shed its hydration shell and enter a highly unfavourable nonpolar environment. The energy cost (Born energy) is prohibitively high, making passive diffusion of ions essentially impossible.
B7
What is the role of cholesterol in the plasma membrane of animal cells?
Sample answer: Cholesterol intercalates between phospholipid tails, with its hydroxyl group at the aqueous interface and its rigid sterol ring aligning with fatty acid chains. At high temperatures, it reduces excess fluidity by restraining tail movement. At low temperatures, it prevents tight crystalline packing (gel phase). It also contributes to lipid raft formation.
B8
Describe the structure of sphingomyelin and its enrichment in lipid rafts.
Sample answer: Sphingomyelin is a sphingolipid with a ceramide backbone (sphingosine + fatty acid) and a phosphocholine head group. Its long, saturated fatty acid chains pack tightly with cholesterol to form ordered lipid microdomains (lipid rafts) in the outer leaflet of the plasma membrane, which concentrate GPI-anchored proteins and signalling molecules.
B9
What structural feature of cis-unsaturated fatty acids makes membranes more fluid?
Sample answer: Each cis double bond introduces a ~30° kink (bend) in the fatty acid chain. This geometric irregularity prevents the chains from packing closely together in an ordered array, increasing the space between lipid molecules and reducing van der Waals interactions, thereby increasing fluidity.
B10
How does the lipid composition of the inner mitochondrial membrane differ from the plasma membrane, and why?
Sample answer: The inner mitochondrial membrane is unusually rich in cardiolipin (a di-phosphatidylglycerol lipid with four acyl chains), which stabilises the curvature of cristae and supports the function of respiratory chain complexes and ATP synthase. This unique lipid contributes to the membrane's impermeability to protons, essential for maintaining the proton gradient used by ATP synthase.
Section C · Critical Thinking · Part 1
Develop analytical responses, then compare with the sample.
C1
A researcher discovers a new antibiotic that inserts into bacterial phospholipid bilayers and creates non-specific pores. Explain why this mechanism might selectively kill bacteria without harming human cells.
Sample answer: Bacteria lack cholesterol in their membranes, which stiffens bilayers and could physically exclude the antibiotic. Additionally, bacteria have different phospholipid compositions (e.g., phosphatidylglycerol predominates) and the antibiotic may have selectivity for these lipids. Human cell membranes contain cholesterol and different lipid profiles that may reduce antibiotic insertion efficiency. However, the lack of selectivity of pore-forming antibiotics is a concern for toxicity to host cells.
C2
FRAP experiments on a membrane protein show 40% fluorescence recovery after 10 minutes. Interpret these results in terms of membrane protein mobility.
Sample answer: Only 40% of the labelled proteins are mobile (the 'mobile fraction'), diffusing back into the bleached region within 10 minutes. The remaining 60% are 'immobile' — likely anchored to the cytoskeleton (e.g., via spectrin-ankyrin interactions), clustered in lipid rafts, or forming stable oligomeric complexes. This result demonstrates that not all membrane proteins are free to diffuse laterally — many are constrained.
C3
How might increasing dietary intake of omega-3 polyunsaturated fatty acids affect the plasma membranes of neurons, and why might this be beneficial?
Sample answer: Omega-3 fatty acids (e.g., DHA) are highly polyunsaturated and, when incorporated into membrane phospholipids, dramatically increase membrane fluidity. In neurons, this increases the fluidity of synaptic membranes and can modulate the activity of membrane-associated proteins (ion channels, G protein-coupled receptors). Enhanced membrane fluidity may improve signal transduction and neurotransmitter release, with potential benefits for cognitive function and anti-inflammatory signalling.
C4
Explain why PS exposure on the outer leaflet is a reliable signal for apoptosis, given that PS is normally confined to the inner leaflet.
Sample answer: PS is actively maintained in the inner leaflet by ATP-dependent flippases. During apoptosis, flippase activity is inhibited while scramblase activity is activated (by caspase cleavage), allowing PS to equilibrate across the bilayer and appear on the outer surface. Because PS exposure is an active, energy-requiring process to maintain and an energy-requiring process to break down, it provides a reliable binary signal. Macrophages have dedicated receptors for outer-leaflet PS, enabling precise recognition of apoptotic cells.
C5
Design an experiment to test whether a newly identified protein is an integral or peripheral membrane protein.
Sample answer: Treat membrane preparations with: (1) a high-salt buffer or carbonate (pH 11) — this removes peripheral proteins that associate via electrostatic interactions, while integral proteins remain membrane-associated; (2) a non-ionic detergent (e.g., Triton X-100) — this solubilises the bilayer and releases integral proteins. Run both fractions on SDS-PAGE and western blot for the protein. If the protein is released by high-salt/carbonate but not detergent, it is peripheral. If it requires detergent for solubilisation, it is integral.
Section D · Interactive Questions · Part 1
Enter your answer and click Check for instant feedback.
D1
The process by which lipids spontaneously move from the outer to the inner leaflet using ATP is called ________.
D2
Small nonpolar molecules like O₂ cross the lipid bilayer by ___________ diffusion.
D3
The experimental model describing proteins embedded in a fluid lipid bilayer is the ________ model.
D4
The lipid that both prevents gel formation at low temperatures and restricts excess fluidity at high temperatures is ___________.
D5
Bilayer lipid asymmetry is broken down during apoptosis by enzymes called ___________.
Part 2 →
Having established the lipid foundation of biological membranes, we now turn to the proteins that give membranes their functional diversity — from transmembrane channels to GPI-anchored receptors — and examine how cells organise membrane components into specialised domains.
Part 2 of 2
Membrane Proteins, Dynamics, and Specialised Domains
10.5 Types of Membrane Proteins
Membrane proteins are classified as integral (transmembrane or monotopic) or peripheral.
Integral transmembrane proteins span the bilayer via hydrophobic α-helices (most common) or β-barrel structures (found in outer membranes of gram-negative bacteria and mitochondria/chloroplasts).
Glycosylphosphatidylinositol (GPI)-anchored proteins are attached to the outer leaflet via a lipid anchor; they lack a cytoplasmic domain and are enriched in lipid rafts.
Peripheral membrane proteins associate non-covalently with the membrane surface (via electrostatic or hydrophobic interactions) or with transmembrane proteins.
Key term
Transmembrane domain
A stretch of ~20 hydrophobic amino acids that forms an α-helix and spans the bilayer; integral membrane proteins typically contain one or more transmembrane helices.
10.6 Lipid Rafts
Lipid rafts are small (10–200 nm), dynamic, ordered microdomains in the outer leaflet enriched in cholesterol, sphingomyelin, and GPI-anchored proteins. They concentrate signalling molecules and receptors, potentially facilitating signal transduction.
✒
Pause & Recall
What is the evidence for lipid rafts?
Biochemical: some membrane proteins fractionate into detergent-resistant membranes (DRMs) with cholesterol and sphingomyelin. Imaging: super-resolution microscopy reveals nanoscale protein clusters. Functional: depletion of cholesterol (with methyl-β-cyclodextrin) disrupts signalling. However, DRMs may be artefacts of detergent treatment, and in-vivo raft size/lifetime is debated.
10.7 Protein Mobility and Membrane Domains
Not all membrane proteins diffuse freely; many are corralled into domains by interactions with the underlying cortical cytoskeleton (membrane skeleton). The 'picket-fence' model proposes that transmembrane proteins anchored to the cytoskeleton act as pickets, and actin-filament corrals act as fences, restricting lateral diffusion of membrane proteins and lipids.
Cell-cell junctions (tight junctions, gap junctions) further restrict membrane protein mobility and create distinct apical and basolateral membrane domains in epithelial cells.
10.8 Membrane Permeability and Biological Functions
The selective permeability of biological membranes underpins cellular homeostasis. While the bilayer itself bars passage of ions, sugars, and amino acids, specialised membrane proteins (channels, carriers, pumps) provide pathways.
Membrane curvature is actively generated and sensed: BAR domain proteins sense or induce curvature; this is important for vesicle budding and tubulation.
Membrane composition is tightly regulated; defects in membrane lipid metabolism cause diseases (e.g., Niemann-Pick disease from sphingomyelinase deficiency; Tangier disease from ABCA1 cholesterol transporter defects).
Practice Questions — Part 2Score: 0 / 10
1. Which structural feature characterises most integral membrane proteins?
Transmembrane α-helices, not GPI anchors or disulfide bonds, are the hallmark structural feature. Most integral membrane proteins span the bilayer via one or more hydrophobic α-helices of ~20 residues.
2. GPI-anchored proteins are exclusively found on which leaflet of the plasma membrane?
GPI anchors connect proteins exclusively to the outer (extracellular) leaflet. GPI anchors are added in the ER and remain on the outer leaflet, concentrating in lipid rafts. They lack a transmembrane or cytoplasmic domain.
3. Which of the following best describes the 'picket-fence' model of membrane organisation?
The picket-fence model specifically invokes cytoskeletal interactions to explain restricted lateral diffusion. The picket-fence model (Kusumi et al.) proposes that cytoskeleton-anchored transmembrane proteins (pickets) and actin corrals (fences) restrict diffusion of membrane proteins and lipids.
4. Depletion of cholesterol from the plasma membrane using methyl-β-cyclodextrin would be expected to:
Cholesterol extraction disrupts rafts and the signalling concentrated in them. Cholesterol is essential for lipid raft integrity; its depletion disrupts rafts, disperses raft-associated proteins, and can inhibit signalling pathways localised to rafts.
5. How do epithelial cells maintain distinct apical and basolateral membrane domains?
Tight junctions are the key structural element that seals epithelial membrane domains. Tight junctions form continuous belts of claudin/occludin strands that seal the paracellular space AND create a diffusion barrier in the membrane, preventing mixing of apical and basolateral proteins and lipids.
6. A β-barrel protein structure in a biological membrane is most likely found in:
β-barrels are a feature of gram-negative bacterial outer membranes and organelles derived from endosymbiosis. β-barrel integral membrane proteins (porins) are found in the outer membranes of gram-negative bacteria, mitochondria, and chloroplasts.
7. A peripheral membrane protein can be selectively removed from membrane preparations by:
High salt or carbonate strips peripheral proteins; detergent is needed to remove integral proteins. Peripheral proteins bind membranes via electrostatic interactions or protein-protein contacts that are disrupted by high ionic strength (salt) or high pH (carbonate, pH 11), without solubilising the bilayer.
8. Niemann-Pick disease type A/B is caused by a deficiency of which enzyme?
Niemann-Pick type A/B is a lysosomal storage disorder caused by acid sphingomyelinase deficiency. Sphingomyelinase deficiency leads to sphingomyelin accumulation in lysosomes. Sphingomyelin is normally hydrolysed to ceramide and phosphocholine.
9. Which domain of peripheral membrane proteins is responsible for sensing membrane curvature?
BAR domain proteins are the canonical curvature-sensing and -generating proteins. BAR (Bin/Amphiphysin/Rvs) domain proteins are banana-shaped dimers that both sense and induce membrane curvature, playing key roles in vesicle budding and tubulation.
10. Which disease is caused by defects in the ABCA1 transporter that exports cholesterol from cells?
Tangier disease = ABCA1 deficiency → cholesterol export failure → HDL deficiency. ABCA1 transports cholesterol and phospholipids to apolipoprotein A-I (forming HDL). Its loss in Tangier disease causes cholesterol accumulation in macrophages and near-absence of HDL.
Part 2 complete! Score: 0 / 10
Section B · Recall Questions · Part 2
Type your answer, then click Check to reveal the sample answer.
B1
Describe two structural differences between integral and peripheral membrane proteins.
Sample answer: Integral proteins span the bilayer via hydrophobic transmembrane domains (α-helices or β-barrels) and require detergent for solubilisation. Peripheral proteins associate with the membrane surface via electrostatic interactions with lipid head groups or via protein-protein contacts, and are removed by salt or high-pH treatments without disrupting the bilayer.
B2
What is a GPI anchor and how is it added to a protein?
Sample answer: A GPI (glycosylphosphatidylinositol) anchor is a lipid modification in which the C-terminus of a protein is cleaved and reattached to a pre-assembled GPI structure in the ER lumen by the GPI transamidase complex. The GPI-anchored protein is then exclusively on the outer leaflet and is enriched in lipid rafts.
B3
What are lipid rafts and what proteins do they concentrate?
Sample answer: Lipid rafts are dynamic, ordered microdomains in the outer leaflet enriched in cholesterol and sphingomyelin. They concentrate GPI-anchored proteins, certain receptor tyrosine kinases, and signalling proteins (e.g., Src-family kinases, doubly acylated proteins). They may function as platforms for signal transduction and membrane trafficking.
B4
Explain how tight junctions maintain membrane polarity in epithelial cells.
Sample answer: Tight junctions form continuous belts of claudin and occludin strands that encircle each epithelial cell near the apical surface. They seal the paracellular space (preventing paracellular ion flow) and create a fence in the plane of the membrane that prevents mixing of apical and basolateral membrane proteins, maintaining distinct domains with different compositions.
B5
What is the 'picket-fence' model of membrane protein diffusion?
Sample answer: The picket-fence model proposes that the membrane is subdivided into ~40–300 nm corrals by a meshwork of sub-membranous actin filaments and associated proteins. Transmembrane proteins anchored to these filaments (pickets) further restrict diffusion of other membrane proteins and lipids. Single-particle tracking reveals 'hop diffusion' as molecules jump between corrals.
B6
Describe the main evidence used to identify lipid rafts.
Sample answer: Main evidence: (1) detergent-resistant membranes (DRMs) — treatment with cold Triton X-100 leaves a cholesterol/sphingomyelin-rich fraction containing GPI-anchored proteins; (2) cholesterol depletion by methyl-β-cyclodextrin disrupts raft-associated signalling; (3) super-resolution microscopy (STED, PALM) reveals nanoscale clusters of raft markers. Limitations: DRMs may be artefacts of detergent treatment.
B7
How do BAR domain proteins generate membrane curvature?
Sample answer: BAR domain proteins form crescent-shaped dimers that bind to negatively charged membrane surfaces through electrostatic interactions. The curved BAR dimer scaffold imposes or senses curvature on the bilayer. Some BAR proteins also insert amphipathic helices into one leaflet, wedging the bilayer to induce curvature. They act during endocytosis and tubule formation.
B8
What is the clinical significance of phosphatidylserine exposure in platelets?
Sample answer: When platelets are activated, scramblases expose PS on the outer leaflet. PS provides a negatively charged surface that binds the prothrombinase complex (factors Xa and Va) and the tenase complex, greatly accelerating thrombin generation and fibrin clot formation. This PS-dependent procoagulant activity is essential for normal haemostasis.
B9
Describe the structural basis for the β-barrel architecture of outer-membrane proteins compared with those in mitochondria.
Sample answer: In gram-negative bacteria, trimeric porins form water-filled β-barrel channels that allow passive diffusion of small molecules (<600 Da) across the outer membrane. Mitochondrial outer membrane porins (VDAC) are also β-barrels but are larger and voltage-gated. The β-barrel architecture, with hydrophobic exterior residues spanning the bilayer and a hydrophilic interior channel, is distinct from the α-helical bundles of plasma membrane proteins.
B10
What disease results from deficiency of acid sphingomyelinase, and what accumulates?
Sample answer: Deficiency of acid sphingomyelinase causes Niemann-Pick disease types A (severe neurological) and B (visceral only). Sphingomyelin cannot be degraded in lysosomes and accumulates in lysosomes of macrophages, liver, spleen, and neurons, causing progressive organ dysfunction. Treatment for type B uses enzyme replacement therapy.
Section C · Critical Thinking · Part 2
Develop analytical responses, then compare with the sample.
C1
A mutant cell line lacks functional flippases. Predict the consequences for membrane lipid asymmetry and cell biology.
Sample answer: Without flippases, PS and PE would not be concentrated in the inner leaflet; they would equilibrate across the bilayer. PS on the outer leaflet would constitutively signal apoptosis, causing macrophages to phagocytose the cells. This would be lethal. Additionally, loss of PS/PE from the inner leaflet would disrupt PI(4,5)P₂ signalling, cytoskeletal attachment, and coagulation regulation on platelets. This experiment highlights the biological importance of lipid asymmetry.
C2
Why might depletion of cholesterol from the membrane affect the function of a receptor tyrosine kinase located in a lipid raft?
Sample answer: Cholesterol is essential for lipid raft integrity. Depletion (e.g., with methyl-β-cyclodextrin) disrupts rafts, dispersing their protein residents. If a receptor tyrosine kinase is raft-localised, it may be separated from its co-receptors, adaptor proteins, or downstream signalling components (which are also raft-enriched). This could reduce the local concentration of signalling partners, impair receptor dimerisation or phosphorylation, and attenuate the downstream signalling cascade.
C3
Explain how a GPI-anchored protein on the outer leaflet could influence signalling on the inner leaflet, given that it has no transmembrane or cytoplasmic domain.
Sample answer: GPI-anchored proteins can cluster in lipid rafts on the outer leaflet, and the raft region of the inner leaflet may be enriched in signalling proteins (e.g., Src-family kinases, which are doubly acylated and associate with the inner leaflet of rafts). Clustering of outer-leaflet GPI proteins could reorganise the inner leaflet raft composition, promoting co-localisation of inner-leaflet signalling proteins and facilitating transactivation. This 'transmembrane raft signalling' remains an active research area.
C4
In epithelial cells of the kidney tubule, Na⁺/K⁺-ATPase pumps are restricted to the basolateral membrane. What structural mechanisms ensure this polarised distribution?
Sample answer: Tight junctions form a diffusion fence in the membrane that prevents mixing of apical and basolateral proteins. Additionally, the Na⁺/K⁺-ATPase has a cytoplasmic domain that interacts with ankyrin, which links it to the spectrin-actin cytoskeleton preferentially at the basolateral surface. Vesicular trafficking also delivers the pump specifically to the basolateral domain. This polarised distribution is essential for vectorial ion transport across the epithelium.
C5
How might a mutation that converts a charged residue in the interior of a lipid raft to a hydrophobic one affect raft association of that protein?
Sample answer: If the mutation is in a GPI-anchored protein, changing a polar/charged residue in its ectodomain to a hydrophobic one would not directly affect raft affinity (which is determined by the GPI anchor and cholesterol/SM interactions). However, if the mutation is in a transmembrane protein, adding hydrophobicity could increase van der Waals packing with cholesterol and sphingolipids, potentially increasing raft affinity. Conversely, if a previously charged residue was part of a sorting signal, its mutation could redirect trafficking away from raft domains.
Section D · Interactive Questions · Part 2
Enter your answer and click Check for instant feedback.
D1
Proteins anchored to the outer leaflet via a phosphoinositol-glycan linkage are called ___________ proteins.
D2
The crescent-shaped protein domain that senses and induces membrane curvature is the ___________ domain.
D3
In gram-negative bacteria, outer membrane channels formed by β-barrel proteins that allow passive diffusion of small molecules are called ___________.
D4
The disease caused by deficiency of acid sphingomyelinase is called ___________ disease.
D5
The junction type that forms a diffusion fence between apical and basolateral membrane domains in epithelia is the ___________ junction.