Molecular Biology · End-of-chapter questions below · Part 1 of 2 · 10 questions per part
Part 1 of 2
The Secretory Pathway: From ER to Golgi
Every protein your cells secrete—insulin, antibodies, collagen—travels an assembly-line route through membranes before reaching its destination. Understanding this route explains how cells build, tag, and deliver molecular cargo with remarkable precision.
In this part, you will explore:
The secretory pathway and its major compartments
COPII vesicle formation and ER-to-Golgi transport
Golgi apparatus structure: cis, medial, and trans cisternae
Glycosylation reactions within the Golgi
Lysosome biogenesis and the mannose-6-phosphate receptor
13.1 The Secretory Pathway
Eukaryotic cells maintain elaborate internal membrane systems that compartmentalize biochemical reactions and direct newly synthesized proteins to precise destinations. The secretory pathway begins at the endoplasmic reticulum (ER), where ribosomes translating signal-sequence-bearing mRNAs dock at the ER membrane and thread the growing polypeptide into the ER lumen. Once inside, proteins are folded with the help of chaperones such as BiP and may receive N-linked glycans co-translationally.
Only correctly folded proteins pass the ER quality-control checkpoint and are packaged into transport vesicles for forward movement. Misfolded proteins are retrotranslocated back into the cytosol and degraded by the proteasome—a process called ER-associated degradation (ERAD).
Key term
Secretory pathway
The series of membrane-enclosed compartments—ER, ER-Golgi intermediate compartment, Golgi, and post-Golgi vesicles—through which newly synthesized proteins destined for secretion or membrane insertion are processed and transported.
13.2 COPII Vesicles and ER-to-Golgi Transport
Cargo leaves the ER in COPII-coated vesicles that bud from specialized regions called ER exit sites. The COPII coat is assembled by the sequential recruitment of the small GTPase Sar1 (activated by the GEF Sec12), followed by the Sec23/24 inner coat complex that selects cargo, and finally the Sec13/31 outer cage that curves the membrane. GTP hydrolysis by Sar1 triggers coat disassembly after budding.
COPII vesicles travel to the ER-Golgi intermediate compartment (ERGIC), also called the cis-Golgi network, where COPI-coated vesicles retrieve escaped ER-resident proteins bearing the KDEL retrieval sequence back to the ER.
Key term
COPII vesicle
A coated transport vesicle that carries newly synthesized cargo from ER exit sites toward the Golgi apparatus; the coat is assembled from Sar1 GTPase and the Sec23/24 and Sec13/31 complexes.
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Pause & Recall
What role does GTP hydrolysis by Sar1 play in COPII vesicle formation?
Sar1-GTP binding to the ER membrane initiates coat assembly; subsequent GTP hydrolysis to Sar1-GDP triggers coat disassembly after the vesicle has budded, releasing the coat so the vesicle can fuse with its target.
13.3 The Golgi Apparatus
The Golgi apparatus is a polarized stack of flattened membrane sacs (cisternae) organized into functionally distinct regions: the cis face (receiving side, near the ER), medial compartments, and the trans face (shipping side, facing the plasma membrane). The trans-Golgi network (TGN) is the major sorting station from which proteins are dispatched to lysosomes, secretory vesicles, or the plasma membrane.
Two models describe how cargo traverses the Golgi: the vesicular transport model proposes that cargo moves in vesicles between stable cisternae, while the cisternal maturation model holds that entire cisternae mature and move forward as new cis cisternae form continuously from ERGIC membranes. Current evidence supports cisternal maturation as the primary mechanism for large cargo such as collagen.
13.4 Glycosylation in the Golgi
As proteins traverse the Golgi, they undergo extensive modification. The N-linked oligosaccharides added in the ER are trimmed and extended in an ordered sequence: cis Golgi enzymes remove mannose residues; medial Golgi enzymes add GlcNAc, and trans Golgi enzymes add galactose and sialic acid. Many membrane proteins also acquire O-linked glycans in the Golgi. The specific glycan pattern on a protein affects its folding stability, recognition by lectins, and half-life in the circulation.
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Pause & Recall
Why is the sequential positioning of glycosylation enzymes across Golgi cisternae functionally important?
Each enzyme modifies the product of the previous one; their sequential arrangement ensures that each reaction can only proceed after the preceding step is complete, producing complex oligosaccharides with the correct structure in an assembly-line manner.
13.5 Lysosome Biogenesis and the Mannose-6-Phosphate Receptor
Lysosomes are acidic organelles (pH ~5) containing more than 50 hydrolytic enzymes that degrade macromolecules. Lysosomal enzymes are synthesized in the ER and tagged in the cis Golgi by the addition of mannose-6-phosphate (M6P) groups to their N-linked glycans—a signal recognized by mannose-6-phosphate receptors (M6PRs) in the TGN.
M6PRs capture their enzyme cargo and are incorporated into clathrin-coated vesicles budding from the TGN. These vesicles deliver enzymes to late endosomes, where low pH causes M6PR to release its cargo. The receptor is then recycled back to the TGN via retromer complexes. Patients with I-cell disease lack the enzyme that adds M6P to lysosomal proteins; their hydrolases are secreted extracellularly instead of reaching lysosomes, causing a severe storage disorder.
Key term
Mannose-6-phosphate receptor
A transmembrane receptor in the trans-Golgi network that binds M6P-tagged lysosomal enzymes and directs them into clathrin-coated vesicles destined for late endosomes/lysosomes.
Practice questions — Part 1Score: 0 / 10
1. Which coat protein complex mediates vesicle budding from ER exit sites toward the Golgi?
COPII coats mediate anterograde (forward) transport from ER exit sites to the ERGIC/cis-Golgi. COPI mediates retrograde retrieval back to the ER, and clathrin mediates TGN-to-endosome and endocytosis events.
2. What is the function of the KDEL sequence on soluble ER-resident proteins?
KDEL (Lys-Asp-Glu-Leu) is a C-terminal retrieval signal recognized by KDEL receptors in the ERGIC/cis-Golgi. When a soluble ER-resident protein escapes forward, KDEL receptors capture it and include it in COPI vesicles for retrograde return to the ER.
3. Which face of the Golgi apparatus receives incoming vesicles from the ER?
The cis face of the Golgi (closest to the ER) receives cargo arriving from the ERGIC. The trans face and TGN are the exit side, from which processed cargo is dispatched to its final destination.
4. Which chemical modification in the cis-Golgi acts as the primary sorting signal for lysosomal hydrolases?
Mannose-6-phosphate (M6P) is added to N-linked glycans of lysosomal enzymes in the cis-Golgi. This tag is recognized by M6P receptors in the TGN, which package the enzymes into vesicles destined for late endosomes and lysosomes.
5. I-cell disease results from a deficiency in which enzymatic activity?
In I-cell disease, the enzyme GlcNAc-1-phosphotransferase, which phosphorylates mannose residues on lysosomal enzyme precursors, is defective. Without the M6P tag, lysosomal enzymes are secreted extracellularly rather than delivered to lysosomes.
6. Where in the Golgi are sialic acid residues added to glycoproteins?
Glycosylation in the Golgi is spatially ordered: cis enzymes trim mannose, medial enzymes add GlcNAc, and trans enzymes/TGN add the terminal sugars galactose and sialic acid. This order ensures correct complex oligosaccharide assembly.
7. Which model of Golgi transport proposes that entire cisternae move progressively from cis to trans?
The cisternal maturation model holds that new cis cisternae form continuously from incoming ERGIC vesicles, mature as they progress toward the trans face, and ultimately disperse as TGN. This explains how large cargo like collagen fibrils (too big for vesicles) can traverse the Golgi.
8. What is the role of ER-associated degradation (ERAD)?
ERAD is the ER quality-control mechanism that identifies misfolded proteins, retrotranslocates them through the Sec61 channel (or other retrotranslocons) back into the cytosol, and targets them for ubiquitin-proteasome degradation, preventing secretion of defective proteins.
9. Which small GTPase initiates COPII coat assembly by inserting into the ER membrane?
Sar1 (a member of the ARF GTPase family) is activated by the GEF Sec12 at the ER membrane. Sar1-GTP inserts its amphipathic helix into the ER membrane and recruits the Sec23/24 and Sec13/31 COPII coat subcomplexes, initiating vesicle budding.
10. In the Golgi, where are mannose residues first trimmed from N-linked glycans?
cis-Golgi mannosidases remove mannose residues from the high-mannose N-linked glycans that arrive from the ER. This trimming is the first step in glycan remodeling that produces the complex or hybrid oligosaccharide structures found on mature glycoproteins.
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Part 1 complete
Part 1 → 2
You have traced how newly synthesized proteins travel from the ER through the Golgi, acquiring glycan modifications and lysosomal sorting tags. Part 2 turns to the endocytic arm of membrane traffic—how cells take material in from outside, process it through endosomes, and either degrade it in lysosomes or recycle components back to the surface. We also cover autophagy and regulated exocytosis.
Part 2 of 2
Endocytosis, Endosomes, Autophagy, and Exocytosis
13.6 Endocytosis and Clathrin-Coated Pits
Cells internalize extracellular material and plasma-membrane components through endocytosis. The best-characterized pathway is receptor-mediated endocytosis, in which cell-surface receptors concentrate ligands at clathrin-coated pits—specialized plasma-membrane regions where the adaptor protein AP2 links receptor cytoplasmic tails to clathrin triskelia. The GTPase dynamin polymerizes around the neck of the invaginating pit and pinches off a clathrin-coated vesicle by GTP hydrolysis.
A classic example is internalization of LDL by the LDL receptor: LDL particles carrying cholesterol bind the receptor at neutral pH, the receptor–LDL complex is endocytosed, and in the acidic early endosome the complex dissociates. The receptor is recycled to the plasma membrane in recycling endosome vesicles, while LDL is delivered to lysosomes for cholesterol release. Mutations in the LDL receptor cause familial hypercholesterolemia.
Key term
Clathrin-coated pit
A specialized plasma-membrane invagination coated on its cytosolic face with clathrin and adaptor proteins; it concentrates receptor–ligand complexes and buds off as a clathrin-coated vesicle upon dynamin-mediated scission.
13.7 Early and Late Endosomes; Multivesicular Bodies
After clathrin coat shedding, the internalized vesicle fuses with the early endosome, a mildly acidic (pH ~6.5) sorting station. Ligands destined for degradation move to late endosomes (pH ~5.5), also called multivesicular bodies (MVBs), where ubiquitinated membrane proteins are sorted into intraluminal vesicles by the ESCRT (Endosomal Sorting Complexes Required for Transport) machinery. MVBs ultimately fuse with lysosomes, releasing their intraluminal vesicles—and the proteins they carry—for hydrolytic degradation.
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Pause & Recall
Why is ubiquitination important for sorting membrane proteins at the MVB stage?
Monoubiquitin (or K63-linked chains) on receptor cytoplasmic tails serves as a signal recognized by ESCRT-0 and subsequent ESCRT complexes, which concentrate the receptor and bud it into an intraluminal vesicle. Once inside the MVB lumen, the receptor cannot be recycled and is irreversibly targeted for lysosomal degradation.
13.8 Autophagy
Autophagy ("self-eating") allows cells to degrade their own cytosolic contents—protein aggregates, damaged organelles, or even intracellular pathogens. In macroautophagy, a double-membrane phagophore engulfs a portion of cytoplasm to form an autophagosome. This double-membrane vesicle then fuses with a lysosome to become an autolysosome, in which the contents are degraded and the products returned to the cytosol for reuse. Autophagy is induced by nutrient starvation and regulated by mTOR kinase (active mTOR suppresses autophagy) and the ULK1 kinase complex.
13.9 Exocytosis: Constitutive and Regulated Secretion
Membrane traffic toward the cell exterior occurs by exocytosis, in which secretory vesicles fuse with the plasma membrane. In constitutive secretion, vesicles bud from the TGN and fuse with the plasma membrane continuously without any external trigger—supplying membrane proteins and extracellular matrix components. In regulated secretion, specialized secretory cells (neurons, endocrine cells, mast cells) store cargo in dense-core or synaptic vesicles and release it only upon a specific stimulus—typically a rise in cytosolic Ca²⁺.
Vesicle–target membrane fusion is driven by SNARE proteins: v-SNAREs on the vesicle pair with cognate t-SNAREs on the target membrane, forming a four-helix bundle that draws the two membranes together. NSF ATPase and α-SNAP disassemble spent SNARE complexes for recycling. Botulinum toxin causes paralysis by cleaving neuronal SNAREs, blocking acetylcholine release at neuromuscular junctions.
Key term
SNARE proteins
Soluble NSF Attachment Protein REceptors; paired v-SNARE/t-SNARE complexes that drive membrane fusion by forming a tight four-helix bundle that forces the vesicle and target bilayers together.
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Pause & Recall
How does regulated exocytosis differ from constitutive exocytosis in terms of stimulus and cargo?
Constitutive exocytosis occurs continuously without a specific trigger and delivers membrane proteins and matrix components. Regulated exocytosis requires a specific extracellular signal (usually leading to a Ca²⁺ rise) to trigger fusion, and releases concentrated cargo (hormones, neurotransmitters) stored in secretory or synaptic vesicles.
Practice questions — Part 2Score: 0 / 10
1. Which GTPase pinches off clathrin-coated vesicles from the plasma membrane?
Dynamin is a large GTPase that forms a helical ring around the neck of a deeply invaginated clathrin-coated pit. GTP hydrolysis drives a conformational change that severs the membrane neck, releasing the coated vesicle into the cytosol.
2. In receptor-mediated endocytosis of LDL, what happens to the LDL receptor after the receptor–LDL complex enters an early endosome?
The mildly acidic pH (~6.5) of early endosomes causes the LDL receptor to release its LDL cargo. The free receptor is then incorporated into recycling endosome tubules that return it to the plasma membrane, while LDL moves to late endosomes and then lysosomes for cholesterol extraction.
3. Which machinery sorts ubiquitinated membrane cargo into intraluminal vesicles of multivesicular bodies?
The ESCRT pathway (Endosomal Sorting Complexes Required for Transport) recognizes ubiquitinated cargo on endosomal membranes, concentrates it, and drives its deformation into inwardly budding vesicles. This topology (budding into the lumen) is topologically equivalent to retroviral budding—HIV hijacks ESCRT for its own egress.
4. Which organelle forms when an autophagosome fuses with a lysosome?
Fusion of a double-membrane autophagosome with a lysosome produces an autolysosome, in which lysosomal acid hydrolases degrade the sequestered cytoplasmic material. The breakdown products (amino acids, fatty acids, nucleotides) are transported back to the cytosol for anabolic reuse.
5. What kinase suppresses autophagy under nutrient-rich conditions?
mTOR (mechanistic Target Of Rapamycin) kinase is active when nutrients are abundant. Active mTOR phosphorylates and inhibits ULK1 (the initiating kinase of autophagy), keeping autophagy off. Starvation inactivates mTOR, allowing ULK1 to activate and initiate phagophore formation.
6. Which type of exocytosis delivers cargo only upon a specific extracellular signal such as elevated Ca²⁺?
Regulated exocytosis occurs in specialized secretory cells. Cargo is stored in secretory or synaptic vesicles poised near the plasma membrane. An extracellular signal triggers a rise in cytosolic Ca²⁺, which activates synaptotagmin (a Ca²⁺ sensor) to promote rapid SNARE-driven fusion.
7. How does botulinum toxin block neurotransmitter release?
Botulinum toxin is a zinc protease that specifically cleaves neuronal SNAREs—SNAP-25 or synaptobrevin (VAMP) depending on the serotype. Without functional SNARE proteins, synaptic vesicles cannot fuse with the presynaptic membrane, blocking acetylcholine release and causing flaccid paralysis.
8. What structural feature distinguishes an autophagosome from most other transport vesicles?
The autophagosome is bounded by two lipid bilayers (a double membrane), reflecting its origin from the phagophore—a cup-shaped membrane sac that closes around cytoplasmic cargo. Standard transport vesicles have a single bilayer. The outer membrane of the autophagosome fuses with the lysosome.
9. Which adaptor protein links the cytoplasmic tails of endocytic receptors to clathrin during receptor-mediated endocytosis?
AP2 (adaptor protein complex 2) is the heterotetrameric adaptor that operates at the plasma membrane during receptor-mediated endocytosis. Its subunits bind tyrosine-based (YXXΦ) or dileucine motifs in receptor cytoplasmic tails and simultaneously bind clathrin, nucleating coat assembly at the plasma membrane.
10. Familial hypercholesterolemia is caused by mutations in which protein?
Familial hypercholesterolemia results from loss-of-function mutations in the LDL receptor gene. Without functional LDL receptors, cholesterol-carrying LDL cannot be cleared from the bloodstream by receptor-mediated endocytosis, leading to extremely elevated plasma LDL and premature cardiovascular disease.
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Part 2 complete
Chapter 13 takeaways
COPII vesicles carry newly synthesized proteins from ER exit sites to the Golgi; COPI vesicles retrieve escaped ER-resident proteins.
The Golgi is polarized (cis → trans) and processes glycans in an ordered sequence ending in terminal sugar additions at the TGN.
Mannose-6-phosphate tagging in the cis-Golgi is the master signal routing lysosomal hydrolases to lysosomes; its absence causes I-cell disease.
Clathrin/dynamin mediate receptor-mediated endocytosis; ESCRT complexes sort ubiquitinated cargo into MVB intraluminal vesicles for lysosomal degradation.
Autophagy is suppressed by mTOR; starvation inactivates mTOR, allowing double-membrane autophagosomes to engulf and deliver cytoplasmic contents to lysosomes.
SNARE proteins drive all intracellular membrane fusion events; regulated exocytosis is additionally controlled by Ca²⁺ and synaptotagmin.
End-of-chapter questions
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Section B — Recall Questions
B1
Describe the sequence of events that leads to COPII vesicle budding from the ER membrane.
Sample answer: The GEF Sec12 activates Sar1 by exchanging GDP for GTP; Sar1-GTP inserts its amphipathic helix into the ER membrane. Sar1-GTP recruits Sec23/24 (inner coat, cargo selection) then Sec13/31 (outer cage, membrane curvature). After budding, Sar1 hydrolyzes GTP and the coat disassembles.
B2
What is the KDEL retrieval mechanism and why is it necessary for ER homeostasis?
Sample answer: ER-resident soluble proteins bear C-terminal KDEL sequences. If they escape into the Golgi, KDEL receptors in the ERGIC/cis-Golgi bind them and include them in COPI retrograde vesicles back to the ER. Without this retrieval, chaperones like BiP would be depleted from the ER and protein folding would fail.
B3
Explain how the mannose-6-phosphate receptor directs lysosomal hydrolases from the TGN to lysosomes.
Sample answer: M6P-tagged hydrolases are recognized by M6P receptors in the TGN at neutral pH. These receptor-cargo complexes are incorporated into clathrin/AP1-coated vesicles and delivered to late endosomes (pH ~5.5), where acidity causes the receptor to release its cargo. The receptor is recycled via retromer to the TGN while the hydrolases move to lysosomes.
B4
Describe the spatial order of N-linked glycan processing across the Golgi cisternae.
Sample answer: In the cis-Golgi, mannosidases trim mannose residues. In the medial Golgi, GlcNAc transferases add GlcNAc. In the trans-Golgi/TGN, galactosyltransferases add galactose and sialyltransferases add sialic acid. This ordered assembly creates complex, branched oligosaccharides.
B5
What are the main molecular components of a clathrin-coated pit and what are their respective roles?
Sample answer: Clathrin triskelia form the outer polygonal coat that drives membrane curvature. AP2 adaptors link receptor cytoplasmic tails to clathrin and initiate coat assembly. Dynamin polymerizes around the vesicle neck and uses GTP hydrolysis to sever the membrane. Eps15 and epsin are accessory proteins that aid curvature and coat nucleation.
B6
How do ESCRT complexes sort downregulated receptors into multivesicular bodies?
Sample answer: Ubiquitinated receptors on endosomal membranes are recognized by ESCRT-0 (Hrs/STAM). ESCRT-I and -II cluster the cargo and bend the membrane inward (opposite topology to normal vesicle budding). ESCRT-III (CHMP proteins) drives membrane scission, releasing intraluminal vesicles. Vps4 ATPase disassembles ESCRT-III for reuse.
B7
Outline the steps of macroautophagy from initiation to cargo degradation.
Sample answer: 1) Starvation inactivates mTOR → ULK1 complex activates. 2) A cup-shaped phagophore (isolation membrane) forms, likely from ER-derived membranes. 3) The phagophore expands and closes around cytoplasmic cargo to form a double-membrane autophagosome. 4) The autophagosome fuses with a lysosome to form an autolysosome. 5) Lysosomal hydrolases degrade the cargo; products are exported to the cytosol.
B8
Explain the SNARE hypothesis of membrane fusion and the role of NSF in recycling SNARE complexes.
Sample answer: v-SNAREs on vesicles pair with cognate t-SNAREs on the target membrane. Zippering of the four-helix bundle from the N-terminus toward the membrane pulls the bilayers into close apposition, overcoming the energy barrier to fusion. After fusion, the stable cis-SNARE complex is disassembled by the ATPase NSF aided by α-SNAP, releasing the individual SNAREs for reuse in subsequent rounds of fusion.
B9
Compare constitutive and regulated secretion in terms of cell types, cargo, and triggering mechanisms.
Sample answer: Constitutive: all cells; cargo includes membrane proteins and ECM components; vesicles from TGN fuse continuously with the plasma membrane without needing a trigger. Regulated: specialized secretory cells (neurons, endocrine, mast); cargo (neurotransmitters, hormones, histamine) stored in dense-core or synaptic vesicles; fusion triggered by a specific stimulus (e.g. action potential) causing cytosolic Ca²⁺ rise that activates synaptotagmin.
B10
Why does the cisternal maturation model better explain the transport of large cargo like collagen through the Golgi than the vesicular transport model?
Sample answer: Collagen procollagen trimers are too large (~300 nm) to fit inside standard (60–80 nm) COPI transport vesicles. The cisternal maturation model avoids this problem because entire cisternae—carrying any size cargo within their lumen—progressively convert from cis to trans identity, while Golgi-resident enzymes are recycled retrogradely in COPI vesicles. Electron-microscopy and live-cell imaging confirm that large cargo remains stationary within cisternae as the Golgi apparatus matures around it.
Section C — Critical Thinking
C1
A patient with I-cell disease has elevated plasma levels of lysosomal hydrolases and accumulates undegraded material in lysosomes. Explain why both observations follow from a single enzyme deficiency.
Sample answer: GlcNAc-1-phosphotransferase normally adds the M6P tag that routes lysosomal hydrolases to lysosomes. Without this enzyme, the hydrolases lack M6P and cannot be captured by M6P receptors in the TGN; instead they follow the default constitutive secretory pathway to the extracellular space—explaining elevated plasma hydrolase levels. Without hydrolases inside lysosomes, macromolecules cannot be degraded and accumulate, causing the storage phenotype (I-cells are so named for the intracellular inclusions visible by microscopy).
C2
Why does the LDL receptor recycle back to the plasma membrane rather than being degraded in the lysosome alongside its LDL cargo? What would happen to cholesterol uptake if the recycling were blocked?
Sample answer: The LDL receptor has high affinity for LDL at neutral pH (plasma membrane) but releases it at the mildly acidic pH of early endosomes. The free receptor lacks ubiquitin marks and is sorted into recycling endosome tubules rather than into ESCRT-driven intraluminal vesicles. Rapid recycling allows each receptor to make hundreds of endocytic trips, amplifying the efficiency of cholesterol uptake. Blocking recycling (e.g., by disrupting the recycling endosome pathway) would reduce surface receptor density, diminish LDL uptake, elevate plasma LDL, and potentially cause hypercholesterolemia similar to familial hypercholesterolemia.
C3
Different botulinum toxin serotypes cleave different SNARE proteins (SNAP-25, synaptobrevin, or syntaxin) yet all cause flaccid paralysis. What does this tell us about the mechanism of SNARE-mediated membrane fusion?
Sample answer: That inactivating any single component of the ternary SNARE complex is sufficient to abolish fusion demonstrates that the complete four-helix bundle is essential—no individual SNARE can compensate for the loss of another. This supports the view that SNARE zippering is the core energetic driver of membrane fusion, not merely a docking event. The identity of which SNARE is cleaved matters pharmacologically (different serotypes have different intoxication kinetics) but not mechanistically: all prevent the synaptic vesicle from fusing with the presynaptic membrane, blocking acetylcholine release.
C4
Autophagy can be either tumor-suppressive or tumor-promoting depending on context. Explain both roles and the paradox this creates for anti-cancer therapy.
Sample answer: Tumor suppressive: autophagy eliminates damaged organelles and protein aggregates that would otherwise cause genomic instability; monoallelic Beclin1 loss (a key autophagy gene) is common in breast and ovarian cancers, suggesting autophagy limits early tumor growth. Tumor promoting: established tumors in hypoxic, nutrient-poor microenvironments upregulate autophagy as a survival mechanism, recycling intracellular macromolecules to maintain energy. Therapeutic paradox: autophagy inhibitors (e.g., chloroquine) might help in established tumors by blocking this survival pathway, but could accelerate progression of early lesions. Combining autophagy inhibition with chemotherapy is an active clinical research area.
C5
The Golgi apparatus maintains its distinct cis–trans polarity even though membrane flows continuously through it. How is this steady-state polarity maintained despite constant anterograde and retrograde flux?
Sample answer: Golgi-resident enzymes are selectively retrieved from more trans compartments back to more cis ones by COPI retrograde vesicles, keeping each enzyme at its functional position. Rab GTPases and coiled-coil tethering factors (golgins) confer compartment identity and ensure that vesicles fuse only with the correct target membrane. The combination of anterograde cisternal maturation (carrying luminal cargo forward) and retrograde COPI transport (recycling resident enzymes) creates a dynamic steady state that sustains compositional asymmetry between cis and trans Golgi despite continuous flux.
Section D — Interactive Questions
D1
What small GTPase initiates COPII coat assembly by inserting into the ER membrane? (one word)
D2
What C-terminal sequence on soluble ER-resident proteins mediates their retrieval from the Golgi? (four letters)
D3
Which GTPase polymerizes around the neck of a clathrin-coated pit to pinch it off? (one word)
D4
What lysosomal sorting tag is added to hydrolases in the cis-Golgi? (abbreviated form, e.g. "m6p")
D5
What type of vesicle (named for its two concentric membranes) engulfs cytoplasmic cargo during autophagy?