Lecture 25

Question 1

Internal membrane compartments vastly outweigh the cell plasma membrane, T or F

Answer 1

T

Question 2

What is the role of Rab proteins in exocytosis

Answer 2

Rab proteins identify target membranes for fusion and Rab-GTPs play a role in tethering and docking of vesicles to the membrane

Question 3

Explain the mechanism of action of the botulinum toxin

Answer 3

Botulinum binds first to gangliosides on neuronal membranes. It then enters the luminal space of recycling synaptic vesicles and following endocytosis, one subunit, the SNARE Protease escapes the vesicle and enters the synaptic cytosol. Here the SNARE protease subunit cleaves a specific SNARE protein. The cleaved SNARE protein cannot support the fusion of synaptic vesicles which results in a long blockade of neurotransmission

Question 4

SNARE proteins help to overcome the force the prevents membrane fusion, what are the two main types of SNAREs

Answer 4

Vesicular or v-SNAREs such as synaptobrevin are found on the vesicle membrane. Target SNAREs or t-SNAREs are found on the target membrane of the compartment

Question 5

From which organelle(s) does the COPI coat allows the budding of vesicles from

Answer 5

The Golgi cisterne (or cis-Golgi)

Question 6

Give an example of endocytosis in nutrient uptake

Answer 6

The protein portion of LDL is recognised by LDL receptor on the cell surface which binds with high affinity to LDL. The adapter molecule called adaptin binds to the intracellular domain of the LDL receptor. Adaptin recruits clathrin molecules which binds to it and coats the inside of the membrane. Assembly of the clathrin coat causes the membrane to invaginate. This forms a vesicle that buds off the inside of the cell taking the LDL receptors and the bound LDL with it along with clathrin and adaptin. Inside the cell the clathrin coated vesicle uncoats and fuses with the endosome. The endosome has an acidic pH which causes the LDL receptors to release the LDL cargo. Empty LDL receptors are recycled back to the plasma membrane in recycling vesicles that bud off from the endosome. Meanwhile the endosomal content delivered to the lysosome where hydrolytic enzymes digest the LDL releasing cholesterol, amino acids and small peptides

Question 7

Give an example of a human disease characterised by defects in an endocytotic mechanism

Answer 7

Defects in endocytosis can cause atherosclerosis. This occurs due to mutations in LDL receptor account for familial cases of atherosclerosis. This results in an accumulation of lipoproteins in blood and the formation of atheromatous plaques which block arteries

Question 8

Phagocytosis is a different type of endocytosis, how is it classified and why

Answer 8

Phagocytosis is an example of actin-driven vesicle formation. This is because actin drives membrane engulfment in phagocytosis by forming a pseudopod around the bacterium

Question 9

Newly-synthesized ER lipids and proteins are packaged into COPI vesicles which snip off the SER, T or F

Answer 9

F – these vesicles are COPII coated

Question 10

Explain how SNAREs act to force membrane fusion

Answer 10

SNARE proteins form a tight 4-helical coiled-coil with hydrophobic faces (containing leucine, valine etc.) coming together away from the cytosol. This coiled-coil structure acts as a zipper all the way to the target membrane and the force produced by SNARE protein coiling into two opposing membranes is translated and forces their fusion

Question 11

Describe the process of lipid synthesis in the SER membrane

Answer 11

Fatty acid binding protein delivers a fatty acid to the plasma membrane. Here, CoA transferase allows synthesis of two fatty acids at the glycerol head group. A phosphatase then releases the phosphate bound to the glycerol molecule which allows for hydroxylation. This hydroxyl group is then replaced by the lipid head group (i.e. choline, serine, ethanolamine).

Question 12

What is seen in mutants in dynamin

Answer 12

Mutated dynamin cannot hydrolyse GTP and thus cannot pinch off endocytic vesicles

Question 13

How do the botulinum and tetanus toxins lead to muscular paralysis

Answer 13

Botulinum and tetanus toxins attack SNARE proteins with high specificity. SNAREs are responsible for acetylcholine release at the neuromuscular junction

Question 14

Which subtypes of SNAREs are Syntaxin and SNAP25

Answer 14

Syntaxin and SNAP25 are t-SNAREs

Question 15

Exocytosis is responsible for secretion of hormones, digestive enzymes, recycling of plasma membrane receptors and neuronal communication. What are the 2 exocytotic pathways and how do they differ

Answer 15

Constitutive exocytosis is always occurring with little to no regulation and occurs in non-polarised cells. In contrast, regulated secretion involves the accumulation and storage of a molecule before a signal-triggered release

Question 16

How is botulism and tetanus contracted

Answer 16

Botulism happens upon consumption of contaminated food whereas tetanus infections can happen after skin cuts, during childbirth or dirty needle injections

Question 17

Other than the cell membrane, what structure do clathrin coats mediate the budding of vesicles from

Answer 17

The trans-golgi network

Question 18

Which type of vesicle coat allows the budding of vesicles from the endoplasmic reticulum

Answer 18

COPII

Question 19

When are LDL receptors actively synthesised

Answer 19

When blood cholesterol is low

Question 20

What is the role of adapter proteins in endocytosis

Answer 20

Adapter proteins such as adaptin link selected cargo to the clathrin lattice

Question 21

What different roles do the SER and RER have

Answer 21

The RER is involved in protein synthesis whereas the SER is responsible for lipid synthesis and the formation of vesicles

Question 22

What is the problem that prevents the fusion of vesicles with the target membrane

Answer 22

The target membranes and the membrane of the vesicle are negatively charged due to the presence of phosphatidylserine. This causes a repulsion between the two membranes that prevents fusion

Question 23

Insulin release is an example of one type of exocytosis. Which type is it and how does it work

Answer 23

Release of highly-concentrated insulin from pancreatic beta cells is an example of regulated exocytosis and happens only in response to high glucose. Insulin is tightly packaged inside secretory granules before secretion. Vesicles move from compartment to compartment with excess lipid material being removed and recycled back to the golgi. This acts to concentrate insulin so much that it is nearly crystalline. Upon release, insulin binds to its receptor and triggers delivery of glucose transporters into the plasma membrane of muscle cells

Question 24

Give examples of cells that are phagocytotic

Answer 24

Neutrophils and macrophages

Question 25

How long does the paralysis caused by botulinum toxin last for if untreated

Answer 25

4-6 months

Question 26

What is autophagy and how does it work

Answer 26

Autophagy is the third pathway towards lysosomal digestion. It acts to help eliminate malfunctioning cell elements and occurs via vesicle fusion and engulfment of organelles

Question 27

What is the name of the mechanism by which LDL is transported into cells

Answer 27

Receptor-mediate endocytosis

Question 28

Outline the process of phagocytosis

Answer 28

The microbe adheres to the phagocyte which forms pseudopods that eventually engulf the particle. The phagocytic vesicle is then fused with a lysosome forming a phagosome. The microbe within the vesicle is killed and digested by lysosomal enzymes within the phagosome leaving a residual body. The ingestible and residual material is removed by exocytosis

Question 29

What are the effects of the tetanus and botulinum neurotoxins produced by some bacterial infections

Answer 29

They can result in complete neuromuscular paralysis

Question 30

What is the role of dynamin in endocytosis

Answer 30

Dynamin is a large molecular weight GTPase that is required for coated pit scission to form coated vesicles during endocytosis. GTP hydrolysis allows the vesicle to bud off from the membrane

Question 31

Compare the tendency of fatty acids to stay in the membrane

Answer 31

A single fatty acid sitting in the ER membrane can easily escape whereas two fatty acids cannot because they are bound much more strongly

Question 32

Describe the structure of clathrin and how is causes membrane invagination

Answer 32

Clathrin has a lattice structure that forms cage-like structures when in complexes. Clathrin itself is said to have a triskelia structure consisting of a tri-legged curved shape which forces membrane invagination. A molecule of clathrin consists of 3 heavy and 3 light chains.

Question 33

Explain the use of botulinum toxin in medicine

Answer 33

Botulinum neurotoxin can be used in medicine for local muscle paralysis. Due to its long-lasting muscle inactivation (4-6 months), the neurotoxin became a successful medicine. BOTOX is being used to treat muscular spasms, dystonias, cerebral palsy, Parkinsonian symptoms, and many other muscular disorders

Question 34

Which enzyme catalyses the dissociation of SNARE coils by hydrolysis ATP and forcing synpatobrevin or VAMP-1 away from the t-SNAREs

Answer 34

NSF enzyme