Vesicular Transport Flashcards
What does endocytosis allow?
The capture of molecules from the outside of the compartment/cell
What does exocytosis allow?
The secretion from compartment/cell to the outside
What are the 3 main components in the plasma membrane?
Lipids
Carbohydrates
Proteins
What do lipids contribute to the membrane?
Continuity and flexibility
What do proteins contribute to the membrane?
Transport
What do carboydrates contribute to the membrane?
Cell protection and tagging of cells
Where are sugars present on the cell?
On the extracellular surface
How are sugars attached to the cell?
To either lipids or proteins
How do sugars protect the cell in damaging environments?
Forms a protective layer of the extracellular surface of the cell
What is the structure of a phosopholipid?
Polar head group (hydrophillic) made of:
Amino acid (which can change)
Phosphate
Glycerol
2 hydrocarbon tails - one which has a cis-double bond to produce a kink
Ester bond between the glycerol and phosphate
What are examples of phospholipids?
Phosphatidyl ethanolamine
Phosphatidyl serine
Phosphatidyl choline
Which phospholipid is of net negative charge?
What does this mean?
Phosphatidyl serine
This phospholipid must be localised to the inside of the membrane
What does phosphatidyl serine play a huge part in?
Cell function and cell death
What is the difference between phospholipids and sphingolipids?
What is an example of a sphingolipid?
Sphingolipids have sphingosine instead of phosphate
Sphingomyelin
What are needed to allow the membrane to be flexible?
Why?
Unsaturated fatty acids
Have a kink in there structure - providing disorder to the membrane
What are unsaturated fatty acids?
Fatty acids which have a CIS double bond
Don’t have the maximum amount of hydrocarbons bound
How many cis double bonds can be present in a hydrocarbon tail?
Many
How many double bonds does Omega-3 have?
Omega-6?
3
6
Where is Omega-3 produced?
What is it eaten by?
By sea plants
Eaten by fish
Where is Omega-6 produced?
What is it eaten by?
By land plants
Eaten by land animals
Why is it important to eat Omega-3 and Omega-6?
These cannot be made in the body
Must consume them to keep the membrane of cells flexible (they have many double bonds)
What does cholesterol do in the membrane?
Helps to seal the porous plasma membrane and preserve internal molecules
Stabilises the membrane
What is the structure of cholesterol?
- 4 rings
- Polar head group
- Ridges hydrophobic tail
What is the percentage of cholesterol in the membrane?
17%
Which phospholipid has the highest percentage in the membrane and what is it?
Phosphatidyl choline
24%
Which phospholipid has the lowest percentage in the membrane and what is it?
Phosphatidyl serine
4%
How are intracellular molecule and vesicles kept away from each other and the plasma membrane?
They have negative charges
What percentage of the inner leaflet is phosphatidylserine and why?
8%
Only found on the inner leaflet, due to its negative charge
What does it mean if phosphatidylserine is exposed to the outside of the cell?
Signals that the cell is dying
Only occurs during apoptosis
What does the exposure of phosphatidylserine to the outside of the cell label?
Why?
The dead cell and its remnants
So they are rapidly consumed by other cells
What can phosphatidylserine flipover be detected by?
Where does this bind?
What colour is this?
Fluorescent Annexin V test
Binds to the heads of phosphatidyl serine
Red in colour
What 2 things does vesicular transport allow?
- Movement of material between distinct compartments of the cell
- Movement to the outside environment
What are transmembrane proteins?
What can the function of these proteins be?
Proteins which are inserted into the membrane
Ion channels/carriers/pumps
What are peripheral proteins?
Proteins that are attached to one leaflet of the membrane through:
- Lipid tails
- Transmembrane proteins
How is cholesterol transported into the cell?
By receptor-mediated endocytosis
What carries cholesterol in he blood?
LDL (low-density lipoproteins)
Describe the process of cholesterol into the cell
1) LDL binds to an LDL-receptor which has accumulated in a coated pit
2) ADAPTIN binds to the tails of the LDL receptors and recruits CLATHERIN
3) Clatherin coats the membrane and causes the membrane to invaginate
4) Coated pit forms a coated vesicle inside the cell
5) Coat dissasembles inside the cell - allowing the vesicle to fuse with the early endosome
6) Acidic environment of the early endosome causes LDL to dissociate from the LDL receptor
7) Receptor is recycles back to the plasma membrane in vesicles that bud off from the early endosome
8) Contents of the early endosoem is delivered to the lysosome, where the LDL proteins are digested
Why is the early endosome important?
Receives ALL endocytosed material
What type of molecule is adaptin?
An adaptor protein
Why is there an acidic environment in the early endosome?
Protein pump - pumps protons in from the cytosol
What is the environment of the lysosome?
Highly acidic with hydrolytic enzymes
What does digesting the LDL protein in the lysosome cause?
Release of cholesterol, amino acids and small peptides into the cytosol
What is cholesterol used for?
Synthesis of new membranes
What do mutations in the LDL receptor result in?
Why?
Defective endocytosis
Leading to atherosclerosis (CVD)
Due to accumulation of lipoproteins in the blood and formation of plaques - blocking arteries
How do mutations in the LDL receptor result in defective endocytosis?
- Mutation in the tail
- Can’t bind to adaptin
- Coated pit cannot form
What is the structure of clatherin?
Tri-legged and curved
What does clatherin help with?
Membrane invagination, in order to form vesicles
What molecules pinches the vesicles off from the planar cell membrane?
What does this molecule require in order to do this?
Dynamin
Requires GTP hydrolysis to produce energy
What occurs when dynamin is mutated?
- Defective endocytosis (vesicles cannot be pinched off
- Vesicles lie along the plasma membrane
What do the ER and golgi use to pinch-off vesicles?
Clatherin-like coat proteins
What Clatherin-like coat proteins is used to created vesicles from the golgi?
COPI
What Clatherin-like coat proteins is used to created vesicles from the ER?
COPII
Where is clatherin used from vesicle formation?
At the membrane and the early endosome
What are newly synthesised proteins and lipids packages into?
What adaptor protein is used?
COPII vesicles
Sar1-GTP is the adaptor protein used
What vesicle formation doesn’t require clatherin?
Phagocytosis
What type of membrane invagination is required in phagocytosis?
How does this occur?
Actin-driven membrane invagination:
1) Microbe adheres to phagocyte
2) Phagocyte forms pseudopods that eventually engulfs the particle
3) Formation of a phagosome
4) Phagosome fuses with lysosome to form a phagolysosome
5) Microbe is killed and digested by lysosomal enzymes, leaving a residual body
6) Residual body is excreted from the cell and the useful material is released into the cytosome
What is a phagocyte?
A cell which is capable of engulfing bacteria through phagocytosis
What are pseudopods?
Temporary cytoplasmic projections
Created by actin
What is a phagosome?
A phagocytic vesicle
What are residual bodies?
How are they removed?
Vesicles containing indigestible materials
Secreted by the cell via exocytosis
What is autophagy?
Elimination of malfunctioning cell elements
How does autophagy occur?
- Small vesicles fusing together around the disease organelle to form a continuous membrane (autophagosome)
- Autophagosome then fuses to the lysosome
How does fusion of vesicles to compartments occur when they are both intrinsically negatively charged?
Due to SNARE proteins
How are membrane proteins transported from the ER?
- Transport vesicles from the ER fuse to form transport intermediates
- Transport intermediates move along microtubule tracts to the golgi
- Membrane proteins leave the golgi in transport vesicles, which are delivered to the plasma membrane by microtubules
What is exocytosis responsible for?
Secretion of:
- Hormones
- Digestive enzymes
Recycling of membrane receptors
Neuronal communication
What are the 2 types of exocytosis?
Constitutive (all of the time)
Regulated (in response to a stimulus)
Why must membranes come very close together in order to fuse?
Hydrophobic regions must interact and fuse
What are v-SNAREs?
Example
SNAREs that are present on the vesicular membrane
Synaptobrevein (VAMP)
What are t-SNAREs?
Examples
Target membrane SNARE
Syntaxin
SNAP25
Which SNARE protein doesn’t have a transmembrane domain?
SNAP25
What is formed on the initial contact between SNARE proteins and what does each SNARE contribute?
A tight 4-helical coiled coil
VAMP - one helice
Syntaxin - one helice
SNAP25 - 2 helices
What forces the fusion of 2 compartments?
Coiling of the SNARE proteins in both membranes
What happens to the SNARE coils post fusion?
What does this require
They are dissociated
Requires NSF with the help of:
- Accessory proteins
- ATP hydrolysis
What does Botulinum neurotoxin do?
Attacks SNARE proteins
What can botulinum and tetanus bacterial infections result in?
How?
Complete neuromuscular paralysis
By blocking exocytosis (important in Neuronal communication)
Blocks the release of Ach from presynapse to activate muscle contraction
How can Botulism occur?
Through consumption of contaminated foods
How can tetanus infection occur?
Dirty needles
Skin cuts
What does BOTOX type A do?
Cleaves SNAP-25
What does BOTOX type B do?
Synaptobrevin
What does BOTOX type C do?
Syntaxin
What is the mechanism of action of BOTOX?
1) Botulinum binds to the gangliosides on the presynaptic neuronal membranes
2) B enters the luminal space of recycling synaptic vesicles (endocytosed)
3) Following endocytosis - one subunit (SNARE protease) escapes the vesicle and cleaves specific SNARE protein
4) Cleaves SNARE can no longer support the fusion of synaptic vesicles - blocking neurotransmission for several months
After infection with BOTOX, what leads to resumption of normal neuronal transmission?
Re-synthesis of the damaged protein
What can BOTOX be used for medically?
How?
Local muscle paralysis:
- Wrinkles
- Muscle spasms
Injection with picogram amounts of BOTOX type A - cleaves SNAP-25
How is SNARE peptidase released from BOTOX following endocytosis?
Vesicle is acidified - changing the structure of BOTOX and releasing the SNARE peptidase subunit
