vesicular transport Flashcards
what are the 3 types of phospholipids and what are their charges?
phosphatidyl-serine = negative charge phosphatidyl-ethanolamine = neutral charge phosphatidyl-choline = neutral charge
what is the charge of sphingomyelin
neutral
what is an unsaturated fatty acid?
fatty acid not fully saturated by hydrogen
contain a cis double bond –> kink in fatty acid tail
what is the role of cholesterol in the cell membrane?
fills gaps between phospholipids
stabilisation of bilayer
seals plasma membrane to preserve internal molecules
where is phosphatidylserine found in the cell membrane?
inner leaflet of cell membrane
what is the role of phosphatidylserine during apoptosis?
phosphatidylserine flips to the outer surface of the cell membrane
labels dead cell and its remnants so they can be consumed by other cells
how do peripheral proteins interact with the cell membrane?
hydrophobic residues or lipid tail attaches protein to one leaflet of membrane
OR
attach to other proteins embedded in the membrane
receptor mediated endocytosis of cholesterol
- LDL particles carry cholesterol in the blood
- LDL binds to LDL receptor
- adaptin binds to intracellular tail of LDL receptor
- adaptin recruits clathrin molecules which coat the membrane to form a coated pit
- membrane invagination to form a coated vesicle which buds off inside the cell and takes LDL bound to LDL receptor with it
- coat disassembles
- vesicle fuses with early endosome
- early endosome has an acidic environment due to a proton pump - breaks LDL binding to its receptor
- vesicle containing receptor buds off from early endosome and returns LDL receptor to plasma membrane vesicles containing LDL fuse with lysosome
- lysosome digests LDL, releasing free cholesterol + amino acids and small peptides
- cholesterol is used for the synthesis of new membranes
what is the lysosome
highly acidic organelle containing hydrolytic enzymes
breaks down proteins, lipids and sugars
how can defective endocytosis cause atherosclerosis
mutation in LDL receptor affecting: - LDL binding - interaction with adaptor proteins coated pit cannot form accumulation of lipoproteins in the blood ---> plaques blocking arteries
what does dynamin do?
dynamin pinches the vesicle off the cell membrane by hydrolysing GTP to GDP
which clathrin-like coat protein is used to pinch vesicles off the ER
COPII
which clathrin-like coat protein is used to pinch vesicles off the Golgi
COPI
describe the process of phagocytosis
- microbe adheres to phagocyte
- actin forms pseudopod which engulfs the microbe
- phagosome formed = vesicle containing antigen
- phagosome fuses with the lysosome to form a phagolysosome
- microbe in the phagolysosome is digested by lysosomal enzymes - leaves a residual body
useful material released in cytosol
indigestible + residual material removed by endocytosis
describe the process of autophagy
vesicles fuse to form a membrane around the diseased organelle
rounded structure formed which will be transported to lysosomes
lysosomes digest the diseased organelle
lipids + amino acids released into cytosol
what is constitutive exocytosis and provide an example
continuous secretion of material
e.g. secretory vesicles in mucus secreting lung cells contain mucus -> constitutively released into airways
what is regulated exocytosis and provide an example
secretion in response to a stimulus
e.g. pancreatic beta cells only release insulin in response to high blood glycose
what is the role of SNARE proteins?
aid fusion of vesicles with plasma membrane by forcing 2 repulsing, negatively charged membranes together
play a role in exocytosis
do this by forming a tight 4 helical coiled coil on the initial contact
V-SNARE
located on vesicles
synaptobrevin/VAMP
T-SNARE
located on plasma membrane
syntaxin and SNAP25
coiled-coil formation of SNARE proteins
SNAP-25 contributes 2 helices
VAMP contributes 1 helix
syntaxin contributes 1 helix
describe a typical coiled coil
2 alpha helices wrap around each other:
- hydrophobic sides of each helix (containing aliphatic AAs = leucine and valine) are tightly together in the centre
- polar sides exposed to the side
what is NSF
enzyme catalysing dissociation of SNARE coils by ATP hydrolysis + use of accessory proteins
which SNARE protein does type A botulinum neurotoxin cleave?
SNAP-25
which SNARE protein does type B botulinum neurotoxin cleave?
synaptobrevin
which SNARE protein does type C botulinum neurotoxin cleave?
syntaxin
what is the mechanism of action for botulinum toxin?
- botulinum toxin binds to gangliosides on neuronal membranes
- botulinum is endocytosed into a synaptic vesicle
- acidification results in botulinum changing structure and releasing SNARE protease (subunit) into the synaptic cytosol
- cleaved SNARE protein cannot support fusion of synaptic vesicles –> blocks neurotransmission
Botox
cleaves SNAP25
used for treat muscle spasms, dystonias + is a cosmetic agent
