Vesicular transport (L10) Flashcards
What does vesicular transport allow?
Vesicular transport allows movement of molecules between compartments. Endocytsosis allows capture of molecules from the outside. Exocytosis allows for the secretion of molecules from inside.
What is the function of the plasma membrane?
The plasma membrane provides the key barrier to the movement of material. It contains lipids for continuity and flexibility, proteins for transport and signalling, and extracellular carbohydrates for cell protection and tagging. The lipid bilayer is rich in amphipathic molecules carrying 2 fatty acids (phospholipids and sphingomyelin). Phospholipids include phosphatidylethanolamine, serine and choline. These phospholipids can have different charges, e.g. serine is negative. To function properly, membranes have to be flexible (fully saturated lipids are too rigid) Cholesterol helps to seal the plasma membrane, preserving internal molecules
Explain the topology of the plasma membrane.
The inside and outside of the membrane are not the same. There are sugars on the outside, and there are more phosphatidylserine molecules on the inside to make the inner surface negatively charged. Therefore, most intracellular molecules and vesicles are negatively charged and thus are intrinsically repulsed by each other in the plasma membrane. The presence of PS on the inside of the cell is actually a matter of life or death. Upon apoptosis, PS flips to the outer surface - this can be detected by a fluorescent Annexin V test (cell outside goes from green to red)
How is cholesterol transported into the cell?
Cholesterol is transported into the cells by receptor-mediated endocytosis. Low-density lipoproteins (LDLs) carry cholesterol in the blood. This binds to LDL receptors on the plasma and the membrane invaginates around the LDL and receptor, creating a coated vesicle. The vesicle then uncoats and fuses with the endosome
A proton pump acidifies the endosome thereby breaking LIL binding to its receptor. The receptor is taken back to the membrane surface. the vesicle of cholesterol fuses with lysosomes that contain hydrolytic enzymes and that release cholesterol.
What can defective endocytosis cause?
Defective endocytosis can cause atherosclerosis. Mutations in LDL account for familial cases of atherosclerosis, a CVS disease. This is due to an accumulation of lipoproteins in blood and formation or plaques blocking blood arteries. The coated pit cannot form properly due to a mutation in the receptor and the inability of the receptor to interact with adaptor proteins.
What is the function of clathrin and dynamin?
Clathrin helos to form vesicles from the planar cell membrane. The tri-legged, curved structure of clathrin forces membrane invagination. Clathrin shapes the vesicle, dynamin pinches it off the cell membrane by hydrolysing GTP into GDP. Mutated dynamin cannot hydrolyse GTP and thus cannot pinch off endocytic vesicles. ER and Golgi also use clathrin like coat proteins to pinch off vesicles (COPI and COPII) Newly synthesised ER lipids and proteins are packaged into COPII vesicles.
Explain phagocytosis and autophagy
Phagocytosis is an example of vesicle formation without clathrin. Actin-driven membrane invagination is important in phagocytosis. Actin drives membrane engulfment and endocytosis. Fusion of phagosomes with lysosomes destroys bacteria due to them containing hydrolases. Autophagy is the 3rd pathway towards lysosomal digestion. Autophagy helps to eliminate malfunctioning cell elements. It happens by vesicle fusion and engulfment of organelles. Fusion merges internal membrane compartments despite the fact they are intrinsically negatively charged.
What is the role of SNAREs in merging membranes?
SNARE proteins play a key role in merging the membranes. Secretory pathways also rely on SNARE-mediated membrane fusion. SNARE proteins were discovered when studying exocytosis. The fusion of the vesicle with the target membrane must overcome the repulsion of negatively charged membranes - SNAREs help with docking. Three SNAREs coil around each other to force the membranes together. V-SNAREs are vesicular and t-SNAREs are on the target membrane e.g. VAMP or synaptobrevin) (e.g. syntaxin and SNAP25). SNARE proteins form a tight 4-helical coiled coil on initial contact. SNAP25 contributes 2 helices, syntaxin 1 and VAMP 1. SNARE coiling proceeds into 2 opposing membranes thereby forcing their fusion. After fusion, NSF enzyme catalyses the dissociation of the SNARE coils by hydrolysing
Explain how Botulinum toxin acts
Botulinum neurotoxin attacks SNARE proteins. Botulinum bacteria are present everywhere in soil and they produce toxins which block exocytosis. Botulism happens upon the consumption of contaminated food. SNAREs are responsible for ACh release in NMJ. Botulinum first binds to gangliosides on neuronal membranes. It then enters the luminal space of recycling synaptic vesicles. Following endocytosis, one subunit (SNARE protease) escapses the vesicle, enters the synaptic cytosol and cleaves specific SNARE protein cannot support anymore, the fusion of synaptic vesicles resulting in a long blockade of neurotransmission. Botulinum neurotoxin can be used in medicine for local muscle paralysis e.g. treating spasms. Its also used in plastic surgery (BOTOX) (cleaves SNAP25)
