M&R 6.2 Receptor-mediated endocytosis Flashcards
Describe phagocytosis
Internalisation of material by a (specialised) cell (e.g. macrophages & neutrophils)
Membrane EVAGINATES and particle is internalised by a membrane zippering mechanism
Clears damaged cellular material and invading organisms for destruction
Describe pinocytosis
INVAGINATION of plasma membrane to form a lipid vesicle
Permits uptake of impermeable extracellular SOLUTES and retrieval of plasma membrane
2 types: Fluid phase & Receptor-mediated endocytosis (RME)
What is receptor-mediated endocytosis (RME)?
Selective internalisation of molecules into a cell via binding to specific cell surface receptors
Describe the structure of LDL cholesterol and how this facilitates its transport into cells that need it
Core of esterified cholesterol ester
Covered by phospholipid & cholesterol monolayer containing Apoprotein B
Animal cells that need cholesterol synthesis LDL receptors which specifically recognises ApoB
LDL particle internalised and delivered to lysosomes –> cholesterol released from cholesterol esters
Where are the LDL receptors localised on the membrane? (for RME)
In clathrin-coated pits
Describe what happens when LDL binds to LDL receptors in the clathrin-coated pits, up until LDL dissociates from the receptor
Coated pit invaginates and pinches off, forming a coated vesicle
Vesicles are uncoated (via ATP-dependent uncoating)
Uncoated vesicle fuses with endosome/CURL (compartment for uncoupling of receptor and ligand)
pH in endosome is maintained low (5.5-6) relative to the cytoplasm (7.2)
At the lower pH the receptor has less affinity for the LDL particle so they dissociated
Describe how the dissocated LDL receptors and LDL are sorted in the endosome
The receptors are sequestered to an area of endosome membrane that buds off and recycles the LDL receptor to the plasma membrane
The endosomes containing LDL fuse with lysosomes, so cholesterol can be hydrolysed from the cholesterol esters and released into the cell
Describe the structure of a clathrin coat
Clathrin forms 3-legged triskelions (which each contain 3 clathrin chains (the ‘legs’) and 3 light chains which orient them
The triskelions associate to form a basket-like structure made of hexagons & pentagons
Do clathrin coats require energy to form?
No - they form spontaneously.
(It is thought that clathrin is attached to the cytosolic side of the membrane. Clathrin molecules start to form cages spontaneously which drives the invagination of membrane & pinching off as a coated vesicle)
Does vesicle uncoating require energy?
Yes
the assembly is spontaneous so the uncoating must be energy-driven
How are clathrin-coated vesicles uncoated?
ATP-dependent uncoating protein binds and stabilises the freed coat proteins
Clathrin triskelions are recycled back to newly-forming clathrin coated pits
Overall, what happens to the ligand and what happens to the receptor in LDL endocytosis?
Ligand degraded
Receptor recycled
Describe 3 types of LDL receptor mutations that have been found in homozygous individuals with hyperchoelsterolaemia
- Non-functional receptor (associated with pits so internalised as normal, but has low affinity for LDL so doesn’t bring LDL with it)
- Receptor deficiency (mutation prevents expression of LDL receptor)
- Receptor binding normal (receptor can bind LDL normally but is missing cytoplasmic domain which interacts with clathrin coat. Therefore LDL receptors distributed over whole cell surface - not localised over pits so not internalised)
Describe the process of RME of Fe3+ up until fusion with the endosome
2 x Fe3+ ions bind to apotransferrin in the circulation = transferrin
Transferrin binds to the transferrin receptor in the clathrin-coated pits (apotransferrin cannot bind to this receptor)
Transferrin internalised in a coated vesicle, then uncoated and fuses with the endosome
In RME of Fe3+, describe what happens after fusion with the endosome
Receptor with transferrin (apotransferrin + 2xFe3+) attached enters acidic endosome
Fe3+ ions dissociate, leaving apotransferrin
Apotransferrin remains attached to the transferrin receptor because of the low pH
Apotransferrin-receptor complex is sorted for recycling back to plasma membrane
Neutral pH at plasma membrane allows apotransferrin to dissociate from transferring receptor
in RME of Fe3+, what is the fate of the ligand and the fate of the receptor?
Ligand recycled (apotransferrin) Receptor recycled (transferrin receptor)
In RME of insulin, are the receptors always localised over the coated pits?
No - the insulin receptor only congregates over the coated pits when insulin is bound
(binding probably induces a conformational change in the receptor which allows it to be recognised by the coated pit)
In RME of insulin, what happens after the uncoated vesicle fuses with the endosome?
What is the advantage of this?
Insulin remains bound to the insulin receptor in the endosome
The entire insulin-receptor complex is targeted to the lysosome for degradation
This allows down-regulation of number of insulin receptors on the cell surface in response to high circulating insulin levels -> desensitises cell
In RME of insulin what is the fate of the ligand and what is the fate of the receptor?
Ligand degraded
Receptor degraded
What is transcytosis?
A process by which macromolecules are uptaken into vesicles via RME, then transported across the cell and ejected from the opposite cell membrane via exocytosis
What is the purpose of transcytosis?
To transport large molecules across cells
Name some molecules which are transported by transcytosis, and where they are transported between
Maternal immunoglobulins transported from mother to fetus via placenta
Immunoglobulin A (IgA) transported from the circulation into bile (in the liver)
Describe the process of transcytosis in relation to ligands and receptors
Ligand binds to receptor, uptaken as a coated vesicle, uncoated & fuses with endosome
Ligand remains bound to receptor at endosome pH
Transfer vesicle transports the ligand-receptor complex across cell
During transport the receptor is cleaved - results in release of ligand with bound ‘secretory component’ - part of original receptor
In transcytosis, what is the fate of the ligand and the fate of the receptor?
Ligand transported
Receptor transported
Explain how membrane-enveloped viruses can take advantage of RME
They can bind to cells via fortuitous association with receptors in the clathrin-coated pits - this tricks cells into internalising them by RME
Then enter endosome and use pH to alter their membrane and reveal membrane fusion domains
This allows viral membrane to fuse with endosome membrane and release viral RNA into cytoplasm
Then uses host cell machinery to replicate RNA & proteins in order to bud new viruses at the cell membrane (using membrane of invaded cell to envelop new virus particles)
How do cholera toxin and diptheria toxin enter cells?
Both bind GM1 ganglioside (a phospholipid-like structure found in the plasma membrane) then enter cell via RME
