Tomlinson - Lipid Rafts Flashcards
Describe the structure of Sphingolipids wrt glycerolipids
Sphingolipids possess longer hydrocarbon tails, are fully saturated and therefore more straighter and stiffer than the glycerolipids counterparts
Glycerolipids greater resemble which phase of the bilayer, the ordered or disordered phase?
Disordered
What are the three components of a lipid raft?
Sphingolipid
Cholesterol
Protein
Most phospholipid bilayers are electrically polarised at..?
-60 mV
Phospholipids are amphipathic and form bimolecular sheets, what does “amphipathic” mean?
It has a hydrophobic interior (fatty acid tails) and hydrophilic exterior (polar head groups)
What is the phospholipid bilayer generically comprised of?
Wholly lipids and proteins, with a few associated carbohydrates
What is the thickness of a phospholipid bilayer?
5 nM wide
Describe the structure of glycerolipids wrt sphinoglipids
Glycerolipids possess shorter hydrocarbon chains, tend to be unsaturated with C=C Cis double bonds and therefore are bulkier
Sphingolipids greater resemble which phase of the bilayer, the ordered or disordered phase?
Ordered
What are the three major classes of membrane residing lipids?
Glycerolipids
Sphingolipids
Cholesterol
How do lipid rafts help with efficient signal transduction?
For efficient signal transduction to occur, it helps if the associated machinery is co-localised or compartmentalised.
What does cholesterol associate better with, glycerolipids or sphinoglipids?
Sphingolipids
In what two ways may a lipid raft micro-domain coalesce to form a larger raft platform?
1) via external signal transduction
2) membrane trafficking events which induce:
- lipid-lipid interactions
- lipid-protein interactions
- protein-protein interactions
What is the current definition of a lipid raft?
- Sphingolipid-Cholesterol-Protein enriched micro domains
- Less than 100 nm in length
- Preferentially recruit GPI-anchored proteins
- Preferentially recruit Palmitoylated proteins (PTM)
From a molecular perspective, why does Cholesterol interact well with Sphingolipid?
The Sphingolipids possess long hydrocarbon fully saturated fatty acid chains which are stiff and straight in structure. This accommodates the contrastingly bulkier sterol cholesterol 4-ring structure. There is also opportunity for an extensive hydrogen bonded network to occur, given the dipole-dipole interactions between cholesterol and Sphingolipids
What is the take home message regarding controversies in lipid raft research?
“Membrane heterogeneity can be induced or modified when trying to observe it.”
State the name of four cholesterol depleting drugs
Methyl-beta-cyclodextrin, which has a sugar ring structure wit a hydrophobic interior and interacts well with the sterol group of Cholesterol
Polyene antibiotics such as Filipin and Nystatin
Cholesterol synthesis inhibitors such as HMG-CoA Reductase
What is the in vitro evidence to suggest cholesterol interacts well with Sphingolipids?
Phosphatidylcholine + Sphingomyelin = freely diffuse structure
Phosphatidylcholine + Sphingomyelin + Cholesterol = Sphingomyelin and Cholesterol assemblies compartmentalised away from PC
Are lipid rafts artefacts of detergent use in studies? Discuss
Early studies defined raft assemblies as “Detergent Resistance Membranes”, following detergent solubilisation at 4 degrees. Papers often published suggested their proteins resided within the DRM and therefore if they had signalling capability would do so involving raft machinery. However these results were not replicable and detergent solubilisation is artificial. It depends on:
- The detergent: Triton X-100, CHAPS, SDS
- Critical Micelle concentration
- The temperature
- Solubilisation period
DRMs are therefore not direct descendants of pre-existing lipid rafts
State the components of the phospholipid bilayer
Peripheral proteins Surface proteins Integral proteins (globular) Integral proteins (alpha helical) Cholesterol Protein channels Glycolipids Glycoproteins
State the names of two Polyene antibiotics which deplete cholesterol levels
Filipin and Nystatin
Should cholesterol extracting drugs be used to study rafts?
In order to determine whether lipid rafts are important for a particular proces, studies have utilised cholesterol extracting drugs I.e. Filipin, nystatin, methylbetacyclodextrin to remove cholesterol from rafts and observe the effects. However it also removes cholesterol from the rest of the membrane too (40% cholesterol component) which can have effects such as unwanted membrane depolarisation and calcium signalling
Describe how lipid rafts are used in HIV assembly
- Gag binds to PIP2, and flips its own myristoylation into the inner leaflet of the plasma membrane, replacing the unsaturated fatty acids of PIP2.
- This drives lipid raft formation at the plasma membrane of the host cell.
- Gag multimerisation drives the assembly of the virus particle.
State the name of three detergents
Triton-X100, SDS, CHAPS
