Lecture 6 Lipid Bilayer And Rafts Flashcards
Membrane microdomains: lipid rafts
Cholesterol and sphingolipids cluster with specific proteins in membrane microdomains
Membrane lipids are not randomly distributed (evenly mixed) in each leaflet of the bilayer
The existence of stable cholesterol and sphingolipids suggests more rigid structures - rafts? That form and unform bringing proteins and lipids together to interact
Rafts in natural membranes typically 50nm in diameter
Lipid rafts
Typically 50nm - thicker and more defined than surrounding membrane areas due to richness in sphingolipid cholesterol.
Evidence for rafts
The lipids that remain after extraction (solubilisation) of mammalian plasma membranes with non ionic detergent e.g. Triton X-100 contain cholesterol and sphingolipid
Rafts can be disrupted by methyl-beta cyclodextrin that extracts cholesterol - this shows that cholesterol plays a structural role
Raft fragments are insoluble in ionic detergents as they contain a subset of plasma membrane proteins thought to transmit extracellular signals into the cytosol
Rafts are rich in glycolipids which can be viewed under a microscope by adding fluorocholera toxin that bonds to gangliosides
Proteins interact with membranes in 3 ways
Lipid anchored: soluble, attached to membrane
Peripheral - attached to other proteins
Integral(transmembrane) - have hydrophobic domain(s) and hydrophilic areas
Transmembrane proteins
Most:
have membrane spanning alpha helices
Are stably imbedded in membrane because of energetically favourable hydrophobic and Van Der Waals interactions of hydrophobic chains with specific lipids and ionic interactions with phospholipid head groups
Coiled coiled dimer stabilised by Van Der Waals interactions between adjacent side chains
Peripheral membrane proteins
Do not directly connect to hydrophobic core
Bind to integral or lipid anchored proteins or directly to lipid head groups
Adapter proteins may link membrane to cytoskeleton
Attached to ECM
Lipid anchored proteins
Covalently linked to phospholipid membrane
Hydrophobic tail embedded in phospholipid membrane
Covalently attached lipids
Anchor some otherwise water soluble proteins to one or the other plasma membrane leaflet in eukaryotes
Proteins anchored to cytosolic membrane by one of 3 methods
Acylation - single fatty acyl chain
Prenylation - prenyl group thioether bond
GPI (glycophosphatidylinositol) lipid anchor, phosphatidylinositol consists of 2 fatty acid chains inserted into the bilayer. Phosphoethanolamine links protein to anchor
GPI anchored proteins may cluster in rafts
Caveola
In some cases a second geranylgeranyl group or a fatty acyl plamitate group is linked to a nearby cysteine residue.
The additional hydrocarbon anchor is thought to reinforce the attachment of the protein to the membrane.
This forms a caveola. Caveolin is inserted into the caveola membrane creating a hairpin domain which may have a role in cholesterol interactions.
SDF - 1/CXCL12 - mediated chemotaxis
Requires dual acylation and lipid raft association of Src-family protein tyrosine kinases in the Jurkat human T cell lymphoma cell line
Lipid raft function
By bringing many key proteins into close proximity and stabilising interactions lipid rafts may facilitate cell surface receptors and subsequent activation of cytosolic events.
Much is still not known
Some lipid raft associated lipids such as glycolipids may permit interaction of their tails across the hydrophobic core and help organise lipids in the leaflet and formation of signalling platforms
Caveola
Caveolin inserted into caveola membrane with N and C termini facing cytoplasm and a putative “hairpin” intramembrane domain embedded within the membrane bilayer.
Scaffolding domain, highly conserved region of caveolin might have a role in cholesterorol interactions through conserved basic (+) and bulky hydrophobic residues.
C terminal domain close to intramembrane domain is modified by palmitolyl groups that insert into the bilayer l.
The complex structures formed by interconnected caveola can occupy a large area of the plasma membrane
