Membrane Rafts

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Membrane Rafts
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Membrane Microdomains
Raft is a specific type of microdomain
sphingolipid/cholesterol rich region
Separation of discrete liquid-ordered and
liquid-disordered phase domains occurring with
sufficient amounts of cholesterol

• Microdomain formation is believed to be involved
  in following cellular processes
• Cell sorting
• Signal transduction
• Endocytosis
• Calcium homeostasis
• And others

Rafts liquid ordered domain lipids are fluid
in that they have a high degree of lateral
diffusion, but the acyl chains are closed packed
and ordered. Glycosphingolipids (particularly
sphingolmyelin and glycosylphosphoinositol-GPI
anchored proteins preferentially partition into
rafts.)
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The debate Rafts in model membranes vs Rafts in
Biological Membranes
Origin TritonX 100 insoluble components isolated
from biological membranes Detergent Resistant
Membranes DRM. Does DRM always equal a raft
TritonX100 can solubilize DOPCchol but Not
DPPCchol
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Sphingomyelin cholesterol interactions
Sphingomyelin
POPC
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December 2005
4 reviews on domain formation in model membranes
and physical properties that underlie raft
formation 2 reviews to describe techniques used
for studying rafts (FRET) and uncertainty for
detecting rafts in cell membranes Raft Function
in Cells 4 on signal transduction(IgE receptor
signaling, Growth factors, Ras signaling) Ceremid
e Raft function in apoptotic signaling 3 reviews
on raft involvement in Endocytosis (mammalian
viruses, bacterial infections, bacterial
toxins) 2 reviews of caveloae
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Membrane raft Organization
DRMs detergent resistant membranes DIGs detergent
insoluble glycolipid-enriched membranes GEMs
glycolipid enriched membranes TIFFs Triton
insoluble membranes
Raft is more generic as the microdomain can be
caused by protein association, not just
physical properties of the lipids themselves
Rafts may or may not contain caveolin
Caveolin1, caveolin2, caveolin 3, hemagglutinin
and GPI anchored proteins serve as markers for
raft formation
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A liquid domains enriched in cholesterol and
sphingolipids large scale gt 50 nm B lipid
shells, small dynamic, regulated processes C
mosaic of domains, maybe regulated by
cholesterol-based mechanism D small dynamic
multimeric lipid assemblies, dynamic and transient
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Protein sorting
Melittin 26 aa cationic bee venom channels
Role of structural and mechanical properties of
bilayers on peptide-lipid partitioning
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111 mixture of DOPCSPMCHOL, the detergent
insoluble fraction has a thickness that is 9Å
greater than that of the DSM
Role of Bilayer thickness in protein-lipid
interactions possible role in sorting of
proteins via hydrophobic mismatch of the
transmembrane domain (TMD).
Hydrophobic mismatch if there is a mismatch
between the length of the TMD and the hydrocarbon
thickness, then the bilayer would need to deform
to prevent exposure of the hydrophobic amino
acids to water. This would be energetically
unfavorable. So, if the protein can move to a
raft of different thickness, there would be a
driving force for such partitioning.
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Bio significance in GOLGI, proteins with short
TMDs reside in non raft regions, whereas
proteins with longer TMDs reside in raft regions
destined to the plasma membrane (rich in
cholesterol and SPM). Length of TMD has been
indicated to be an important factor in
controlling protein trafficing.
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Experimental studies of peptide sorting by length
Thermal kT 0.6 kcal/mol at 37oC.
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Big Question We can see rafts in Model
Membranes (GUVs or Supported Lipid Bilayers, LM),
but how to study in cells? Do rafts really exist
in cells? Are they static large structures? Are
they small transient structures?
FRET and FRET based Microscopy Techniques
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FRET fluorescence resonance energy transfer
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