Membrane Rafts‘70s

‘40s ‘50s

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.)

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 DOPC:chol butNot DPPC:chol

Sphingomyelin – cholesterol interactions

Sphingomyelin

POPC

4 reviews on domain formation in model membranes and physical properties that underlie raft formation2 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)

Ceremide Raft function in apoptotic signaling3 reviews on raft involvement in Endocytosis (mammalian viruses, bacterial infections, bacterial toxins)2 reviews of caveloae

December 2005

Membrane raft Organization

DRMs detergent resistant membranesDIGs detergent insoluble glycolipid-enriched membranesGEMs glycolipid enriched membranesTIFFs 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

A: liquid domains enriched in cholesterol and sphingolipids – large scale > 50 nmB: lipid shells, small dynamic, regulated processesC: mosaic of domains, maybe regulated by cholesterol-based mechanismD: small dynamic multimeric lipid assemblies, dynamic and transient

Role of structural and mechanical properties of bilayers on peptide-lipid partitioning

Melittin: 26 aa cationic bee venom: channels

Protein sorting

1:1:1 mixture of DOPC:SPM:CHOL, 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.

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.

Experimental studies of peptide sorting by length

Thermal kT = 0.6 kcal/mol at 37oC.

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

FRET fluorescence resonance energy transfer