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#membrane microdomains

3 public questions tagged with this topic.

Plasma Membrane

Which of the following statements about lipid rafts is correct?

Lipid rafts are defined as transient, nanoscale domains 10-200 nm enriched in saturated sphingolipids including sphingomyelin and glycosphingolipids, cholesterol, and GPI-anchored proteins plus acylated inner leaflet proteins like Src kinases, exhibiting liquid-ordered phase where acyl chains are extended and ordered yet laterally mobile, surrounded by liquid-disordered PC-rich matrix. Their cholesterol dependence arises from hydrogen bonding between cholesterol hydroxyl and sphingolipid amide, plus van der Waals interaction with saturated chains, increasing order and thickness. Importantly rafts are highly dynamic, assembling and disassembling on millisecond timescales, merging upon receptor crosslinking to concentrate signaling components like B cell receptor, LAT in T cells, and endothelial nitric oxide synthase. They do not lack cholesterol nor are restricted to bacteria which lack cholesterol but use hopanoids analogously. Techniques including FRET, super-resolution STED, single-particle tracking, and isolation of detergent-resistant membranes support their existence as platforms for endocytosis, viral entry, and immune synapse organization, regulating protein-protein interactions without requiring higher overall protein density.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Lipid Rafts and Membrane Domains.

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Glycolipid and Phospholipid

Which of the following is NOT abundant in lipid rafts?

Isolation detergent resistant membranes quantitative shotgun lipidomics show selective lipid sorting excluding certain anionic glycerophospholipids and enriching sphingolipid cholesterol species. Sphingomyelin cholesterol ceramide cerebrosides galactosylceramide gangliosides GM1 associate preferentially due saturated acyl chains fourteen to twenty four carbons hydrogen bonding amide hydroxyl enabling tight packing into liquid ordered phase high chain order low fluidity lateral diffusion moderate. Glycolipids project bulky headgroups outward stabilizing raft extracellular leaflet sterically hindering phospholipase access. Phosphatidylserine with often unsaturated oleoyl arachidonoyl tail serine carboxyl amine conferring net negative charge pH seven favors liquid disordered phase interior leaflet interacting annexins PKC KRAS polybasic clusters via electrostatic attraction membrane potential negative inside. Hence PS consistently underrepresented raft fractions measured TLC mass spec relative enrichment factor zero point three versus SM factor three fold enrichment. Differential partitioning underpins transbilayer coupling model inner PS clustering reciprocally stabilizes outer raft via long chain interdigitation cholesterol flipping. Recognizing PS depletion explains why calcium induced PS externalization disrupts raft organization triggers coagulation tenase assembly apoptosis signaling distinct raft functions interleaflet communication examined biophysics and cell biology questions advanced level.

Ref: Levental I et al., PNAS 2010, Lipid raft composition PS depleted not enriched and coupling.

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Glycolipid and Phospholipid

Which feature allows lipid rafts to regulate signal transduction?

Signal compartmentalization arises because cholesterol sphingolipid rich liquid ordered platforms concentrate specific proteins while excluding others lowering activation threshold facilitating downstream signaling. Cholesterol fills interstitial gaps between sphingomyelin saturated chains raising order parameter decreasing fluidity variance thickening bilayer promoting sequestration GPI anchored proteins dual acylated Src family kinases Fyn Lyn palmitoylated LAT adaptor PAG and flotillins. Extracellular ganglioside glycans form lattice galectins stabilizing clusters via multivalent interactions. Upon receptor ligation partitioning increases local kinase substrate concentration facilitating trans phosphorylation triggering Ras MAPK PI3K cascades crucial TCR BCR Fc receptor signaling immune activation. Depletion cyclodextrin disperses clusters blunts T cell activation influenza virus entry eNOS regulation insulin receptor signaling GLUT4 translocation. High phospholipid content without cholesterol fails forming ordered domains; high water solubility precludes membrane residence preventing anchoring. Absence protein interactions prevents function raft mediated scaffolding. Thus cholesterol sphingolipid co enrichment mechanistic determinant raft driven signaling essential immunology neurobiology frequently tested cell biology describing microdomain functions and experimental dependence cholesterol depletion assays for raft involvement in signal transduction mechanisms and drug discovery.

Ref: Simons & Toomre, Nature Rev Mol Cell Biol 2000, Raft signaling dependence cholesterol sphingolipids and kinases.

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