Practice question

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.