also been reported during the multi- valent binding of SV40 virus to its GM1 rec

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also been reported during the multi- valent binding of SV40 virus to its GM1 receptor (35). Invagination from the plasma membrane was dependent on having longer reoeptor hydrocarbon chains, which are com- mon to sphingolipid, and suggests that the effect is mediated by line tension arising between membrane domains of different compositional order (35). Coalescence of dynamic beterogeneity also oocurs during sig- Raft maling, for example, during the for- mation of B cell receptor (BCR) or T cell receptor (TCR) foci. Antigen binding induces the dynamic asso- ciation of BCR to its sigmaling ef fector Lyn kinase and leads to the formation of an immune sympase. The interaction is dependent on the nature of Lyn lipid anchorage, with drocarbon chains preventing associa- tion with the BCR (36) During TCR activation, raft components of this roceptor complex (e.g, GPl- anchored proteins) becone selectively immo- bilized in nanoscale clusters (37). seeding the accumulation of choles- terol, sphingomyelin, and saturated and long-chain phosphatidyl choline into the synapse, effectively sorting proteins according to their affinity for raft domains (38), Rafts in this "activated" or coalesced condition constitute a more ordered assen- blage: a fluid membrune enviroement in which proteins can be modulated specifically (39 yet the exists sopa nk y from the surrounding membrane Fig. 2. Hierarchy of raft-based heterogeneity in cell membranes ? Fuctuating nanoscale assemblies of sterol, and rich in unsaturated glycerophospho- sphingolipid-related biases in lateral composition. This sphingolipid/sterol assemblage potential can be accessed lpid. Raft activation is oftien stabi andior modulated by GPHanchared proteins, certain TM proteins, acylated cytosolic effectors, and cortical actin lized or nucleaded by scaffolding Gray proteins do not possess the chemical or physical specificity to associate with this membrane connectivity and glycerophospholipid SM, sphingomyelin. (B) Nanoscale heterogeneity is functionatized t protein-mediated activation events (eg, multivalent ligand binding, synapse for- mation, protein oligomerization) that trigger the coalescence of membrane arder-forming lipids with their accom panying selective chemical and physical specificities for protein. This level of lateral sorting can also be buttressed by cortical actin. (O The membrane basis for heterogenelity as revealed by the acthvation of raft phase coalescence at equlibrium in plasma-membrane spheres. Separated from the influence of cortical actin and in the absence of membrane traffic, multivalent clustering of raft tipids can amplity the functional level to a microscopk membrane Membrane constituents are laterally sorted according to preferences for membrane order and chemical elements such as cortical actin (40) are considered non-raft. GPL the mole fraction of sphingolipids and cholesterol increases, as is the case in the apical membrane of epithelial cells (47) Phase Separation in with raft markers at the cell surface, raft TM phase (47) Remarikably, this phase coexistence fom the influence of cytoskelesal, endocytie, or proteins are depleted from the tightly packod Lo indicates that afler chemical modification of exocytic processes in a cell line enriched in the phase when reconstituted in moded systems (42,43 procin (eg. Sormaldehyde cross-inking, thiol aft ganglioside GM1. Pentavalent clustering by membrunes is unlikely to be identical to raf-based based liquid-iquid phase sepiration can be cence of a micron-scale raft phase" at 37 c beterogeneity in plasma membranes that selec- manifested by the plasma membans, depite ts selectively reorganining the lateral distributioe of tively includes TM proteins (44, 45). Giant compositional conplexity Now the question i proteins and lipids according to their predicted plasma-membrane vesicles isolated by a dem bw mi phase ag ale separaton ake affinity for raft danans (4.) In this ca e sele. ical membrane blebbing procedure can be place in plasena membranes physiological tive incorporation of TM prolcins was achieved cooled to phase separate into Lo- and Ld-like teetres? phases (46), and here also, raft TM proteins are Sone insight has come from a cell-swelling model-membrane Lo phasos (45) Thus, where typically excluded froem the oederod membeane proce toseparte plasma-mambene spheres as preferential lipid-lipid associasions do under at a lipid-ordering level far below that observed

Explanation / Answer

The diagram represents the hierarchy of raft based heterogeneity in cell membranes. It depicts the fluctuating nanoscale assemblies of sterols and sphingolipids. These molecules can be regulated by GPI anchored proteins and certain TM proteins. This kind of lateral sorting of sterols and sphingolipids is mediated by cortical actin. Finally, the membrane constituents are sorted according to preferences for membrane order and chemical interactions.

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