Practice question

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.