LipidRafts

Lipid rafts are dynamic, nanoscale domains within cellular membranes that are enriched in cholesterol and sphingolipids. Compared with the surrounding phospholipid bilayer, raft regions exhibit a more ordered, tightly packed state known as liquid-ordered (Lo) domains, while excluding many unsaturated phospholipids. This compositional bias is associated with specific lipid and protein constituents that partition preferentially into rafts.

Common raft components include glycosphingolipids, cholesterol, and proteins anchored by glycosylphosphatidylinositol (GPI) or palmitoylation, as well as certain signaling molecules such as Src-family kinases, G protein-coupled receptors, and receptor tyrosine kinases. Through local clustering, lipid rafts can concentrate signaling complexes, facilitate receptor activation, and influence intracellular trafficking and endocytosis. Rafts have also been implicated in antigen presentation and in the entry of several pathogens.

Size and dynamics: In living cells, rafts are thought to be nanoscale and highly dynamic, with lifetimes ranging from milliseconds to seconds and capable of coalescing into larger assemblies upon stimulation. Their exact dimensions and abundance remain subjects of investigation.

Detection and controversy: Early work used detergent-resistant membranes to infer raft existence, but this approach can artifactually induce domains. Modern methods include cholesterol manipulation, fluorescence resonance energy transfer, single-particle tracking, and super-resolution imaging, which collectively support a model of transient, heterogeneous lipid domains rather than stable, uniform structures. The functional relevance of rafts is recognized, but their in vivo organization and ubiquity remain debated.