Lipid rafts are cholesterol rich domains found on the cell surface, present in the external leaflet of the plasma membrane. These are small micro domains ranging from 10–200 nm in size, characterized by presence of glycosylphosphatidylinositol (GPI)-anchored proteins and proteins involved in signal transduction.
They are dynamic assemblies of phospholipids and glycosphingolipids that contain mostly saturated hydrocarbon chains which allow cholesterol to intercalate between the fatty acyl chains.
Lipid rafts :
- float freely in the membrane bi-layer, hence termed rafts.
- more ordered and tightly packed than the surrounding bi-layer.
- Are the regions of the membranes with a distinct characteristic structural composition (enriched of sphingolipid (< 50%) and cholesterol (3 to 5 folds) and saturated phospholipid that appear to act as platform to co-localize protein involved in the intracellular signaling pathway.
Presence of saturated fatty acid and cholesterol makes the membrane closely packed so lipid rafts are more ordered and less fluid than surrounding. The lipids in the outer leaflet of the raft have an organizing effect on the lipids of the inner leaflet. As a result, the inner‐leaflet raft lipids consist primarily of cholesterol and glycerophospholipids with long saturated fatty acyl tails. The inner leaflet tends to concentrate lipid‐anchored proteins, that are involved in cell signaling.
They modulate membrane distribution of receptors and signaling molecules facilitating assembly of active signaling platforms.