Lipid Rafts

Hanging Ten on the Plasma Membrane

Science's STKE  21 Mar 2000:
Vol. 2000, Issue 24, pp. tw9
DOI: 10.1126/stke.2000.24.tw9

Lipid rafts are insoluble complexes of cholesterol, glycosphingolipids, sphingomyelin, and saturated glycerophospholipids that are located on plasma membranes. The importance of lipid rafts for tethering signaling proteins has recently become appreciated. However, the stability and the mobile diffusion of the rafts upon the plasma membrane have not been well characterized. Pralle et al. used a laser trap to immobilize lipid rafts and determined that glycosylphosphatidylinositol (GPI)-anchored proteins and transmembrane proteins in rafts diffuse to a much lower extent than membrane proteins not contained within lipid rafts. Depleting cholesterol from the rafts greatly increased the rate of diffusion for signaling proteins within rafts. GPI-anchored proteins were never observed leaving the rafts (up to 10 minutes). The mean area of the lipid rafts was determined to be approximately 2100 nm2. The authors estimated that each raft contained about 3500 lipid molecules, and each had fewer than 60 proteins per raft, if densely packed. Thus, the importance of rafts in the signaling process appears to be at least twofold: (i) small numbers of signaling proteins are tethered at the plasma membrane where they cannot diffuse, and (ii) clustering of the rafts recruits enough signaling proteins in close proximity to form a competent signal system.

Pralle, A., Keller, P., Florin, E.-L., Simons, K., and Hörber, J.K.H. (2000) Sphingolipid-cholesterol rafts diffuse as small entities in the plasma membrane of mammalian cells. J. Cell Biol. 148: 997-1008. [Abstract] [Full Text]