Lipid raft

Lipid rafts are cholesterol-rich microdomains in cell membranes.

Since 1972, it has been believed that, in cell membranes, phospholipids and membrane proteins are ubiquitously distributed according to a fluid mosaic model. However, in 1998, Kai Simons at the European Molecular Biology Laboratory (EMBL) in Germany suggested the novel idea that there exist microdomains, which are enriched with many kinds of lipids such as cholesterol, glycolipids, and sphingolipids, present in cell membranes. He called these microdomains "lipid rafts". The concept of lipid rafts has been related to the liquid-liquid immiscibility observed within model membranes. This immmiscibility has been reported between two liquid crystalline phases, the liquid ordered phase (Lo phase) and the liquid disordered phase (Ld or Lalpha phase).

The evidence for lipid rafts is still very controversial and has yet to be confirmed despite many experiments involving several different methods e.g. detergent extraction and fluorescence cell imaging. Furthermore, recent studies of cell membranes reported that lipid rafts contain not only lipid components but also numerous signaling molecules such as GPI-anchored proteins and receptor or non-receptor type tyrosine kinase, which modulate various physiological processes in cells. Therefore, lipid rafts are thought to act as a platform for protein segregation and signaling in cells. However, the functional role of lipid rafts still remains very uncertain because it has not been determined whether they have a positive effect or negative effect on signal transduction in cells. Investigating the effect of the lipid components on cellular signaling pathways may provide a novel understanding of the roles of lipid rafts in cells.