Lipid Rafts in Binary Lipid/Cholesterol Bilayers

Abstract

The lipid membrane was once modelled as a homogeneous two-dimensional fluid, however, it is in fact a patchy, inhomogeneous state of matter. Inhomogeneities in membranes are now understood to be crucial to their function. These so-called lipid rafts are described as lateral lipid assemblies enriched in cholesterol, which have a greater degree of molecular order. They are believed to play a role in membrane bound protein insertion, sorting, and function. However, size, lifetime, and even the existence of rafts are still the subject of debate. This chapter discusses a role for lipid rafts in membrane homeostasis, experimental techniques to detect rafts, and our current physical models of how rafts may form. In particular, recent X-ray and neutron scattering experiments will be highlighted, which have revealed the lateral molecular ordering of cholesterol in lipid membranes, and have enhanced our understanding of lipid rafts. Those experiments, in particular, demonstrated that a drug, such as aspirin, can interrupt membrane homeostasis by influencing the formation of lipid rafts.